ninja: break unnecessary target dependencies

Previously, given two libraries, X and Y where X depends on Y, all
object compilations of X would require the Y library to have been linked
before being compiled. This is not necessary and can instead be loosened
such that object compilations of X only depend on the order-only
dependencies of Y to be completed. This is to ensure that generated
sources, headers, custom commands, etc. are completed before X starts to
compile its objects.

This should help build performance in projects with many libraries which
cause a deep library dependency chain. Previously, a library at the
bottom would not start compilation until after all other libraries
completed, but now only its link step needs to wait and its compilation
jobs can be run in parallel with other tasks.

Fixes: #15555

1 files changed, 8 insertions, 10 deletions

diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index daad3fc..2c14a3e 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -715,8 +715,8 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements()
   }
 
   cmNinjaDeps orderOnlyDeps;
-  this->GetLocalGenerator()->AppendTargetDepends(this->GeneratorTarget,
-                                                 orderOnlyDeps);
+  this->GetLocalGenerator()->AppendTargetDepends(
+    this->GeneratorTarget, orderOnlyDeps, DependOnTargetOrdering);
 
   // Add order-only dependencies on other files associated with the target.
   orderOnlyDeps.insert(orderOnlyDeps.end(), this->ExtraFiles.begin(),
@@ -741,7 +741,7 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements()
   orderOnlyDeps.erase(std::unique(orderOnlyDeps.begin(), orderOnlyDeps.end()),
                       orderOnlyDeps.end());
 
-  if (!orderOnlyDeps.empty()) {
+  {
     cmNinjaDeps orderOnlyTarget;
     orderOnlyTarget.push_back(this->OrderDependsTargetForTarget());
     this->GetGlobalGenerator()->WritePhonyBuild(
@@ -754,7 +754,7 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements()
   for (std::vector<cmSourceFile const*>::const_iterator si =
          objectSources.begin();
        si != objectSources.end(); ++si) {
-    this->WriteObjectBuildStatement(*si, !orderOnlyDeps.empty());
+    this->WriteObjectBuildStatement(*si);
   }
 
   if (!this->DDIFiles.empty()) {
@@ -779,8 +779,8 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements()
     // our dependencies produces them.  Fixing this will require
     // refactoring the Ninja generator to generate targets in
     // dependency order so that we can collect the needed information.
-    this->GetLocalGenerator()->AppendTargetDepends(this->GeneratorTarget,
-                                                   ddOrderOnlyDeps);
+    this->GetLocalGenerator()->AppendTargetDepends(
+      this->GeneratorTarget, ddOrderOnlyDeps, DependOnTargetArtifact);
 
     this->GetGlobalGenerator()->WriteBuild(
       this->GetBuildFileStream(), ddComment, ddRule, ddOutputs, ddImplicitOuts,
@@ -791,7 +791,7 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatements()
 }
 
 void cmNinjaTargetGenerator::WriteObjectBuildStatement(
-  cmSourceFile const* source, bool writeOrderDependsTargetForTarget)
+  cmSourceFile const* source)
 {
   std::string const language = source->GetLanguage();
   std::string const sourceFileName =
@@ -842,9 +842,7 @@ void cmNinjaTargetGenerator::WriteObjectBuildStatement(
   }
 
   cmNinjaDeps orderOnlyDeps;
-  if (writeOrderDependsTargetForTarget) {
-    orderOnlyDeps.push_back(this->OrderDependsTargetForTarget());
-  }
+  orderOnlyDeps.push_back(this->OrderDependsTargetForTarget());
 
   // If the source file is GENERATED and does not have a custom command
   // (either attached to this source file or another one), assume that one of