Constant folding support for load from GlobalAlias

[andrew.zhogin]

Summary

Clang generates alias for getelementptr for vtable access. But the function ConstantFoldLoadFromConstPtr fails for GlobalAlias. It breaks devirtualization even in simple cases.
This patch fixes the problem.

Devirtualization fails only for MicrosoftCXXABI because it generates aliases for vtable access.

Problem was found during investigation of https://llvm.org/bugs/show_bug.cgi?id=20801.

[majnemer]

LGTM

lib/Analysis/ConstantFolding.cpp

535

You can use auto for the LHS.

535–538

No need for the braces.

[andrew.zhogin]

Addressed the review comments.

[andrew.zhogin]

I don't have commit access so could you please commit the patch?

[andrew.zhogin]

Ping. Could someone please commit approved patch?

lib/Analysis/ConstantFolding.cpp

 /// determinable. If this is not determinable, return null.
 Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *C,
                                              const DataLayout &DL) {
   // First, try the easy cases:
   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(C))
     if (GV->isConstant() && GV->hasDefinitiveInitializer())
       return GV->getInitializer();
 
+  if (auto *GA = dyn_cast<GlobalAlias>(C))
+    if (GA->getAliasee() && !GA->mayBeOverridden())
+      return ConstantFoldLoadFromConstPtr(GA->getAliasee(), DL);
+
   // If the loaded value isn't a constant expr, we can't handle it.
   ConstantExpr *CE = dyn_cast<ConstantExpr>(C);
   if (!CE)
     return nullptr;
 
   if (CE->getOpcode() == Instruction::GetElementPtr) {
     if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0))) {
       if (GV->isConstant() && GV->hasDefinitiveInitializer()) {

test/Analysis/ConstantFolding/global-alias-constprop.ll

+; RUN: opt < %s -sccp -S | FileCheck %s
+
+@0 = private unnamed_addr constant [2 x i32] [i32 -1, i32 1] 
+@"\01??_7A@@6B@" = unnamed_addr alias getelementptr inbounds ([2 x i32], [2 x i32]* @0, i32 0, i32 1) 
+
+; CHECK: ret i32 1
+
+define i32 @main() {
+  %a = load i32, i32* @"\01??_7A@@6B@"
+  ret i32 %a
+}