Add option to optimize link dependencies for static libraries

Add an `OPTIMIZE_DEPENDENCIES` target property and supporting
`CMAKE_OPTIMIZE_DEPENDENCIES` variable to optionally enable pruning and
flattening of outgoing dependencies from static libraries.  Since they
do not actually link, they only depend on side effects of their
dependencies.  Therefore we can drop dependencies that contribute no
side effects.

1 files changed, 31 insertions, 0 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 889ad7c..9611e14 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -1084,6 +1084,37 @@ std::vector<cmCustomCommand> const& cmGeneratorTarget::GetPostBuildCommands()
   return this->Target->GetPostBuildCommands();
 }
 
+void cmGeneratorTarget::AppendCustomCommandSideEffects(
+  std::set<cmGeneratorTarget const*>& sideEffects) const
+{
+  if (!this->GetPreBuildCommands().empty() ||
+      !this->GetPreLinkCommands().empty() ||
+      !this->GetPostBuildCommands().empty()) {
+    sideEffects.insert(this);
+  } else {
+    for (auto const& source : this->GetAllConfigSources()) {
+      if (source.Source->GetCustomCommand() != nullptr) {
+        sideEffects.insert(this);
+        break;
+      }
+    }
+  }
+}
+
+void cmGeneratorTarget::AppendLanguageSideEffects(
+  std::map<std::string, std::set<cmGeneratorTarget const*>>& sideEffects) const
+{
+  static const std::set<cm::string_view> LANGS_WITH_NO_SIDE_EFFECTS = {
+    "C"_s, "CXX"_s, "OBJC"_s, "OBJCXX"_s, "ASM"_s, "CUDA"_s,
+  };
+
+  for (auto const& lang : this->GetAllConfigCompileLanguages()) {
+    if (!LANGS_WITH_NO_SIDE_EFFECTS.count(lang)) {
+      sideEffects[lang].insert(this);
+    }
+  }
+}
+
 bool cmGeneratorTarget::IsInBuildSystem() const
 {
   if (this->IsImported()) {