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[SCEV] identical instructions don't compute equal values

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Before this change `HasSameValue` would return true for distinct
`alloca` instructions if they happened to be allocating the same
type (`alloca` instructions are not specified as reading memory).  This
change adds an explicit whitelist of instruction types for which
"identical" instructions compute the same value.

Fixes PR24952.

llvm-svn: 248690
This commit is contained in:
Sanjoy Das 2015-09-27 21:09:48 +00:00
parent c39648a72a
commit a9895bca61
2 changed files with 35 additions and 1 deletions
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9
lib/Analysis/ScalarEvolution.cpp
View File

@ -6412,13 +6412,20 @@ static bool HasSameValue(const SCEV *A, const SCEV *B) {
// Quick check to see if they are the same SCEV.
if (A == B) return true;
auto ComputesEqualValues = [](const Instruction *A, const Instruction *B) {
// Not all instructions that are "identical" compute the same value. For
// instance, two distinct alloca instructions allocating the same type are
// identical and do not read memory; but compute distinct values.
return A->isIdenticalTo(B) && (isa<BinaryOperator>(A) || isa<GetElementPtrInst>(A));
};
// Otherwise, if they're both SCEVUnknown, it's possible that they hold
// two different instructions with the same value. Check for this case.
if (const SCEVUnknown *AU = dyn_cast<SCEVUnknown>(A))
if (const SCEVUnknown *BU = dyn_cast<SCEVUnknown>(B))
if (const Instruction *AI = dyn_cast<Instruction>(AU->getValue()))
if (const Instruction *BI = dyn_cast<Instruction>(BU->getValue()))
if (AI->isIdenticalTo(BI) && !AI->mayReadFromMemory())
if (ComputesEqualValues(AI, BI))
return true;
// Otherwise assume they may have a different value.

27
test/Transforms/IndVarSimplify/pr24952.ll Normal file
View File

@ -0,0 +1,27 @@
; RUN: opt -indvars -S < %s | FileCheck %s
declare void @use(i1)
define void @f() {
; CHECK-LABEL: @f(
entry:
%x = alloca i32
%y = alloca i32
br label %loop
loop:
%iv = phi i32 [ 0, %entry ], [ %iv.inc, %loop ]
%iv.inc = add i32 %iv, 1
%x.gep = getelementptr i32, i32* %x, i32 %iv
%eql = icmp eq i32* %x.gep, %y
; CHECK-NOT: @use(i1 true)
call void @use(i1 %eql)
; %be.cond deliberately 'false' -- we want want the trip count to be 0.
%be.cond = icmp ult i32 %iv, 0
br i1 %be.cond, label %loop, label %leave
leave:
ret void
}