[PATCH] D82987: [BasicAA] Fix recursive phi MustAlias calculations

[Dave Green via Phabricator via llvm-commits]

dmgreen created this revision.
dmgreen added reviewers: hfinkel, efriedma, tobiasvk, fhahn.
Herald added a subscriber: hiraditya.
Herald added a project: LLVM.

With the option -basic-aa-recphi we can detect recursive phis's 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 operand are MustAlias though, we cannot presume that every element in the loop is also MustAlias. We need to instead be conservative and return NoAlias.


https://reviews.llvm.org/D82987

Files:
  llvm/lib/Analysis/BasicAliasAnalysis.cpp
  llvm/test/Analysis/BasicAA/recphi.ll


Index: llvm/test/Analysis/BasicAA/recphi.ll
===================================================================
--- llvm/test/Analysis/BasicAA/recphi.ll
+++ llvm/test/Analysis/BasicAA/recphi.ll
@@ -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 @@
 ; 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 {
Index: llvm/lib/Analysis/BasicAliasAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1724,6 +1724,10 @@
   // Other results are not possible.
   if (Alias == MayAlias)
     return MayAlias;
+  // With recursive phis we cannot guarantee that MustAlias will remain equal to
+  // all elements and needs to conservatively return MayAlias.
+  if (isRecursive && Alias == MustAlias)
+    return MayAlias;
 
   // If all sources of the PHI node NoAlias or MustAlias V2, then returns
   // NoAlias / MustAlias. Otherwise, returns MayAlias.


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