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Fix BasicAA incorrect assumption on GEP

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This is fixing pr31761: BasicAA is deducing NoAlias
on the result of the GEP if the base pointer is itself NoAlias.

This is possible only if the NoAlias on the base pointer is
deduced with a non-sized query: this should guarantee that
the pointers are belonging to different memory allocation
and that the GEP can't legally jump from one to another.

Differential Revision: https://reviews.llvm.org/D29216

llvm-svn: 293293
This commit is contained in:
Mehdi Amini 2017-01-27 16:12:22 +00:00
parent 3f19a46ff1
commit 6aa2c55163
2 changed files with 24 additions and 5 deletions
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10
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -1191,14 +1191,14 @@ AliasResult BasicAAResult::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
return MayAlias;
AliasResult R = aliasCheck(UnderlyingV1, MemoryLocation::UnknownSize,
AAMDNodes(), V2, V2Size, V2AAInfo,
nullptr, UnderlyingV2);
AAMDNodes(), V2, MemoryLocation::UnknownSize,
V2AAInfo, nullptr, UnderlyingV2);
if (R != MustAlias)
// If V2 may alias GEP base pointer, conservatively returns MayAlias.
// If V2 is known not to alias GEP base pointer, then the two values
// cannot alias per GEP semantics: "A pointer value formed from a
// getelementptr instruction is associated with the addresses associated
// with the first operand of the getelementptr".
// cannot alias per GEP semantics: "Any memory access must be done through
// a pointer value associated with an address range of the memory access,
// otherwise the behavior is undefined.".
return R;
// If the max search depth is reached the result is undefined

19
test/Analysis/BasicAA/pr31761.ll Normal file
View File

@ -0,0 +1,19 @@
; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx10.12.0"
%struct.blam = type { i32, i32 }
; CHECK-DAG: MayAlias: i32* %tmp, i32* %tmp3
define i1 @ham(%struct.blam* %arg) {
%isNull = icmp eq %struct.blam* %arg, null
%tmp = getelementptr %struct.blam, %struct.blam* %arg, i64 0, i32 0
%tmp2 = getelementptr %struct.blam, %struct.blam* %arg, i64 0, i32 1
%select = select i1 %isNull, i32* null, i32* %tmp2
%tmp3 = getelementptr i32, i32* %select, i32 -1
ret i1 true
}