[ConstantFolding] Support folding loads from a GlobalAlias
The MSVC ABI requires that we generate an alias for the vtable which
means looking through a GlobalAlias which cannot be overridden improves
our ability to devirtualize.
Found while investigating PR20801.
Patch by Andrew Zhogin!
Differential Revision: http://reviews.llvm.org/D11306
llvm-svn: 242955
diff --git a/llvm/lib/Analysis/ConstantFolding.cpp b/llvm/lib/Analysis/ConstantFolding.cpp
index bd9439e..70c929b 100644
--- a/llvm/lib/Analysis/ConstantFolding.cpp
+++ b/llvm/lib/Analysis/ConstantFolding.cpp
@@ -532,6 +532,10 @@
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)