aboutsummaryrefslogtreecommitdiff
path: root/libbuild2/cc/guess.cxx
blob: 40fb14f797154a5a5ef238e2c6a74f91ab713b40 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
// file      : libbuild2/cc/guess.cxx -*- C++ -*-
// copyright : Copyright (c) 2014-2019 Code Synthesis Ltd
// license   : MIT; see accompanying LICENSE file

#include <libbuild2/cc/guess.hxx>

#include <map>
#include <cstring>  // strlen(), strchr()

#include <libbuild2/diagnostics.hxx>

using namespace std;

namespace build2
{
  namespace cc
  {
    string
    to_string (compiler_type t)
    {
      string r;

      switch (t)
      {
      case compiler_type::clang: r = "clang"; break;
      case compiler_type::gcc:   r = "gcc";   break;
      case compiler_type::msvc:  r = "msvc";  break;
      case compiler_type::icc:   r = "icc";   break;
      }

      return r;
    }

    compiler_id::
    compiler_id (const std::string& id)
    {
      using std::string;

      size_t p (id.find ('-'));

      if      (id.compare (0, p, "gcc"  ) == 0) type = compiler_type::gcc;
      else if (id.compare (0, p, "clang") == 0) type = compiler_type::clang;
      else if (id.compare (0, p, "msvc" ) == 0) type = compiler_type::msvc;
      else if (id.compare (0, p, "icc"  ) == 0) type = compiler_type::icc;
      else
        throw invalid_argument (
          "invalid compiler type '" + string (id, 0, p) + "'");

      if (p != string::npos)
      {
        variant.assign (id, p + 1, string::npos);

        if (variant.empty ())
          throw invalid_argument ("empty compiler variant");
      }
    }

    string compiler_id::
    string () const
    {
      std::string r (to_string (type));

      if (!variant.empty ())
      {
        r += '-';
        r += variant;
      }

      return r;
    }

    string
    to_string (compiler_class c)
    {
      string r;

      switch (c)
      {
      case compiler_class::gcc:  r = "gcc";  break;
      case compiler_class::msvc: r = "msvc"; break;
      }

      return r;
    }

    // Standard library detection for GCC-class compilers.
    //
    // The src argument should detect the standard library based on the
    // preprocessor macros and output the result in the stdlib:="XXX" form.
    //
    static string
    stdlib (lang xl,
            const process_path& xp,
            const strings* c_po, const strings* x_po,
            const strings* c_co, const strings* x_co,
            const char* src)
    {
      cstrings args {xp.recall_string ()};
      if (c_po != nullptr) append_options (args, *c_po);
      if (x_po != nullptr) append_options (args, *x_po);
      if (c_co != nullptr) append_options (args, *c_co);
      if (x_co != nullptr) append_options (args, *x_co);
      args.push_back ("-x");
      switch (xl)
      {
      case lang::c:   args.push_back ("c");   break;
      case lang::cxx: args.push_back ("c++"); break;
      }
      args.push_back ("-E");
      args.push_back ("-");  // Read stdin.
      args.push_back (nullptr);

      // The source we are going to preprocess may contains #include's which
      // may fail to resolve if, for example, there is no standard library
      // (-nostdinc/-nostdinc++). So we are going to suppress diagnostics and
      // assume the error exit code means no standard library (of course it
      // could also be because there is something wrong with the compiler or
      // options but that we simply leave to blow up later).
      //
      process pr (run_start (3     /* verbosity */,
                             xp,
                             args.data (),
                             -1    /* stdin */,
                             -1    /* stdout */,
                             false /* error  */));
      string l, r;
      try
      {
        // Here we have to simultaneously write to stdin and read from stdout
        // with both operations having the potential to block. For now we
        // assume that src fits into the pipe's buffer.
        //
        ofdstream os (move (pr.out_fd));
        ifdstream is (move (pr.in_ofd),
                      fdstream_mode::skip,
                      ifdstream::badbit);

        os << src << endl;
        os.close ();

        while (!eof (getline (is, l)))
        {
          size_t p (l.find_first_not_of (' '));

          if (p != string::npos && l.compare (p, 9, "stdlib:=\"") == 0)
          {
            p += 9;
            r = string (l, p, l.size () - p - 1); // One for closing \".
            break;
          }
        }

        is.close ();
      }
      catch (const io_error&)
      {
        // Presumably the child process failed. Let run_finish() deal with
        // that.
      }

      if (!run_finish (args.data (), pr, false /* error */, l))
        r = "none";

      if (r.empty ())
        fail << "unable to determine " << xl << " standard library";

      return r;
    }

    // C standard library detection on POSIX (i.e., non-Windows) systems.
    // Notes:
    //
    // - We place platform macro-based checks (__FreeBSD__, __APPLE__, etc)
    //   after library macro-based ones in case a non-default libc is used.
    //
    static const char* c_stdlib_src =
"#if !defined(__STDC_HOSTED__) || __STDC_HOSTED__ == 1                      \n"
"#  include <stddef.h>    /* Forces defining __KLIBC__ for klibc.        */ \n"
"#  include <limits.h>    /* Includes features.h for glibc.              */ \n"
"#  include <sys/types.h> /* Includes sys/cdefs.h for bionic.            */ \n"
"                         /* Includes sys/features.h for newlib.         */ \n"
"                         /* Includes features.h for uclibc.             */ \n"
"#    if defined(__KLIBC__)                                                 \n"
"     stdlib:=\"klibc\"                                                     \n"
"#  elif defined(__BIONIC__)                                                \n"
"     stdlib:=\"bionic\"                                                    \n"
"#  elif defined(__NEWLIB__)                                                \n"
"     stdlib:=\"newlib\"                                                    \n"
"#  elif defined(__UCLIBC__)                                                \n"
"     stdlib:=\"uclibc\"                                                    \n"
"#  elif defined(__dietlibc__) /* Also has to be defined manually by     */ \n"
"     stdlib:=\"dietlibc\"     /* or some wrapper.                       */ \n"
"#  elif defined(__MUSL__)     /* This libc refuses to define __MUSL__   */ \n"
"     stdlib:=\"musl\"         /* so it has to be defined by user.       */ \n"
"#  elif defined(__GLIBC__)    /* Check for glibc last since some libc's */ \n"
"     stdlib:=\"glibc\"        /* pretend to be it.                      */ \n"
"#  elif defined(__FreeBSD__)                                               \n"
"     stdlib:=\"freebsd\"                                                   \n"
"#  elif defined(__APPLE__)                                                 \n"
"     stdlib:=\"apple\"                                                     \n"
"#  else                                                                    \n"
"     stdlib:=\"other\"                                                     \n"
"#  endif                                                                   \n"
"#else                                                                      \n"
"  stdlib:=\"none\"                                                         \n"
"#endif                                                                     \n";

    // Pre-guess the compiler type based on the compiler executable name and
    // also return the start of that name in the path (used to derive the
    // toolchain pattern). Return empty string/npos if can't make a guess (for
    // example, because the compiler name is a generic 'c++'). Note that it
    // only guesses the type, not the variant.
    //
    static pair<compiler_type, size_t>
    pre_guess (lang xl, const path& xc, const optional<compiler_id>& xi)
    {
      tracer trace ("cc::pre_guess");

      // Analyze the last path component only.
      //
      const string& s (xc.string ());
      size_t s_p (path::traits_type::find_leaf (s));
      size_t s_n (s.size ());

      // Name separator characters (e.g., '-' in 'g++-4.8').
      //
      auto sep = [] (char c) -> bool
      {
        return c == '-' || c == '_' || c == '.';
      };

      auto stem = [&sep, &s, s_p, s_n] (const char* x) -> size_t
      {
        size_t m (strlen (x));
        size_t p (s.find (x, s_p, m));

        return (p != string::npos &&
                (      p == s_p || sep (s[p - 1])) && // Separated beginning.
                ((p + m) == s_n || sep (s[p + m])))   // Separated end.
        ? p
        : string::npos;
      };

      using type = compiler_type;
      using pair = std::pair<type, size_t>;

      // If the user specified the compiler id, then only check the stem for
      // that compiler.
      //
      auto check = [&xi, &stem] (type t, const char* s) -> optional<pair>
      {
        if (!xi || xi->type == t)
        {
          size_t p (stem (s));

          if (p != string::npos)
            return pair (t, p);
        }

        return nullopt;
      };

      // Warn if the user specified a C compiler instead of C++ or vice versa.
      //
      lang o;                   // Other language.
      const char* as (nullptr); // Actual stem.
      const char* es (nullptr); // Expected stem.

      switch (xl)
      {
      case lang::c:
        {
          // Keep msvc last since 'cl' is very generic.
          //
          if (auto r = check (type::gcc,   "gcc")  ) return *r;
          if (auto r = check (type::clang, "clang")) return *r;
          if (auto r = check (type::icc,   "icc")  ) return *r;
          if (auto r = check (type::msvc,  "cl")   ) return *r;

          if      (check (type::gcc,   as = "g++")    ) es = "gcc";
          else if (check (type::clang, as = "clang++")) es = "clang";
          else if (check (type::icc,   as = "icpc")   ) es = "icc";
          else if (check (type::msvc,  as = "c++")    ) es = "cc";

          o = lang::cxx;
          break;
        }
      case lang::cxx:
        {
          // Keep msvc last since 'cl' is very generic.
          //
          if (auto r = check (type::gcc,   "g++")    ) return *r;
          if (auto r = check (type::clang, "clang++")) return *r;
          if (auto r = check (type::icc,   "icpc")   ) return *r;
          if (auto r = check (type::msvc,  "cl")     ) return *r;

          if      (check (type::gcc,   as = "gcc")  ) es = "g++";
          else if (check (type::clang, as = "clang")) es = "clang++";
          else if (check (type::icc,   as = "icc")  ) es = "icpc";
          else if (check (type::msvc,  as = "cc")   ) es = "c++";

          o = lang::c;
          break;
        }
      }

      if (es != nullptr)
        warn << xc << " looks like a " << o << " compiler" <<
          info << "should it be '" << es << "' instead of '" << as << "'?";

      // If the user specified the id, then continue as if we pre-guessed.
      //
      if (xi)
        return pair (xi->type, string::npos);

      l4 ([&]{trace << "unable to guess compiler type of " << xc;});

      return pair (invalid_compiler_type, string::npos);
    }

    // Guess the compiler type and variant by running it. If the pre argument
    // is not empty, then only "confirm" the pre-guess. Return empty result if
    // unable to guess.
    //
    struct guess_result
    {
      compiler_id id;
      string signature;
      string checksum;
      process_path path;

      guess_result () = default;
      guess_result (compiler_id i, string&& s)
          : id (move (i)), signature (move (s)) {}

      bool
      empty () const {return id.empty ();}
    };

    // Allowed to change pre if succeeds.
    //
    static guess_result
    guess (const char* xm,
           lang,
           const path& xc,
           const optional<compiler_id>& xi,
           compiler_type& pre,
           sha256& cs)
    {
      tracer trace ("cc::guess");

      assert (!xi || xi->type == pre);

      using type = compiler_type;
      const type invalid = invalid_compiler_type;

      guess_result r;

      process_path xp;
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << " to override";
          });

        // If we are running in the Visual Studio command prompt, add the
        // potentially bundled Clang directory as a fallback (for some reason
        // the Visual Studio prompts don't add it to PATH themselves).
        //
        dir_path fallback;

#ifdef _WIN32
        if (pre == type::clang)
        {
          if (optional<string> v = getenv ("VCINSTALLDIR"))
          {
            try
            {
              fallback = ((dir_path (move (*v)) /= "Tools") /= "Llvm") /= "bin";
            }
            catch (const invalid_path&)
            {
              // Ignore it.
            }
          }
        }
#endif
        // Only search in PATH (specifically, omitting the current
        // executable's directory on Windows).
        //
        xp = run_search (xc,
                         false     /* init (note: result is cached) */,
                         fallback,
                         true      /* path_only */);
      }

      // Start with -v. This will cover gcc and clang.
      //
      // While icc also writes what may seem like something we can use to
      // detect it:
      //
      // icpc version 16.0.2 (gcc version 4.9.0 compatibility)
      //
      // That first word is actually the executable name. So if we rename
      // icpc to foocpc, we will get:
      //
      // foocpc version 16.0.2 (gcc version 4.9.0 compatibility)
      //
      // In fact, if someone renames icpc to g++, there will be no way for
      // us to detect this. Oh, well, their problem.
      //
      if (r.empty () && (pre == invalid   ||
                         pre == type::gcc ||
                         pre == type::clang))
      {
        auto f = [&xi] (string& l, bool last) -> guess_result
        {
          if (xi)
          {
            // The signature line is first in Clang and last in GCC.
            //
            if (xi->type != type::gcc || last)
              return guess_result (*xi, move (l));
          }

          // The gcc/g++ -v output will have a last line in the form:
          //
          // "gcc version X.Y.Z ..."
          //
          // The "version" word can probably be translated. For example:
          //
          // gcc version 3.4.4
          // gcc version 4.2.1
          // gcc version 4.8.2 (GCC)
          // gcc version 4.8.5 (Ubuntu 4.8.5-2ubuntu1~14.04.1)
          // gcc version 4.9.2 (Ubuntu 4.9.2-0ubuntu1~14.04)
          // gcc version 5.1.0 (Ubuntu 5.1.0-0ubuntu11~14.04.1)
          // gcc version 6.0.0 20160131 (experimental) (GCC)
          //
          if (last && l.compare (0, 4, "gcc ") == 0)
            return guess_result (compiler_id {type::gcc, ""}, move (l));

          // The Apple clang/clang++ -v output will have a line (currently
          // first) in the form:
          //
          // "Apple (LLVM|clang) version X.Y.Z ..."
          //
          // Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
          // Apple clang version 4.0 (tags/Apple/clang-421.0.60) (based on LLVM 3.1svn)
          // Apple clang version 4.1 (tags/Apple/clang-421.11.66) (based on LLVM 3.1svn)
          // Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
          // Apple LLVM version 5.0 (clang-500.2.79) (based on LLVM 3.3svn)
          // Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
          // Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
          // Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
          // Apple LLVM version 7.0.0 (clang-700.0.53)
          // Apple LLVM version 7.0.0 (clang-700.1.76)
          // Apple LLVM version 7.0.2 (clang-700.1.81)
          // Apple LLVM version 7.3.0 (clang-703.0.16.1)
          //
          // Note that the gcc/g++ "aliases" for clang/clang++ also include
          // this line but it is (currently) preceded by "Configured with:
          // ...".
          //
          // Check for Apple clang before the vanilla one since the above line
          // also includes "clang".
          //
          if (l.compare (0, 6, "Apple ") == 0 &&
              (l.compare (6, 5, "LLVM ") == 0 ||
               l.compare (6, 6, "clang ") == 0))
            return guess_result (compiler_id {type::clang, "apple"}, move (l));

          // The vanilla clang/clang++ -v output will have a first line in the
          // form:
          //
          // "[... ]clang version X.Y.Z[-...] ..."
          //
          // The "version" word can probably be translated. For example:
          //
          // FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512
          // Ubuntu clang version 3.5.0-4ubuntu2~trusty2 (tags/RELEASE_350/final) (based on LLVM 3.5.0)
          // Ubuntu clang version 3.6.0-2ubuntu1~trusty1 (tags/RELEASE_360/final) (based on LLVM 3.6.0)
          // clang version 3.7.0 (tags/RELEASE_370/final)
          //
          if (l.find ("clang ") != string::npos)
            return guess_result (compiler_id {type::clang, ""}, move (l));

          return guess_result ();
        };

        // The -v output contains other information (such as the compiler
        // build configuration for gcc or the selected gcc installation for
        // clang) which makes sense to include into the compiler checksum. So
        // ask run() to calculate it for every line of the -v ouput.
        //
        // One notable consequence of this is that if the locale changes
        // (e.g., via LC_ALL), then the compiler signature will most likely
        // change as well because of the translated text.

        // Suppress all the compiler errors because we may be trying an
        // unsupported option (but still consider the exit code).
        //
        r = run<guess_result> (3, xp, "-v", f, false, false, &cs);

        if (r.empty ())
        {
          if (xi)
          {
            // Fallback to --version below in case this GCC/Clang-like
            // compiler doesn't support -v.
            //
            //fail << "unable to obtain " << xc << " signature with -v";
          }

          cs.reset ();
        }
        else
        {
          // If this is clang-apple and pre-guess was gcc then change it so
          // that we don't issue any warnings.
          //
          if (r.id.type == type::clang &&
              r.id.variant == "apple"  &&
              pre == type::gcc)
            pre = type::clang;
        }
      }

      // Next try --version to detect icc. As well as obtain signature for
      // GCC/Clang-like compilers in case -v above didn't work.
      //
      if (r.empty () && (pre == invalid   ||
                         pre == type::icc ||
                         pre == type::gcc ||
                         pre == type::clang))
      {
        auto f = [&xi] (string& l, bool) -> guess_result
        {
          // Assume the first line is the signature.
          //
          if (xi)
            return guess_result (*xi, move (l));

          // The first line has the " (ICC) " in it, for example:
          //
          // icpc (ICC) 9.0 20060120
          // icpc (ICC) 11.1 20100414
          // icpc (ICC) 12.1.0 20110811
          // icpc (ICC) 14.0.0 20130728
          // icpc (ICC) 15.0.2 20150121
          // icpc (ICC) 16.0.2 20160204
          // icc (ICC) 16.0.2 20160204
          //
          if (l.find (" (ICC) ") != string::npos)
            return guess_result (compiler_id {type::icc, ""}, move (l));

          return guess_result ();
        };

        r = run<guess_result> (3, xp, "--version", f, false);

        if (r.empty ())
        {
          if (xi)
            fail << "unable to obtain " << xc << " signature with --version";
        }
      }

      // Finally try to run it without any options to detect msvc.
      //
      if (r.empty () && (pre == invalid || pre == type::msvc))
      {
        auto f = [&xi] (string& l, bool) -> guess_result
        {
          // Assume the first line is the signature.
          //
          if (xi)
            return guess_result (*xi, move (l));

          // Check for "Microsoft (R)" and "C/C++" in the first line as a
          // signature since all other words/positions can be translated. For
          // example:
          //
          // Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 13.10.6030 for 80x86
          // Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 14.00.50727.762 for 80x86
          // Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80x86
          // Compilador de optimizacion de C/C++ de Microsoft (R) version 16.00.30319.01 para x64
          // Microsoft (R) C/C++ Optimizing Compiler Version 17.00.50727.1 for x86
          // Microsoft (R) C/C++ Optimizing Compiler Version 18.00.21005.1 for x86
          // Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23026 for x86
          // Microsoft (R) C/C++ Optimizing Compiler Version 19.10.24629 for x86
          //
          // In the recent versions the architecture is either "x86", "x64",
          // or "ARM".
          //
          if (l.find ("Microsoft (R)") != string::npos &&
              l.find ("C/C++") != string::npos)
            return guess_result (compiler_id {type::msvc, ""}, move (l));

          return guess_result ();
        };

        // One can pass extra options/arguments to cl.exe with the CL and _CL_
        // environment variables. However, if such extra options are passed
        // without anything to compile, then cl.exe no longer prints usage and
        // exits successfully but instead issues an error and fails. So we are
        // going to unset these variables for our test (interestingly, only CL
        // seem to cause the problem but let's unset both, for good measure).
        //
        const char* env[] = {"CL=", "_CL_=", nullptr};

        r = run<guess_result> (3, process_env (xp, env), f, false);

        if (r.empty ())
        {
          if (xi)
            fail << "unable to obtain " << xc << " signature";
        }
      }

      if (!r.empty ())
      {
        if (pre != invalid && r.id.type != pre)
        {
          l4 ([&]{trace << "compiler type guess mismatch"
                        << ", pre-guessed " << pre
                        << ", determined " << r.id.type;});

          r = guess_result ();
        }
        else
        {
          l5 ([&]{trace << xc << " is " << r.id << ": '"
                        << r.signature << "'";});

          r.path = move (xp);
        }
      }
      else
        l4 ([&]{trace << "unable to determine compiler type of " << xc;});

      return r;
    }

    // Try to derive the toolchain pattern.
    //
    // The s argument is the stem to look for in the leaf of the path. The ls
    // and rs arguments are the left/right separator characters. If either is
    // NULL, then the stem should be the prefix/suffix of the leaf,
    // respectively. Note that a path that is equal to stem is not considered
    // a pattern.
    //
    // Note that the default right separator includes digits to handle cases
    // like clang++37 (FreeBSD).
    //
    static string
    pattern (const path& xc,
             const char* s,
             const char* ls = "-_.",
             const char* rs = "-_.0123456789")
    {
      string r;
      size_t sn (strlen (s));

      if (xc.size () > sn)
      {
        string l (xc.leaf ().string ());
        size_t ln (l.size ());

        size_t b;
        if (ln >= sn && (b = l.find (s)) != string::npos)
        {
          // Check left separators.
          //
          if (b == 0 || (ls != nullptr && strchr (ls, l[b - 1]) != nullptr))
          {
            // Check right separators.
            //
            size_t e (b + sn);
            if (e == ln || (rs != nullptr && strchr (rs, l[e]) != nullptr))
            {
              l.replace (b, sn, "*", 1);
              path p (xc.directory ());
              p /= l;
              r = move (p).string ();
            }
          }
        }
      }

      return r;
    }

    static compiler_version
    msvc_compiler_version (string v)
    {
      compiler_version r;

      // Split the version into components.
      //
      size_t b (0), e (b);
      auto next = [&v, &b, &e] (const char* m) -> uint64_t
      {
        try
        {
          if (next_word (v, b, e, '.'))
            return stoull (string (v, b, e - b));
        }
        catch (const invalid_argument&) {}
        catch (const out_of_range&) {}

        fail << "unable to extract MSVC " << m << " version from '"
             << v << "'" << endf;
      };

      r.major = next ("major");
      r.minor = next ("minor");
      r.patch = next ("patch");

      if (next_word (v, b, e, '.'))
        r.build.assign (v, b, e - b);

      r.string = move (v);

      return r;
    }

    static string
    msvc_runtime_version (const compiler_version& v)
    {
      // Mapping of compiler versions to runtime versions:
      //
      // Note that VC 15 has runtime version 14.1 but the DLLs are still
      // called *140.dll (they are said to be backwards-compatible).
      //
      // And VC 16 seems to have the runtime version 14.1 (and not 14.2, as
      // one might expect; DLLs are still *140.dll but there are now _1 and _2
      // variants for, say, msvcp140.dll). We will, however, call it 14.2
      // (which is the version of the "toolset") in our target triplet.
      //
      // year   ver   cl     crt/dll   toolset
      //
      // 2019   16.X  19.2X  14.2/140  14.2X
      // 2017   15.9  19.16  14.1/140  14.16
      // 2017   15.8  19.15  14.1/140
      // 2017   15.7  19.14  14.1/140
      // 2017   15.6  19.13  14.1/140
      // 2017   15.5  19.12  14.1/140
      // 2017   15.3  19.11  14.1/140
      // 2017   15    19.10  14.1/140
      // 2015   14    19.00  14.0/140
      // 2013   12    18.00  12.0/120
      // 2012   11    17.00  11.0/110
      // 2010   10    16.00  10.0/100
      // 2008    9    15.00   9.0/90
      // 2005    8    14.00   8.0/80
      // 2003  7.1    13.10   7.1/71
      //
      // _MSC_VER is the numeric cl version, e.g., 1921 for 19.21.
      //
      /**/ if (v.major == 19 && v.minor >= 20) return "14.2";
      else if (v.major == 19 && v.minor >= 10) return "14.1";
      else if (v.major == 19 && v.minor ==  0) return "14.0";
      else if (v.major == 18 && v.minor ==  0) return "12.0";
      else if (v.major == 17 && v.minor ==  0) return "11.0";
      else if (v.major == 16 && v.minor ==  0) return "10.0";
      else if (v.major == 15 && v.minor ==  0) return "9.0";
      else if (v.major == 14 && v.minor ==  0) return "8.0";
      else if (v.major == 13 && v.minor == 10) return "7.1";

      fail << "unable to map MSVC compiler version '" << v.string
           << "' to runtime version" << endf;
    }


    static compiler_info
    guess_msvc (const char* xm,
                lang xl,
                const path& xc,
                const string* xv,
                const string* xt,
                const strings*, const strings*,
                const strings*, const strings*,
                const strings*, const strings*,
                guess_result&& gr, sha256&)
    {
      // Extract the version. The signature line has the following format
      // though language words can be translated and even rearranged (see
      // examples above).
      //
      // "Microsoft (R) C/C++ Optimizing Compiler Version A.B.C[.D] for CPU"
      //
      // The CPU keywords (based on the above samples) appear to be:
      //
      // "80x86"
      // "x86"
      // "x64"
      // "ARM"
      //
      compiler_version v;
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << ".version to override";
          });

        // Treat the custom version as just a tail of the signature.
        //
        const string& s (xv == nullptr ? gr.signature : *xv);

        // Some overrides for testing.
        //
        //string s;
        //s = "Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80x86";
        //s = "Compilador de optimizacion de C/C++ de Microsoft (R) version 16.00.30319.01 para x64";
        //s = "Compilateur d'optimisation Microsoft (R) C/C++ version 19.16.27026.1 pour x64";

        // Scan the string as words and look for the version.
        //
        size_t b (0), e (0);
        while (next_word (s, b, e, ' ', ','))
        {
          // The third argument to find_first_not_of() is the length of the
          // first argument, not the length of the interval to check. So to
          // limit it to [b, e) we are also going to compare the result to the
          // end of the word position (first space). In fact, we can just
          // check if it is >= e.
          //
          if (s.find_first_not_of ("1234567890.", b, 11) >= e)
            break;
        }

        if (b == e)
          fail << "unable to extract MSVC version from '" << s << "'";

        v = msvc_compiler_version (string (s, b, e - b));
      }


      // Figure out the target architecture.
      //
      string t, ot;

      if (xt == nullptr)
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << ".target to override";
          });

        const string& s (gr.signature);

        // Scan the string as words and look for the CPU.
        //
        string arch;

        for (size_t b (0), e (0), n;
             (n = next_word (s, b, e, ' ', ',')) != 0; )
        {
          if (s.compare (b, n, "x64", 3) == 0 ||
              s.compare (b, n, "x86", 3) == 0 ||
              s.compare (b, n, "ARM", 3) == 0 ||
              s.compare (b, n, "80x86", 5) == 0)
          {
            arch.assign (s, b, n);
            break;
          }
        }

        if (arch.empty ())
          fail << "unable to extract MSVC target architecture from "
               << "'" << s << "'";

        // Now we need to map x86, x64, and ARM to the target triplets. The
        // problem is, there aren't any established ones so we got to invent
        // them ourselves. Based on the discussion in
        // <libbutl/target-triplet.mxx>, we need something in the
        // CPU-VENDOR-OS-ABI form.
        //
        // The CPU part is fairly straightforward with x86 mapped to 'i386'
        // (or maybe 'i686'), x64 to 'x86_64', and ARM to 'arm' (it could also
        // include the version, e.g., 'amrv8').
        //
        // The (toolchain) VENDOR is also straightforward: 'microsoft'. Why
        // not omit it? Two reasons: firstly, there are other compilers with
        // the otherwise same target, for example Intel C/C++, and it could be
        // useful to distinguish between them. Secondly, by having all four
        // components we remove any parsing ambiguity.
        //
        // OS-ABI is where things are not as clear cut. The OS part shouldn't
        // probably be just 'windows' since we have Win32 and WinCE. And
        // WinRT.  And Universal Windows Platform (UWP). So perhaps the
        // following values for OS: 'win32', 'wince', 'winrt', 'winup'.
        //
        // For 'win32' the ABI part could signal the Microsoft C/C++ runtime
        // by calling it 'msvc'. And seeing that the runtimes are incompatible
        // from version to version, we should probably add the 'X.Y' version
        // at the end (so we essentially mimic the DLL name, for example,
        // msvcr120.dll). Some suggested we also encode the runtime type
        // (those pesky /M* options) though I am not sure: the only
        // "redistributable" runtime is multi-threaded release DLL.
        //
        // The ABI part for the other OS values needs thinking. For 'winrt'
        // and 'winup' it probably makes sense to encode the WINAPI_FAMILY
        // macro value (perhaps also with the version). Some of its values:
        //
        // WINAPI_FAMILY_APP        Windows 10
        // WINAPI_FAMILY_PC_APP     Windows 8.1
        // WINAPI_FAMILY_PHONE_APP  Windows Phone 8.1
        //
        // For 'wince' we may also want to add the OS version, for example,
        // 'wince4.2'.
        //
        // Putting it all together, Visual Studio 2015 will then have the
        // following target triplets:
        //
        // x86  i386-microsoft-win32-msvc14.0
        // x64  x86_64-microsoft-win32-msvc14.0
        // ARM  arm-microsoft-winup-???
        //
        if (arch == "ARM")
          fail << "cl.exe ARM/WinRT/UWP target is not yet supported";
        else
        {
          if (arch == "x64")
            t = "x86_64-microsoft-win32-msvc";
          else if (arch == "x86" || arch == "80x86")
            t = "i386-microsoft-win32-msvc";
          else
            assert (false);

          t += msvc_runtime_version (v);
        }

        ot = t;
      }
      else
        ot = t = *xt;

      // Derive the toolchain pattern.
      //
      // If the compiler name is/starts with 'cl' (e.g., cl.exe, cl-14),
      // then replace it with '*' and use it as a pattern for lib, link,
      // etc.
      //
      string cpat (pattern (xc, "cl", nullptr, ".-"));
      string bpat (cpat); // Binutils pattern is the same as toolchain.

      // Runtime and standard library.
      //
      string rt ("msvc");
      string csl ("msvc");
      string xsl;
      switch (xl)
      {
      case lang::c:   xsl = csl;     break;
      case lang::cxx: xsl = "msvcp"; break;
      }

      return compiler_info {
        move (gr.path),
        move (gr.id),
        compiler_class::msvc,
        move (v),
        move (gr.signature),
        "",
        move (t),
        move (ot),
        move (cpat),
        move (bpat),
        move (rt),
        move (csl),
        move (xsl),
        nullopt,
        nullopt};
    }

    static compiler_info
    guess_gcc (const char* xm,
               lang xl,
               const path& xc,
               const string* xv,
               const string* xt,
               const strings* c_po, const strings* x_po,
               const strings* c_co, const strings* x_co,
               const strings*, const strings*,
               guess_result&& gr, sha256&)
    {
      tracer trace ("cc::guess_gcc");

      const process_path& xp (gr.path);

      // Extract the version. The signature line has the following format
      // though language words can be translated and even rearranged (see
      // examples above).
      //
      // "gcc version A.B.C[ ...]"
      //
      compiler_version v;
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << ".version to override";
          });

        // Treat the custom version as just a tail of the signature.
        //
        const string& s (xv == nullptr ? gr.signature : *xv);

        // Scan the string as words and look for one that looks like a
        // version.
        //
        size_t b (0), e (0);
        while (next_word (s, b, e))
        {
          // The third argument to find_first_not_of() is the length of the
          // first argument, not the length of the interval to check. So to
          // limit it to [b, e) we are also going to compare the result to the
          // end of the word position (first space). In fact, we can just
          // check if it is >= e.
          //
          if (s.find_first_not_of ("1234567890.", b, 11) >= e)
            break;
        }

        if (b == e)
          fail << "unable to extract GCC version from '" << s << "'";

        v.string.assign (s, b, string::npos);

        // Split the version into components.
        //
        size_t vb (b), ve (b);
        auto next = [&s, b, e, &vb, &ve] (const char* m) -> uint64_t
        {
          try
          {
            if (next_word (s, e, vb, ve, '.'))
              return stoull (string (s, vb, ve - vb));
          }
          catch (const invalid_argument&) {}
          catch (const out_of_range&) {}

          fail << "unable to extract GCC " << m << " version from '"
               << string (s, b, e - b) << "'" << endf;
        };

        v.major = next ("major");
        v.minor = next ("minor");
        v.patch = next ("patch");

        if (e != s.size ())
          v.build.assign (s, e + 1, string::npos);
      }

      // Figure out the target architecture. This is actually a lot trickier
      // than one would have hoped.
      //
      // There is the -dumpmachine option but gcc doesn't adjust it per the
      // compile options (e.g., -m32). However, starting with 4.6 it has the
      // -print-multiarch option which gives (almost) the right answer. The
      // "almost" part has to do with it not honoring the -arch option (which
      // is really what this compiler is building for). To get to that, we
      // would have to resort to a hack like this:
      //
      // gcc -v -E - 2>&1 | grep cc1
      // .../cc1 ... -mtune=generic -march=x86-64
      //
      // Also, -print-multiarch will print am empty line if the compiler
      // actually wasn't built with multi-arch support.
      //
      // So for now this is what we are going to do for the time being: First
      // try -print-multiarch. If that works out (recent gcc configure with
      // multi-arch support), then use the result. Otherwise, fallback to
      // -dumpmachine (older gcc or not multi-arch).
      //
      string t, ot;

      if (xt == nullptr)
      {
        cstrings args {xp.recall_string (), "-print-multiarch"};
        if (c_co != nullptr) append_options (args, *c_co);
        if (x_co != nullptr) append_options (args, *x_co);
        args.push_back (nullptr);

        // The output of both -print-multiarch and -dumpmachine is a single
        // line containing just the target triplet.
        //
        auto f = [] (string& l, bool) {return move (l);};

        t = run<string> (3, xp, args.data (), f, false);

        if (t.empty ())
        {
          l5 ([&]{trace << xc << " doesn's support -print-multiarch, "
                        << "falling back to -dumpmachine";});

          args[1] = "-dumpmachine";
          t = run<string> (3, xp, args.data (), f, false);
        }

        if (t.empty ())
          fail << "unable to extract target architecture from " << xc
               << " using -print-multiarch or -dumpmachine output" <<
            info << "use config." << xm << ".target to override";

        ot = t;
      }
      else
        ot = t = *xt;

      // Parse the target into triplet (for further tests) ignoring any
      // failures.
      //
      target_triplet tt;
      try {tt = target_triplet (t);} catch (const invalid_argument&) {}

      // Derive the toolchain pattern. Try cc/c++ as a fallback.
      //
      string pat (pattern (xc, xl == lang::c ? "gcc" : "g++"));

      if (pat.empty ())
        pat = pattern (xc, xl == lang::c ? "cc" : "c++");

      // Runtime and standard library.
      //
      // GCC always uses libgcc (even on MinGW). Even with -nostdlib GCC's
      // documentation says that you should usually specify -lgcc.
      //
      string rt  ("libgcc");
      string csl (tt.system == "mingw32"
                  ? "msvc"
                  : stdlib (xl, xp, c_po, x_po, c_co, x_co, c_stdlib_src));
      string xsl;
      switch (xl)
      {
      case lang::c:   xsl = csl;     break;
      case lang::cxx:
        {
          // While GCC only supports it's own C++ standard library (libstdc++)
          // we still run the test to detect the "none" case (-nostdinc++).
          //
          const char* src =
            "#include <bits/c++config.h> \n"
            "stdlib:=\"libstdc++\"       \n";

          xsl = stdlib (xl, xp, c_po, x_po, c_co, x_co, src);
          break;
        }
      }

      return compiler_info {
        move (gr.path),
        move (gr.id),
        compiler_class::gcc,
        move (v),
        move (gr.signature),
        move (gr.checksum), // Calculated on whole -v output.
        move (t),
        move (ot),
        move (pat),
        "",
        move (rt),
        move (csl),
        move (xsl),
        nullopt,
        nullopt};
    }

    struct clang_msvc_info
    {
      string   triple;
      string   msvc_ver;
      dir_path msvc_dir;
      string   psdk_ver;
      dir_path psdk_dir;
    };

    static clang_msvc_info
    guess_clang_msvc (lang xl,
                      const process_path& xp,
                      const strings* c_co, const strings* x_co)
    {
      tracer trace ("cc::guess_clang_msvc");

      cstrings args {xp.recall_string ()};
      if (c_co != nullptr) append_options (args, *c_co);
      if (x_co != nullptr) append_options (args, *x_co);
      args.push_back ("-x");
      switch (xl)
      {
      case lang::c:   args.push_back ("c");   break;
      case lang::cxx: args.push_back ("c++"); break;
      }
      args.push_back ("-v");
      args.push_back ("-E");
      args.push_back ("-");  // Read stdin.
      args.push_back (nullptr);

      // The diagnostics we are interested in goes to stderr but we also get a
      // few lines of the preprocessed boilerplate at the end.
      //
      process pr (run_start (3     /* verbosity */,
                             xp,
                             args.data (),
                             -2    /* stdin  (/dev/null) */,
                             -1    /* stdout             */,
                             false /* error  (2>&1)      */));

      clang_msvc_info r;

      string l;
      try
      {
        // The overall structure of the output is as follows (with some
        // fragments that we are not interested in replaced with `...`):
        //
        // clang version 9.0.0 (tags/RELEASE_900/final)
        // ...
        // ...
        // InstalledDir: C:\Program Files\LLVM\bin
        //  "C:\\Program Files\\LLVM\\bin\\clang++.exe" -cc1 -triple x86_64-pc-windows-msvc19.23.28105 ..."
        // clang -cc1 version 9.0.0 based upon LLVM 9.0.0 default target x86_64-pc-windows-msvc
        // #include "..." search starts here:
        // #include <...> search starts here:
        //  C:\Program Files\LLVM\lib\clang\9.0.0\include
        //  C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include
        //  C:\Program Files (x86)\Windows Kits\10\Include\10.0.18362.0\ucrt
        //  ...
        // End of search list.
        // ...
        // ...
        //
        // Notice also that the version in the target triple and in the
        // ...VC\Tools\MSVC\ subdirectory are not exactly the same (and "how
        // the same" the are guaranteed to be is anyone's guess).
        //
        ifdstream is (move (pr.in_ofd), fdstream_mode::skip, ifdstream::badbit);

        for (bool in_include (false); !eof (getline (is, l)); )
        {
          l6 ([&]{trace << "examining line '" << l << "'";});

          if (r.triple.empty ())
          {
            size_t b, e;
            if ((b = l.find ("-triple "))  != string::npos &&
                (e = l.find (' ', b += 8)) != string::npos)
            {
              r.triple.assign (l, b, e - b);

              if ((b = r.triple.find ("-msvc")) == string::npos)
                fail << "no MSVC version in Clang target " << r.triple;

              r.msvc_ver.assign (r.triple, b += 5, string::npos);

              l5 ([&]{trace << "MSVC target " << r.triple
                            << ", version " << r.msvc_ver;});
            }

            continue;
          }

          // Note: similar logic to gcc_header_search_paths().
          //
          if (!in_include)
            in_include = l.find ("#include <...>") != string::npos;
          else
          {
            if (l[0] != ' ') // End of header search paths.
              break;

            try
            {
              dir_path d (move (trim (l)));

              l6 ([&]{trace << "examining directory " << d;});

              auto b (d.begin ()), e (d.end ());

              if (r.msvc_dir.empty ())
              {
                // Look for the "Tools\MSVC\<ver>\include" component sequence.
                //
                auto i (find_if (b, e,
                                 [] (const string& n)
                                 {
                                   return icasecmp (n, "Tools") == 0;
                                 }));

                if (i != e                                   &&
                    (++i != e && icasecmp (*i, "MSVC") == 0) &&
                    (++i != e                              ) &&
                    (++i != e && icasecmp (*i, "include") == 0))
                {
                  r.msvc_dir = dir_path (b, i);

                  l5 ([&]{trace << "MSVC directory " << r.msvc_dir;});
                }
              }

              if (r.psdk_dir.empty ())
              {
                // Look for the "Windows Kits\<ver>\Include" component
                // sequence.
                //
                // Note that the path structure differs between 10 and pre-10
                // versions:
                //
                // ...\Windows Kits\10\Include\10.0.18362.0\...
                // ...\Windows Kits\8.1\Include\...
                //
                auto i (find_if (b, e,
                                 [] (const string& n)
                                 {
                                   return icasecmp (n, "Windows Kits") == 0;
                                 })), j (i);

                if (i != e                                   &&
                    (++i != e                              ) &&
                    (++i != e && icasecmp (*i, "Include") == 0))
                {
                  r.psdk_dir = dir_path (b, i);

                  if (*++j == "10" && ++i != e)
                    r.psdk_ver = *i;

                  l5 ([&]{trace << "Platform SDK directory " << r.psdk_dir
                                << ", version '" << r.psdk_ver << "'";});
                }
              }
            }
            catch (const invalid_path&)
            {
              // Skip this path.
            }

            if (!r.msvc_dir.empty () && !r.psdk_dir.empty ())
              break;
          }
        }

        is.close ();
      }
      catch (const io_error&)
      {
        // Presumably the child process failed. Let run_finish() deal with
        // that.
      }

      if (!run_finish (args.data (), pr, false /* error */, l))
        fail << "unable to extract MSVC information from " << xp;

      if (const char* w = (r.triple.empty ()   ? "MSVC target" :
                           r.msvc_ver.empty () ? "MSVC version" :
                           r.msvc_dir.empty () ? "MSVC directory" :
                           r.psdk_dir.empty () ? "Platform SDK directory":
                           nullptr))
        fail << "unable to extract " << w << " from " << xp;

      return r;
    }

    const char*
    msvc_cpu (const string&); // msvc.cxx

    static compiler_info
    guess_clang (const char* xm,
                 lang xl,
                 const path& xc,
                 const string* xv,
                 const string* xt,
                 const strings* c_po, const strings* x_po,
                 const strings* c_co, const strings* x_co,
                 const strings* c_lo, const strings* x_lo,
                 guess_result&& gr, sha256& cs)
    {
      const process_path& xp (gr.path);

      // Extract the version. Here we will try to handle both vanilla and
      // Apple clang since the signature lines are fairly similar. They have
      // the following format though language words can probably be translated
      // and even rearranged (see examples above).
      //
      // "[... ]clang version A.B.C[( |-)...]"
      // "Apple (clang|LLVM) version A.B[.C] ..."
      //
      compiler_version v;
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << ".version to override";
          });

        // Treat the custom version as just a tail of the signature.
        //
        const string& s (xv == nullptr ? gr.signature : *xv);

        // Some overrides for testing.
        //
        //s = "clang version 3.7.0 (tags/RELEASE_370/final)";
        //
        //gr.id.variant = "apple";
        //s = "Apple LLVM version 7.3.0 (clang-703.0.16.1)";
        //s = "Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)";

        // Scan the string as words and look for one that looks like a
        // version.  Use '-' as a second delimiter to handle versions like
        // "3.6.0-2ubuntu1~trusty1".
        //
        size_t b (0), e (0);
        while (next_word (s, b, e, ' ', '-'))
        {
          // The third argument to find_first_not_of() is the length of the
          // first argument, not the length of the interval to check. So to
          // limit it to [b, e) we are also going to compare the result to the
          // end of the word position (first space). In fact, we can just
          // check if it is >= e.
          //
          if (s.find_first_not_of ("1234567890.", b, 11) >= e)
            break;
        }

        if (b == e)
          fail << "unable to extract Clang version from '" << s << "'";

        v.string.assign (s, b, string::npos);

        // Split the version into components.
        //
        size_t vb (b), ve (b);
        auto next = [&s, b, e, &vb, &ve] (const char* m, bool opt) -> uint64_t
        {
          try
          {
            if (next_word (s, e, vb, ve, '.'))
              return stoull (string (s, vb, ve - vb));

            if (opt)
              return 0;
          }
          catch (const invalid_argument&) {}
          catch (const out_of_range&) {}

          fail << "unable to extract Clang " << m << " version from '"
               << string (s, b, e - b) << "'" << endf;
        };

        v.major = next ("major", false);
        v.minor = next ("minor", false);
        v.patch = next ("patch", gr.id.variant == "apple");

        if (e != s.size ())
          v.build.assign (s, e + 1, string::npos);
      }

      // Figure out the target architecture.
      //
      // Unlike gcc, clang doesn't have -print-multiarch. Its -dumpmachine,
      // however, respects the compile options (e.g., -m32).
      //
      string t, ot;

      if (xt == nullptr)
      {
        cstrings args {xp.recall_string (), "-dumpmachine"};
        if (c_co != nullptr) append_options (args, *c_co);
        if (x_co != nullptr) append_options (args, *x_co);
        args.push_back (nullptr);

        // The output of -dumpmachine is a single line containing just the
        // target triplet.
        //
        auto f = [] (string& l, bool) {return move (l);};
        t = run<string> (3, xp, args.data (), f, false);

        if (t.empty ())
          fail << "unable to extract target architecture from " << xc
               << " using -dumpmachine output" <<
            info << "use config." << xm << ".target to override";

        ot = t;
      }
      else
        ot = t = *xt;

      // Parse the target into triplet (for further tests) ignoring any
      // failures.
      //
      target_triplet tt;
      try {tt = target_triplet (t);} catch (const invalid_argument&) {}

      // For Clang on Windows targeting MSVC we remap the target to match
      // MSVC's.
      //
      optional<dir_paths> lib_dirs;
      string bin_pat;

      if (tt.system == "windows-msvc")
      {
        // Note that currently there is no straightforward way to determine
        // the VC version Clang is using. See:
        //
        // http://lists.llvm.org/pipermail/cfe-dev/2017-December/056240.html
        //
        // So we have to sniff this information out from Clang's -v output
        // (plus a couple of other useful bits like the VC installation
        // directory and Platform SDK).
        //
        clang_msvc_info mi (guess_clang_msvc (xl, xp, c_co, x_co));

        // Keep the CPU and replace the rest.
        //
        tt.vendor = "microsoft";
        tt.system = "win32-msvc";
        tt.version = msvc_runtime_version (msvc_compiler_version (mi.msvc_ver));
        t = tt.string ();

        // Add the MSVC information to the signature and checksum.
        //
        if (cs.empty ())
          cs.append (gr.signature);

        cs.append (mi.msvc_ver);
        cs.append (mi.msvc_dir.string ());
        cs.append (mi.psdk_ver);
        cs.append (mi.psdk_dir.string ());

        gr.signature += " MSVC version ";
        gr.signature += mi.msvc_ver;

        const char* cpu (msvc_cpu (tt.cpu));

        // Come up with the system library search paths. Ideally we would want
        // to extract this from Clang and -print-search-paths would have been
        // the natural way for Clang to report it. But no luck.
        //
        {
          dir_paths ds;

          ds.push_back ((dir_path (mi.msvc_dir) /= "lib") /= cpu);

          // This path structure only appeared in Platform SDK 10 (if anyone
          // wants to use anything older, they will just have to use the MSVC
          // command prompt).
          //
          if (!mi.psdk_ver.empty ())
          {
            dir_path d ((dir_path (mi.psdk_dir) /= "Lib") /= mi.psdk_ver);

            ds.push_back ((dir_path (d) /= "ucrt") /= cpu);
            ds.push_back ((dir_path (d) /= "um"  ) /= cpu);
          }

          lib_dirs = move (ds);
        }

        // Binutils search paths.
        //
        // We shouldn't need them if we are running from the command prompt
        // and omitting them in this case would also result in tidier command
        // lines. However, reliably detecting this and making sure the result
        // matches Clang's is complex. So let's keep it simple for now.
        //
        // Seeing that we only do 64-bit on Windows, let's always use 64-bit
        // MSVC tools (link.exe, etc). In case of the Platform SDK, it's
        // unclear what the CPU signifies (host, target, both).
        //
        bin_pat  = (((dir_path (mi.msvc_dir) /= "bin") /= "Hostx64") /= cpu).
          representation ();

        bin_pat += path::traits_type::path_separator;

        bin_pat += (((dir_path (mi.psdk_dir) /= "bin") /= mi.psdk_ver) /= cpu).
          representation ();
      }

      // Derive the toolchain pattern. Try clang/clang++, the gcc/g++ alias,
      // as well as cc/c++.
      //
      string pat (pattern (xc, xl == lang::c ? "clang" : "clang++"));

      if (pat.empty ())
        pat = pattern (xc, xl == lang::c ? "gcc" : "g++");

      if (pat.empty ())
        pat = pattern (xc, xl == lang::c ? "cc" : "c++");

      // Runtime and standard library.
      //
      // Clang can use libgcc, its own compiler-rt, or, on Windows targeting
      // MSVC, the VC's runtime. As usual, there is no straightforward way
      // to query this and silence on the mailing list. See:
      //
      // http://lists.llvm.org/pipermail/cfe-dev/2018-January/056494.html
      //
      // So for now we will just look for --rtlib (note: linker option) and if
      // none specified, assume some platform-specific defaults.
      //
      string rt;
      {
        auto find_rtlib = [] (const strings* ops) -> const string*
        {
          return ops != nullptr
          ? find_option_prefix ("--rtlib=", *ops, false)
          : nullptr;
        };

        const string* o;
        if ((o = find_rtlib (x_lo)) != nullptr ||
            (o = find_rtlib (c_lo)) != nullptr)
        {
          rt = string (*o, 8);
        }
        else if (tt.system == "win32-msvc")  rt = "msvc";
        else if (tt.system == "linux-gnu" ||
                 tt.system == "freebsd")     rt = "libgcc";
        else /* Mac OS, etc. */              rt = "compiler-rt";
      }

      string csl (tt.system == "win32-msvc" || tt.system == "mingw32"
                  ? "msvc"
                  : stdlib (xl, xp, c_po, x_po, c_co, x_co, c_stdlib_src));

      string xsl;
      switch (xl)
      {
      case lang::c:   xsl = csl; break;
      case lang::cxx:
        {
          // All Clang versions that we care to support have __has_include()
          // so we use it to determine which standard library is available.
          //
          // Note that we still include the corresponding headers to verify
          // things are usable. For the "other" case we include some
          // standard header to detect the "none" case (e.g, -nostdinc++).
          //
          const char* src =
            "#if __has_include(<__config>)           \n"
            "  #include <__config>                   \n"
            "  stdlib:=\"libc++\"                    \n"
            "#elif __has_include(<bits/c++config.h>) \n"
            "  #include <bits/c++config.h>           \n"
            "  stdlib:=\"libstdc++\"                 \n"
            "#else                                   \n"
            "  #include <cstddef>                    \n"
            "  stdlib:=\"other\"                     \n"
            "#endif                                  \n";

          xsl = tt.system == "win32-msvc"
            ? "msvcp"
            : stdlib (xl, xp, c_po, x_po, c_co, x_co, src);
          break;
        }
      }

      return compiler_info {
        move (gr.path),
        move (gr.id),
        compiler_class::gcc,
        move (v),
        move (gr.signature),
        move (gr.checksum), // Calculated on whole -v output.
        move (t),
        move (ot),
        move (pat),
        move (bin_pat),
        move (rt),
        move (csl),
        move (xsl),
        move (lib_dirs),
        nullopt};
    }

    static compiler_info
    guess_icc (const char* xm,
               lang xl,
               const path& xc,
               const string* xv,
               const string* xt,
               const strings* c_po, const strings* x_po,
               const strings* c_co, const strings* x_co,
               const strings*, const strings*,
               guess_result&& gr, sha256&)
    {
      const process_path& xp (gr.path);

      // Extract the version. If the version has the fourth component, then
      // the signature line (extracted with --version) won't include it. So we
      // will have to get a more elaborate line with -V. We will also have to
      // do it to get the compiler target that respects the -m option: icc
      // doesn't support -print-multiarch like gcc and its -dumpmachine
      // doesn't respect -m like clang. In fact, its -dumpmachine is
      // completely broken as it appears to print the compiler's host and not
      // the target (e.g., .../bin/ia32/icpc prints x86_64-linux-gnu).
      //
      // Some examples of the signature lines from -V output:
      //
      // Intel(R) C++ Compiler for 32-bit applications, Version 9.1 Build 20070215Z Package ID: l_cc_c_9.1.047
      // Intel(R) C++ Compiler for applications running on Intel(R) 64, Version 10.1 Build 20071116
      // Intel(R) C++ Compiler for applications running on IA-32, Version 10.1 Build 20071116 Package ID: l_cc_p_10.1.010
      // Intel C++ Intel 64 Compiler Professional for applications running on Intel 64, Version 11.0 Build 20081105 Package ID: l_cproc_p_11.0.074
      // Intel(R) C++ Intel(R) 64 Compiler Professional for applications running on Intel(R) 64, Version 11.1 Build 20091130 Package ID: l_cproc_p_11.1.064
      // Intel C++ Intel 64 Compiler XE for applications running on Intel 64, Version 12.0.4.191 Build 20110427
      // Intel(R) C++ Intel(R) 64 Compiler for applications running on Intel(R) 64, Version 16.0.2.181 Build 20160204
      // Intel(R) C++ Intel(R) 64 Compiler for applications running on IA-32, Version 16.0.2.181 Build 20160204
      // Intel(R) C++ Intel(R) 64 Compiler for applications running on Intel(R) MIC Architecture, Version 16.0.2.181 Build 20160204
      // Intel(R) C Intel(R) 64 Compiler for applications running on Intel(R) MIC Architecture, Version 16.0.2.181 Build 20160204
      //
      // We should probably also assume the language words can be translated
      // and even rearranged.
      //
      auto f = [] (string& l, bool)
      {
        return l.compare (0, 5, "Intel") == 0 && (l[5] == '(' || l[5] == ' ')
        ? move (l)
        : string ();
      };

      if (xv == nullptr)
      {
        string& s (gr.signature);
        s.clear ();

        // The -V output is sent to STDERR.
        //
        s = run<string> (3, xp, "-V", f, false);

        if (s.empty ())
          fail << "unable to extract signature from " << xc << " -V output";

        if (s.find (xl == lang::c ? " C " : " C++ ") == string::npos)
          fail << xc << " does not appear to be the Intel " << xl
               << " compiler" <<
            info << "extracted signature: '" << s << "'";
      }

      // Scan the string as words and look for the version. It consist of only
      // digits and periods and contains at least one period.
      //
      compiler_version v;
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << ".version to override";
          });

        // Treat the custom version as just a tail of the signature.
        //
        const string& s (xv == nullptr ? gr.signature : *xv);

        // Some overrides for testing.
        //
        //s = "Intel(R) C++ Compiler for 32-bit applications, Version 9.1 Build 20070215Z Package ID: l_cc_c_9.1.047";
        //s = "Intel(R) C++ Compiler for applications running on Intel(R) 64, Version 10.1 Build 20071116";
        //s = "Intel(R) C++ Compiler for applications running on IA-32, Version 10.1 Build 20071116 Package ID: l_cc_p_10.1.010";
        //s = "Intel C++ Intel 64 Compiler Professional for applications running on Intel 64, Version 11.0 Build 20081105 Package ID: l_cproc_p_11.0.074";
        //s = "Intel(R) C++ Intel(R) 64 Compiler Professional for applications running on Intel(R) 64, Version 11.1 Build 20091130 Package ID: l_cproc_p_11.1.064";
        //s = "Intel C++ Intel 64 Compiler XE for applications running on Intel 64, Version 12.0.4.191 Build 20110427";

        size_t b (0), e (0);
        while (next_word (s, b, e, ' ', ',') != 0)
        {
          // The third argument to find_first_not_of() is the length of the
          // first argument, not the length of the interval to check. So to
          // limit it to [b, e) we are also going to compare the result to the
          // end of the word position (first space). In fact, we can just
          // check if it is >= e. Similar logic for find_first_of() except
          // that we add space to the list of character to make sure we don't
          // go too far.
          //
          if (s.find_first_not_of ("1234567890.", b, 11) >= e &&
              s.find_first_of (". ", b, 2) < e)
            break;
        }

        if (b == e)
          fail << "unable to extract ICC version from '" << s << "'";

        v.string.assign (s, b, string::npos);

        // Split the version into components.
        //
        size_t vb (b), ve (b);
        auto next = [&s, b, e, &vb, &ve] (const char* m, bool opt) -> uint64_t
        {
          try
          {
            if (next_word (s, e, vb, ve, '.'))
              return stoull (string (s, vb, ve - vb));

            if (opt)
              return 0;
          }
          catch (const invalid_argument&) {}
          catch (const out_of_range&) {}

          fail << "unable to extract ICC " << m << " version from '"
               << string (s, b, e - b) << "'" << endf;
        };

        v.major = next ("major", false);
        v.minor = next ("minor", false);
        v.patch = next ("patch", true);

        if (vb != ve && next_word (s, e, vb, ve, '.'))
          v.build.assign (s, vb, ve - vb);

        if (e != s.size ())
        {
          if (!v.build.empty ())
            v.build += ' ';

          v.build.append (s, e + 1, string::npos);
        }
      }

      // Figure out the target CPU by re-running the compiler with -V and
      // compile options (which may include, e.g., -m32). The output will
      // contain two CPU keywords: the first is the host and the second is the
      // target (hopefully this won't get rearranged by the translation).
      //
      // The CPU keywords (based on the above samples) appear to be:
      //
      // "32-bit"
      // "IA-32"
      // "Intel"    "64"
      // "Intel(R)" "64"
      // "Intel(R)" "MIC"      (-dumpmachine says: x86_64-k1om-linux)
      //
      string t, ot;

      if (xt == nullptr)
      {
        auto df = make_diag_frame (
          [&xm](const diag_record& dr)
          {
            dr << info << "use config." << xm << ".target to override";
          });

        cstrings args {xp.recall_string (), "-V"};
        if (c_co != nullptr) append_options (args, *c_co);
        if (x_co != nullptr) append_options (args, *x_co);
        args.push_back (nullptr);

        // The -V output is sent to STDERR.
        //
        t = run<string> (3, xp, args.data (), f, false);

        if (t.empty ())
          fail << "unable to extract target architecture from " << xc
               << " -V output";

        string arch;
        for (size_t b (0), e (0), n;
             (n = next_word (t, b, e, ' ', ',')) != 0; )
        {
          if (t.compare (b, n, "Intel(R)", 8) == 0 ||
              t.compare (b, n, "Intel", 5) == 0)
          {
            if ((n = next_word (t, b, e, ' ', ',')) != 0)
            {
              if (t.compare (b, n, "64", 2) == 0)
              {
                arch = "x86_64";
              }
              else if (t.compare (b, n, "MIC", 3) == 0)
              {
                arch = "x86_64"; // Plus "-k1om-linux" from -dumpmachine below.
              }
            }
            else
              break;
          }
          else if (t.compare (b, n, "IA-32", 5) == 0 ||
                 t.compare (b, n, "32-bit", 6) == 0)
          {
            arch = "i386";
          }
        }

        if (arch.empty ())
          fail << "unable to extract ICC target architecture from '"
               << t << "'";

        // So we have the CPU but we still need the rest of the triplet. While
        // icc currently doesn't support cross-compilation (at least on Linux)
        // and we could have just used the build triplet (i.e., the
        // architecture on which we are running), who knows what will happen
        // in the future. So instead we are going to use -dumpmachine and
        // substitute the CPU.
        //
        {
          auto f = [] (string& l, bool) {return move (l);};
          t = run<string> (3, xp, "-dumpmachine", f);
        }

        if (t.empty ())
          fail << "unable to extract target architecture from " << xc
               << " using -dumpmachine output";

        // The first component in the triplet is always CPU.
        //
        size_t p (t.find ('-'));

        if (p == string::npos)
          fail << "unable to parse ICC target architecture '" << t << "'";

        t.swap (arch);
        t.append (arch, p, string::npos);

        ot = t;
      }
      else
        ot = t = *xt;

      // Parse the target into triplet (for further tests) ignoring any
      // failures.
      //
      target_triplet tt;
      try {tt = target_triplet (t);} catch (const invalid_argument&) {}

      // Derive the toolchain pattern.
      //
      string pat (pattern (xc, xl == lang::c ? "icc" : "icpc"));

      // Runtime and standard library.
      //
      // For now we assume that unless it is Windows, we are targeting
      // Linux/GCC.
      //
      string rt  (tt.system == "win32-msvc" ? "msvc" : "libgcc");
      string csl (tt.system == "win32-msvc"
                  ? "msvc"
                  : stdlib (xl, xp, c_po, x_po, c_co, x_co, c_stdlib_src));
      string xsl;
      switch (xl)
      {
      case lang::c:   xsl = csl;     break;
      case lang::cxx:
        {
          xsl = tt.system == "win32-msvc" ? "msvcp" : "libstdc++";
          break;
        }
      }

      return compiler_info {
        move (gr.path),
        move (gr.id),
        compiler_class::gcc, //@@ TODO: msvc on Windows?
        move (v),
        move (gr.signature),
        "",
        move (t),
        move (ot),
        move (pat),
        "",
        move (rt),
        move (csl),
        move (xsl),
        nullopt,
        nullopt};
    }

    // Compiler checks can be expensive (we often need to run the compiler
    // several times) so we cache the result.
    //
    static map<string, compiler_info> cache;

    const compiler_info&
    guess (const char* xm,
           lang xl,
           const path& xc,
           const string* xis,
           const string* xv,
           const string* xt,
           const strings* c_po, const strings* x_po,
           const strings* c_co, const strings* x_co,
           const strings* c_lo, const strings* x_lo)
    {
      // First check the cache.
      //
      string key;
      {
        sha256 cs;
        cs.append (static_cast<size_t> (xl));
        cs.append (xc.string ());
        if (xis != nullptr) cs.append (*xis);
        if (c_po != nullptr) hash_options (cs, *c_po);
        if (x_po != nullptr) hash_options (cs, *x_po);
        if (c_co != nullptr) hash_options (cs, *c_co);
        if (x_co != nullptr) hash_options (cs, *x_co);
        if (c_lo != nullptr) hash_options (cs, *c_lo);
        if (x_lo != nullptr) hash_options (cs, *x_lo);
        key = cs.string ();

        auto i (cache.find (key));
        if (i != cache.end ())
          return i->second;
      }

      // Parse the user-specified compiler id (config.x.id).
      //
      optional<compiler_id> xi;
      if (xis != nullptr)
      {
        try
        {
          xi = compiler_id (*xis);
        }
        catch (const invalid_argument& e)
        {
          fail << "invalid compiler id '" << *xis << "' "
               << "specified in variable config." << xm << ".id: " << e;
        }
      }

      pair<compiler_type, size_t> pre (pre_guess (xl, xc, xi));
      compiler_type& type (pre.first);

      // If we could pre-guess the type based on the excutable name, then
      // try the test just for that compiler.
      //
      guess_result gr;
      sha256 cs;

      if (type != invalid_compiler_type)
      {
        gr = guess (xm, xl, xc, xi, type, cs);

        if (gr.empty ())
        {
          warn << xc << " looks like " << type << " but it is not" <<
            info << "use config." << xm << " to override";

          type = invalid_compiler_type; // Clear pre-guess.
        }
      }

      if (gr.empty ())
        gr = guess (xm, xl, xc, xi, type, cs);

      if (gr.empty ())
        fail << "unable to guess " << xl << " compiler type of " << xc <<
          info << "use config." << xm << ".id to specify explicitly";

      compiler_info r;
      const compiler_id& id (gr.id);

      switch (id.type)
      {
      case compiler_type::gcc:
        {
          r = guess_gcc (xm, xl, xc, xv, xt,
                         c_po, x_po, c_co, x_co, c_lo, x_lo,
                         move (gr), cs);
          break;
        }
      case compiler_type::clang:
        {
          r = guess_clang (xm, xl, xc, xv, xt,
                           c_po, x_po, c_co, x_co, c_lo, x_lo,
                           move (gr), cs);
          break;
        }
      case compiler_type::msvc:
        {
          r = guess_msvc (xm, xl, xc, xv, xt,
                          c_po, x_po, c_co, x_co, c_lo, x_lo,
                          move (gr), cs);
          break;
        }
      case compiler_type::icc:
        {
          r = guess_icc (xm, xl, xc, xv, xt,
                         c_po, x_po, c_co, x_co, c_lo, x_lo,
                         move (gr), cs);
          break;
        }
      }

      // By default use the signature line to generate the checksum.
      //
      if (cs.empty ())
        cs.append (r.signature);

      r.checksum = cs.string ();

      // Derive binutils pattern unless this has already been done by the
      // compiler-specific code.
      //

      // When cross-compiling the whole toolchain is normally prefixed with
      // the target triplet, e.g., x86_64-w64-mingw32-{gcc,g++,ar,ld}. But
      // oftentimes it is not quite canonical (and sometimes -- outright
      // bogus). So instead we are going to first try to derive the prefix
      // using the pre-guessed position of the compiler name. Note that we
      // still want to try the target in case we could not pre-guess (think
      // x86_64-w64-mingw32-c++).
      //
      // BTW, for GCC we also get gcc-{ar,ranlib} (but not gcc-ld) which add
      // support for the LTO plugin though it seems more recent GNU binutils
      // (2.25) are able to load the plugin when needed automatically. So it
      // doesn't seem we should bother trying to support this on our end (one
      // way we could do it is by passing config.bin.{ar,ranlib} as hints).
      //
      // It's also normal for native (i.e., non-cross-compiler) builds of GCC
      // and Clang to not have binutils installed in the same directory and
      // instead relying on the system ones. In this case, if the compiler is
      // specified with the absolute path, the pattern will be the search
      // path.
      //
      if (r.bin_pattern.empty ())
      {
        if (pre.second != 0 &&
            pre.second != string::npos &&
            !path::traits_type::is_separator (xc.string ()[pre.second - 1]))
        {
          r.bin_pattern.assign (xc.string (), 0, pre.second);
          r.bin_pattern += '*'; // '-' or similar is already there.
        }
      }

      if (r.bin_pattern.empty ())
      {
        const string& t (r.target);
        size_t n (t.size ());

        if (xc.size () > n + 1)
        {
          const string& l (xc.leaf ().string ());

          if (l.size () > n + 1 && l.compare (0, n, t) == 0 && l[n] == '-')
          {
            path p (xc.directory ());
            p /= t;
            p += "-*";
            r.bin_pattern = move (p).string ();
          }
        }
      }

      // If we could not derive the pattern, then see if we can come up with a
      // search path.
      //
      if (r.bin_pattern.empty ())
      {
        const path& p (r.path.recall.empty () ? xc : r.path.recall);

        if (!p.simple ())
          r.bin_pattern = p.directory ().representation (); // Trailing slash.
      }

      return (cache[key] = move (r));
    }

    path
    guess_default (lang xl, const string& cid, const string& pat)
    {
      compiler_id id (cid);
      const char* s (nullptr);

      using type = compiler_type;

      switch (xl)
      {
      case lang::c:
        {
          switch (id.type)
          {
          case type::gcc:    s = "gcc";   break;
          case type::clang:  s = "clang"; break;
          case type::icc:    s = "icc";   break;
          case type::msvc:   s = "cl";    break;
          }

          break;
        }
      case lang::cxx:
        {
          switch (id.type)
          {
          case type::gcc:    s = "g++";     break;
          case type::clang:  s = "clang++"; break;
          case type::icc:    s = "icpc";    break;
          case type::msvc:   s = "cl";      break;
          }

          break;
        }
      }

      return path (apply_pattern (s, pat));
    }
  }
}