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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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236511 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Daniel Berlin
2015-05-05 18:10:49 +00:00
parent a67d5abd53
commit 7a5c0e599c
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