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[BasicAA] Fix recursive phi MustAlias calculations 

With the option -basic-aa-recphi we can detect recursive phis that loop
through constant geps, which allows us to detect more no-alias case for
pointer IV's. If the other phi operand and the other alias value are
MustAlias though, we cannot presume that every element in the loop is
also MustAlias. We need to instead be conservative and return MayAlias.

Differential Revision: https://reviews.llvm.org/D82987
This commit is contained in:
David Green 2020-07-02 10:56:58 +01:00
parent 8c5133f185
commit 30bd66544d
2 changed files with 12 additions and 8 deletions
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4
llvm/lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -1724,6 +1724,10 @@ AliasResult BasicAAResult::aliasPHI(const PHINode *PN, LocationSize PNSize,
// Other results are not possible.
if (Alias == MayAlias)
return MayAlias;
// With recursive phis we cannot guarantee that MustAlias/PartialAlias will
// remain valid to all elements and needs to conservatively return MayAlias.
if (isRecursive && Alias != NoAlias)
return MayAlias;
// If all sources of the PHI node NoAlias or MustAlias V2, then returns
// NoAlias / MustAlias. Otherwise, returns MayAlias.

16
llvm/test/Analysis/BasicAA/recphi.ll
View File

@ -3,9 +3,9 @@
; CHECK-LABEL: Function: simple: 5 pointers, 0 call sites
; CHECK: NoAlias: float* %src1, float* %src2
; CHECK: NoAlias: float* %phi, float* %src1
; CHECK: MustAlias: float* %phi, float* %src2
; CHECK: MayAlias: float* %phi, float* %src2
; CHECK: NoAlias: float* %next, float* %src1
; CHECK: NoAlias: float* %next, float* %src2
; CHECK: MayAlias: float* %next, float* %src2
; CHECK: NoAlias: float* %next, float* %phi
; CHECK: NoAlias: float* %g, float* %src1
; CHECK: NoAlias: float* %g, float* %src2
@ -38,13 +38,13 @@ end:
; CHECK: MustAlias: [2 x i32]* %tab, i32* %arrayidx1
; CHECK: MustAlias: i32* %arrayidx1, i8* %0
; CHECK: NoAlias: i32* %arrayidx, i32* %arrayidx1
; CHECK: MustAlias: [2 x i32]* %tab, i32* %p.addr.05.i
; CHECK: MustAlias: i32* %p.addr.05.i, i8* %0
; CHECK: NoAlias: i32* %arrayidx, i32* %p.addr.05.i
; CHECK: MustAlias: i32* %arrayidx1, i32* %p.addr.05.i
; CHECK: PartialAlias: [2 x i32]* %tab, i32* %incdec.ptr.i
; CHECK: MayAlias: [2 x i32]* %tab, i32* %p.addr.05.i
; CHECK: MayAlias: i32* %p.addr.05.i, i8* %0
; CHECK: MayAlias: i32* %arrayidx, i32* %p.addr.05.i
; CHECK: MayAlias: i32* %arrayidx1, i32* %p.addr.05.i
; CHECK: MayAlias: [2 x i32]* %tab, i32* %incdec.ptr.i
; CHECK: NoAlias: i32* %incdec.ptr.i, i8* %0
; CHECK: MustAlias: i32* %arrayidx, i32* %incdec.ptr.i
; CHECK: MayAlias: i32* %arrayidx, i32* %incdec.ptr.i
; CHECK: NoAlias: i32* %arrayidx1, i32* %incdec.ptr.i
; CHECK: NoAlias: i32* %incdec.ptr.i, i32* %p.addr.05.i
define i32 @notmust() nounwind {