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Using various methods, BasicAA tries to determine whether two GetElementPtr memory locations alias when its base pointers are known to be equal. When none of its heuristics are applicable, it falls back to PartialAlias to, according to a comment, protect TBAA making a wrong decision in case of unions and malloc. PartialAlias is not correct, because a PartialAlias result implies that some, but not all, bytes overlap which is not necessarily the case here. AAResults returns the first analysis result that is not MayAlias. BasicAA is always the first alias analysis. When it returns PartialAlias, no other analysis is queried to give a more exact result (which was the intention of returning PartialAlias instead of MayAlias). For instance, ScopedAA could return a more accurate result. The PartialAlias hack was introduced in r131781 (and re-applied in r132632 after some reverts) to fix llvm.org/PR9971 where TBAA returns a wrong NoAlias result due to a union. A test case for the malloc case mentioned in the comment was not provided and I don't think it is affected since it returns an omnipotent char anyway. Since r303851 (https://reviews.llvm.org/D33328) clang does emit specific TBAA for unions anymore (but "omnipotent char" instead). Hence, the PartialAlias workaround is not required anymore. This patch passes the test-suite and check-llvm/check-clang of a self-hoisted build on x64. Reviewed By: hfinkel Differential Revision: https://reviews.llvm.org/D34318 llvm-svn: 305938
18 lines
478 B
LLVM
18 lines
478 B
LLVM
; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
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; PR7959
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target datalayout = "e-p:32:32:32"
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; CHECK: 1 may alias responses
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define i32 @test(i32* %tab, i32 %indvar) nounwind {
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%tmp31 = mul i32 %indvar, -2
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%tmp32 = add i32 %tmp31, 30
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%t.5 = getelementptr i32, i32* %tab, i32 %tmp32
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%loada = load i32, i32* %tab
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store i32 0, i32* %t.5
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%loadb = load i32, i32* %tab
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%rval = add i32 %loada, %loadb
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ret i32 %rval
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}
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