From 86a570c792e692d711dc4dc8530ff54b9980c0a4 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Fri, 8 Apr 2022 12:12:53 +0200
Subject: Optimize update_during_match()

---
 libbuild2/algorithm.cxx | 21 ++++++++++++++-------
 libbuild2/target.ixx    |  4 +++-
 2 files changed, 17 insertions(+), 8 deletions(-)

(limited to 'libbuild2')

diff --git a/libbuild2/algorithm.cxx b/libbuild2/algorithm.cxx
index 8b1d2fc..5011b26 100644
--- a/libbuild2/algorithm.cxx
+++ b/libbuild2/algorithm.cxx
@@ -2113,6 +2113,10 @@ namespace build2
     // causes the target state to be automatically set to unchanged) if the
     // file is known to be up to date. So we do the update "smartly".
     //
+    // Also, now that we do header pre-generation by default, there is a good
+    // chance the header has already been updated. So we also detect that and
+    // avoid switching the phase.
+    //
     const path_target* pt (t.is_a<path_target> ());
 
     if (pt == nullptr)
@@ -2140,11 +2144,14 @@ namespace build2
       // target_state::changed because of the dynamic dependency extraction
       // run for some other target.
       //
-      // @@ MT perf: so we are going to switch the phase and execute for
-      //    any generated header.
-      //
-      phase_switch ps (t.ctx, run_phase::execute);
-      target_state ns (execute_direct_sync (a, t));
+      target_state ns;
+      if (os != target_state::changed)
+      {
+        phase_switch ps (t.ctx, run_phase::execute);
+        ns = execute_direct_sync (a, t);
+      }
+      else
+        ns = os;
 
       if (ns != os && ns != target_state::unchanged)
       {
@@ -2204,9 +2211,9 @@ namespace build2
 
     bool r (false);
 
-    // @@ Maybe we should optimize for n == 1?
+    // @@ Maybe we should optimize for n == 1? Maybe we should just call
+    //    smarter update_during_match() in this case?
     //
-
 #if 0
     for (prerequisite_target& p: pts)
     {
diff --git a/libbuild2/target.ixx b/libbuild2/target.ixx
index af75cd1..ecc16c4 100644
--- a/libbuild2/target.ixx
+++ b/libbuild2/target.ixx
@@ -302,7 +302,9 @@ namespace build2
       return make_pair (false, target_state::unknown);
     else
     {
-      // Normally applied but can also be already executed.
+      // Normally applied but can also be already executed. Note that in the
+      // latter case we are guaranteed to be synchronized since we are in the
+      // match phase.
       //
       assert (o == offset_applied || o == offset_executed);
       return make_pair (true, (group_state (a) ? group->state[a] : s).state);
-- 
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