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[ConstantFolding] Support folding loads from a GlobalAlias

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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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242955 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2015-07-22 22:29:30 +00:00
parent 186b8ca6dc
commit 1b072f2beb
2 changed files with 15 additions and 0 deletions
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4
lib/Analysis/ConstantFolding.cpp
View File

@ -532,6 +532,10 @@ Constant *llvm::ConstantFoldLoadFromConstPtr(Constant *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)

11
test/Transforms/SCCP/global-alias-constprop.ll Normal file
View File

@ -0,0 +1,11 @@
; 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
}