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Update BasicAliasAnalysis to understand that nothing aliases with undef values.

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It got this in some cases (if one of them was an identified object), but not in all cases.

This caused stores to undef to block load-forwarding in some cases, etc.

Added test to Transforms/GVN to verify optimization occurs as expected.

llvm-svn: 236511
This commit is contained in:
Daniel Berlin 2015-05-05 18:10:49 +00:00
parent b888632efe
commit af1954cd08
2 changed files with 20 additions and 0 deletions
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5
lib/Analysis/BasicAliasAnalysis.cpp
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@ -1389,6 +1389,11 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
V1 = V1->stripPointerCasts();
V2 = V2->stripPointerCasts();
// If V1 or V2 is undef, the result is NoAlias because we can always pick a
// value for undef that aliases nothing in the program.
if (isa<UndefValue>(V1) || isa<UndefValue>(V2))
return NoAlias;
// Are we checking for alias of the same value?
// Because we look 'through' phi nodes we could look at "Value" pointers from
// different iterations. We must therefore make sure that this is not the

15
test/Transforms/GVN/basic-undef-test.ll Normal file
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@ -0,0 +1,15 @@
; RUN: opt -basicaa -gvn -S < %s | FileCheck %s
; ModuleID = 'test3.ll'
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
define i32 @main(i32 *%foo) {
entry:
; CHECK: load i32, i32* %foo, align 4
%0 = load i32, i32* %foo, align 4
store i32 5, i32* undef, align 4
; CHECK-NOT: load i32, i32* %foo, align 4
%1 = load i32, i32* %foo, align 4
; CHECK: add i32 %0, %0
%2 = add i32 %0, %1
ret i32 %2
}