[CodeGen] Disable aggressive structor optimizations at -O0, take 3

The previous version of this patch (r332839) was reverted because it was
causing "definition with same mangled name as another definition" errors
in some module builds. This was caused by an unrelated bug in module
importing which it exposed. The importing problem was fixed in r336240,
so this recommits the original patch (r332839).

Differential Revision: https://reviews.llvm.org/D46685

llvm-svn: 337456

1 files changed, 14 insertions, 4 deletions

diff --git a/clang/lib/CodeGen/ItaniumCXXABI.cpp b/clang/lib/CodeGen/ItaniumCXXABI.cpp
index 8dd94e49556..d156ffab562 100644
--- a/clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ b/clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -3737,12 +3737,22 @@ static StructorCodegen getCodegenToUse(CodeGenModule &CGM,
   }
   llvm::GlobalValue::LinkageTypes Linkage = CGM.getFunctionLinkage(AliasDecl);
 
-  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage))
-    return StructorCodegen::RAUW;
+  // All discardable structors can be RAUWed, but we don't want to do that in
+  // unoptimized code, as that makes complete structor symbol disappear
+  // completely, which degrades debugging experience.
+  // Symbols with private linkage can be safely aliased, so we special case them
+  // here.
+  if (llvm::GlobalValue::isLocalLinkage(Linkage))
+    return CGM.getCodeGenOpts().OptimizationLevel > 0 ? StructorCodegen::RAUW
+                                                      : StructorCodegen::Alias;
 
+  // Linkonce structors cannot be aliased nor placed in a comdat, so these need
+  // to be emitted separately.
   // FIXME: Should we allow available_externally aliases?
-  if (!llvm::GlobalAlias::isValidLinkage(Linkage))
-    return StructorCodegen::RAUW;
+  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage) ||
+      !llvm::GlobalAlias::isValidLinkage(Linkage))
+    return CGM.getCodeGenOpts().OptimizationLevel > 0 ? StructorCodegen::RAUW
+                                                      : StructorCodegen::Emit;
 
   if (llvm::GlobalValue::isWeakForLinker(Linkage)) {
     // Only ELF and wasm support COMDATs with arbitrary names (C5/D5).