Emit static constexpr member as available_externally definition

By exposing the constant initializer, the optimizer can fold many
of these constructs.

Differential Revision: https://reviews.llvm.org/D34992

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@311857 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 22 insertions, 0 deletions

diff --git a/lib/CodeGen/CodeGenModule.cpp b/lib/CodeGen/CodeGenModule.cpp
index c726d90f2e..5feb6b4d76 100644
--- a/lib/CodeGen/CodeGenModule.cpp
+++ b/lib/CodeGen/CodeGenModule.cpp
@@ -2437,6 +2437,28 @@ CodeGenModule::GetOrCreateLLVMGlobal(StringRef MangledName,
         D->getType().isConstant(Context) &&
         isExternallyVisible(D->getLinkageAndVisibility().getLinkage()))
       GV->setSection(".cp.rodata");
+
+    // Check if we a have a const declaration with an initializer, we may be
+    // able to emit it as available_externally to expose it's value to the
+    // optimizer.
+    if (Context.getLangOpts().CPlusPlus && GV->hasExternalLinkage() &&
+        D->getType().isConstQualified() && !GV->hasInitializer() &&
+        !D->hasDefinition() && D->hasInit() && !D->hasAttr<DLLImportAttr>()) {
+      const auto *Record =
+          Context.getBaseElementType(D->getType())->getAsCXXRecordDecl();
+      bool HasMutableFields = Record && Record->hasMutableFields();
+      if (!HasMutableFields) {
+        const VarDecl *InitDecl;
+        const Expr *InitExpr = D->getAnyInitializer(InitDecl);
+        if (InitExpr) {
+          GV->setConstant(true);
+          GV->setLinkage(llvm::GlobalValue::AvailableExternallyLinkage);
+          ConstantEmitter emitter(*this);
+          GV->setInitializer(emitter.tryEmitForInitializer(*InitDecl));
+          emitter.finalize(GV);
+        }
+      }
+    }
   }
 
   auto ExpectedAS =