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, 24 insertions, 5 deletions

diff --git a/Source/cmGlobalNinjaGenerator.cxx b/Source/cmGlobalNinjaGenerator.cxx
index e61cbd9..1a77d7c 100644
--- a/Source/cmGlobalNinjaGenerator.cxx
+++ b/Source/cmGlobalNinjaGenerator.cxx
@@ -966,8 +966,14 @@ void cmGlobalNinjaGenerator::WriteAssumedSourceDependencies()
   }
 }
 
+std::string OrderDependsTargetForTarget(cmGeneratorTarget const* target)
+{
+  return "cmake_object_order_depends_target_" + target->GetName();
+}
+
 void cmGlobalNinjaGenerator::AppendTargetOutputs(
-  cmGeneratorTarget const* target, cmNinjaDeps& outputs)
+  cmGeneratorTarget const* target, cmNinjaDeps& outputs,
+  cmNinjaTargetDepends depends)
 {
   std::string configName =
     target->Target->GetMakefile()->GetSafeDefinition("CMAKE_BUILD_TYPE");
@@ -979,15 +985,27 @@ void cmGlobalNinjaGenerator::AppendTargetOutputs(
   bool realname = target->IsFrameworkOnApple();
 
   switch (target->GetType()) {
-    case cmStateEnums::EXECUTABLE:
     case cmStateEnums::SHARED_LIBRARY:
     case cmStateEnums::STATIC_LIBRARY:
     case cmStateEnums::MODULE_LIBRARY: {
+      if (depends == DependOnTargetOrdering) {
+        outputs.push_back(OrderDependsTargetForTarget(target));
+        break;
+      }
+    }
+    // FALLTHROUGH
+    case cmStateEnums::EXECUTABLE: {
       outputs.push_back(this->ConvertToNinjaPath(target->GetFullPath(
         configName, cmStateEnums::RuntimeBinaryArtifact, realname)));
       break;
     }
-    case cmStateEnums::OBJECT_LIBRARY:
+    case cmStateEnums::OBJECT_LIBRARY: {
+      if (depends == DependOnTargetOrdering) {
+        outputs.push_back(OrderDependsTargetForTarget(target));
+        break;
+      }
+    }
+    // FALLTHROUGH
     case cmStateEnums::GLOBAL_TARGET:
     case cmStateEnums::UTILITY: {
       std::string path =
@@ -1003,7 +1021,8 @@ void cmGlobalNinjaGenerator::AppendTargetOutputs(
 }
 
 void cmGlobalNinjaGenerator::AppendTargetDepends(
-  cmGeneratorTarget const* target, cmNinjaDeps& outputs)
+  cmGeneratorTarget const* target, cmNinjaDeps& outputs,
+  cmNinjaTargetDepends depends)
 {
   if (target->GetType() == cmStateEnums::GLOBAL_TARGET) {
     // These depend only on other CMake-provided targets, e.g. "all".
@@ -1023,7 +1042,7 @@ void cmGlobalNinjaGenerator::AppendTargetDepends(
       if ((*i)->GetType() == cmStateEnums::INTERFACE_LIBRARY) {
         continue;
       }
-      this->AppendTargetOutputs(*i, outs);
+      this->AppendTargetOutputs(*i, outs, depends);
     }
     std::sort(outs.begin(), outs.end());
     outputs.insert(outputs.end(), outs.begin(), outs.end());