From d7571f706928dd023097540dfc88448cf80fdcdb Mon Sep 17 00:00:00 2001
From: Matthew Krupcale <mkrupcale@matthewkrupcale.com>
Date: Sun, 9 Aug 2020 09:45:15 -0400
Subject: Adjust linker arguments for LTO parallelization

GCC 10+ and Clang 4+ support controlling the number of LTO threads/jobs used
during linking. Use the build2 scheduler to allocate up to the number of
hardware threads to the GCC or Clang linker processes when -flto=auto or
-flto=thin is specified, respectively. Otherwise, GCC or Clang will attempt to
spawn the number of hardware threads detected for each linker process, which
could result in up to n^2 linker threads on a CPU with n hardware threads.
---
 libbuild2/cc/link-rule.cxx | 54 ++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 54 insertions(+)

diff --git a/libbuild2/cc/link-rule.cxx b/libbuild2/cc/link-rule.cxx
index e94f538..b0457e7 100644
--- a/libbuild2/cc/link-rule.cxx
+++ b/libbuild2/cc/link-rule.cxx
@@ -34,6 +34,7 @@ namespace build2
   namespace cc
   {
     using namespace bin;
+    using build2::to_string;
 
     link_rule::
     link_rule (data&& d)
@@ -2996,6 +2997,58 @@ namespace build2
       else if (verb == 2)
         print_process (args);
 
+      // Adjust linker parallelism.
+      //
+      string jobs_arg;
+      scheduler::alloc_guard jobs_extra;
+
+      if (!lt.static_library ())
+      {
+        switch (ctype)
+        {
+        case compiler_type::gcc:
+          {
+            // Rewrite -flto=auto (available since GCC 10).
+            //
+            // By default GCC 10 splits the optimization into 128 units.
+            //
+            if (cmaj < 10)
+              break;
+
+            auto i (find_option_prefix ("-flto", args.rbegin (), args.rend ()));
+            if (i != args.rend () && strcmp (*i, "-flto=auto") == 0)
+            {
+              jobs_extra = scheduler::alloc_guard (ctx.sched, 0);
+              jobs_arg = "-flto=" + to_string (1 + jobs_extra.n);
+              *i = jobs_arg.c_str ();
+            }
+            break;
+          }
+        case compiler_type::clang:
+          {
+            // If we have -flto=thin and no explicit -flto-jobs=N (available
+            // since Clang 4), then add our own -flto-jobs value.
+            //
+            if (cmaj < 4)
+              break;
+
+            auto i (find_option_prefix ("-flto", args.rbegin (), args.rend ()));
+            if (i != args.rend ()              &&
+                strcmp (*i, "-flto=thin") == 0 &&
+                !find_option_prefix ("-flto-jobs=", args))
+            {
+              jobs_extra = scheduler::alloc_guard (ctx.sched, 0);
+              jobs_arg = "-flto-jobs=" + to_string (1 + jobs_extra.n);
+              args.insert (i.base (), jobs_arg.c_str ()); // After -flto=thin.
+            }
+            break;
+          }
+        case compiler_type::msvc:
+        case compiler_type::icc:
+          break;
+        }
+      }
+
       // Do any necessary fixups to the command line to make it runnable.
       //
       // Notice the split in the diagnostics: at verbosity level 1 we print
@@ -3162,6 +3215,7 @@ namespace build2
           }
 
           run_finish (args, pr);
+          jobs_extra.deallocate ();
         }
         catch (const process_error& e)
         {
-- 
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