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[BasicAA] Fix - missed alias in GEP expressions

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In BasicAA GEP operand values get adjusted ("wrap-around") based on the
pointersize. Otherwise, in non-64b modes, AA could report false negatives.
However, a wrap-around is valid only for a fully evaluated expression.
It had been introduced to fix an alias problem in
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160118/326163.html.
This commit restricts the wrap-around to constant gep operands only where the
value is known at compile-time.

llvm-svn: 284908
This commit is contained in:
Gerolf Hoflehner 2016-10-22 02:41:39 +00:00
parent 986fc6cfb6
commit c01563abdd
2 changed files with 54 additions and 5 deletions
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16
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -409,6 +409,8 @@ bool BasicAAResult::DecomposeGEPExpression(const Value *V,
// Walk the indices of the GEP, accumulating them into BaseOff/VarIndices.
gep_type_iterator GTI = gep_type_begin(GEPOp);
unsigned PointerSize = DL.getPointerSizeInBits(AS);
// Assume all GEP operands are constants until proven otherwise.
bool GepHasConstantOffset = true;
for (User::const_op_iterator I = GEPOp->op_begin() + 1, E = GEPOp->op_end();
I != E; ++I) {
const Value *Index = *I;
@ -433,6 +435,8 @@ bool BasicAAResult::DecomposeGEPExpression(const Value *V,
continue;
}
GepHasConstantOffset = false;
uint64_t Scale = DL.getTypeAllocSize(*GTI);
unsigned ZExtBits = 0, SExtBits = 0;
@ -458,7 +462,7 @@ bool BasicAAResult::DecomposeGEPExpression(const Value *V,
// A[x][x] -> x*16 + x*4 -> x*20
// This also ensures that 'x' only appears in the index list once.
for (unsigned i = 0, e = Decomposed.VarIndices.size(); i != e; ++i) {
if (Decomposed.VarIndices[i].V == Index &&
if (Decomposed.VarIndices[i].V == Index &&
Decomposed.VarIndices[i].ZExtBits == ZExtBits &&
Decomposed.VarIndices[i].SExtBits == SExtBits) {
Scale += Decomposed.VarIndices[i].Scale;
@ -479,10 +483,12 @@ bool BasicAAResult::DecomposeGEPExpression(const Value *V,
}
// Take care of wrap-arounds
Decomposed.StructOffset =
adjustToPointerSize(Decomposed.StructOffset, PointerSize);
Decomposed.OtherOffset =
adjustToPointerSize(Decomposed.OtherOffset, PointerSize);
if (GepHasConstantOffset) {
Decomposed.StructOffset =
adjustToPointerSize(Decomposed.StructOffset, PointerSize);
Decomposed.OtherOffset =
adjustToPointerSize(Decomposed.OtherOffset, PointerSize);
}
// Analyze the base pointer next.
V = GEPOp->getOperand(0);

43
test/Analysis/BasicAA/gep-and-alias.ll Normal file
View File

@ -0,0 +1,43 @@
; RUN: opt -S -basicaa -gvn < %s | FileCheck %s
target datalayout = "e-m:o-p:32:32-f64:32:64-f80:128-n8:16:32-S128"
target triple = "i386-apple-macosx10.6.0"
; The load and store address in the loop body could alias so the load
; can't be hoisted above the store and out of the loop.
declare void @llvm.memset.p0i8.i32(i8* nocapture writeonly, i8, i32, i32, i1)
define i32 @foo(i32 %x, i32 %z, i32 %n) {
entry:
%pool = alloca [59 x i32], align 4
%tmp = bitcast [59 x i32]* %pool to i8*
call void @llvm.memset.p0i8.i32(i8* nonnull %tmp, i8 0, i32 236, i32 4, i1 false)
%cmp3 = icmp eq i32 %n, 0
br i1 %cmp3, label %for.end, label %for.body.lr.ph
for.body.lr.ph: ; preds = %entry
%add = add i32 %z, %x
%and = and i32 %add, 2147483647
%sub = add nsw i32 %and, -2137521902
%arrayidx = getelementptr inbounds [59 x i32], [59 x i32]* %pool, i32 0, i32 %sub
%arrayidx1 = getelementptr inbounds [59 x i32], [59 x i32]* %pool, i32 0, i32 42
br label %for.body
for.body: ; preds = %for.body.lr.ph, %for.body
%i.04 = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
store i32 %i.04, i32* %arrayidx, align 4
%tmp1 = load i32, i32* %arrayidx1, align 4
%inc = add nuw i32 %i.04, 1
%exitcond = icmp ne i32 %inc, %n
br i1 %exitcond, label %for.body, label %for.end.loopexit
for.end.loopexit: ; preds = %for.body
%lcssa = phi i32 [ %tmp1, %for.body ]
br label %for.end
for.end: ; preds = %for.end.loopexit, %entry
%s = phi i32 [ 0, %entry ], [ %lcssa, %for.end.loopexit ]
; CHECK: ret i32 %s
ret i32 %s
}