[llvm] r218714 - [BasicAA] Make better use of zext and sign information
Hal Finkel
hfinkel at anl.gov
Tue Sep 30 15:43:41 PDT 2014
Author: hfinkel
Date: Tue Sep 30 17:43:40 2014
New Revision: 218714
URL: http://llvm.org/viewvc/llvm-project?rev=218714&view=rev
Log:
[BasicAA] Make better use of zext and sign information
Two related things:
1. Fixes a bug when calculating the offset in GetLinearExpression. The code
previously used zext to extend the offset, so negative offsets were converted
to large positive ones.
2. Enhance aliasGEP to deduce that, if the difference between two GEP
allocations is positive and all the variables that govern the offset are also
positive (i.e. the offset is strictly after the higher base pointer), then
locations that fit in the gap between the two base pointers are NoAlias.
Patch by Nick White!
Added:
llvm/trunk/test/Analysis/BasicAA/zext.ll
Modified:
llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
llvm/trunk/test/Analysis/BasicAA/phi-aa.ll
Modified: llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp?rev=218714&r1=218713&r2=218714&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp Tue Sep 30 17:43:40 2014
@@ -254,7 +254,10 @@ static Value *GetLinearExpression(Value
Value *Result = GetLinearExpression(CastOp, Scale, Offset, Extension,
DL, Depth+1, AT, DT);
Scale = Scale.zext(OldWidth);
- Offset = Offset.zext(OldWidth);
+
+ // We have to sign-extend even if Extension == EK_ZeroExt as we can't
+ // decompose a sign extension (i.e. zext(x - 1) != zext(x) - zext(-1)).
+ Offset = Offset.sext(OldWidth);
return Result;
}
@@ -1055,8 +1058,26 @@ BasicAliasAnalysis::aliasGEP(const GEPOp
// Grab the least significant bit set in any of the scales.
if (!GEP1VariableIndices.empty()) {
uint64_t Modulo = 0;
- for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i)
+ bool AllPositive = true;
+ for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) {
+ const Value *V = GEP1VariableIndices[i].V;
Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
+
+ // If the variable's been zero-extended or begins with a zero then
+ // we know it's positive. regardless of whether the value is signed
+ // or unsigned.
+ bool SignKnownZero, SignKnownOne;
+ ComputeSignBit(
+ const_cast<Value *>(V),
+ SignKnownZero, SignKnownOne,
+ DL, 0, AT, nullptr, DT);
+ bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
+ AllPositive &= IsZExt || SignKnownZero;
+
+ // If the Value is currently positive but could change in a cycle,
+ // then we can't guarantee it'll always br positive.
+ AllPositive &= isValueEqualInPotentialCycles(V, V);
+ }
Modulo = Modulo ^ (Modulo & (Modulo - 1));
// We can compute the difference between the two addresses
@@ -1066,6 +1087,12 @@ BasicAliasAnalysis::aliasGEP(const GEPOp
if (V1Size != UnknownSize && V2Size != UnknownSize &&
ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
return NoAlias;
+
+ // If we know all the variables are positive, then GEP1 >= GEP1BasePtr.
+ // If GEP1BasePtr > V2 (GEP1BaseOffset > 0) then we know the pointers
+ // don't alias if V2Size can fit in the gap between V2 and GEP1BasePtr.
+ if (AllPositive && GEP1BaseOffset > 0 && V2Size <= (uint64_t) GEP1BaseOffset)
+ return NoAlias;
}
// Statically, we can see that the base objects are the same, but the
Modified: llvm/trunk/test/Analysis/BasicAA/phi-aa.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/BasicAA/phi-aa.ll?rev=218714&r1=218713&r2=218714&view=diff
==============================================================================
--- llvm/trunk/test/Analysis/BasicAA/phi-aa.ll (original)
+++ llvm/trunk/test/Analysis/BasicAA/phi-aa.ll Tue Sep 30 17:43:40 2014
@@ -39,6 +39,7 @@ return:
; CHECK-LABEL: pr18068
; CHECK: MayAlias: i32* %0, i32* %arrayidx5
+; CHECK: NoAlias: i32* %arrayidx13, i32* %arrayidx5
define i32 @pr18068(i32* %jj7, i32* %j) {
entry:
Added: llvm/trunk/test/Analysis/BasicAA/zext.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/BasicAA/zext.ll?rev=218714&view=auto
==============================================================================
--- llvm/trunk/test/Analysis/BasicAA/zext.ll (added)
+++ llvm/trunk/test/Analysis/BasicAA/zext.ll Tue Sep 30 17:43:40 2014
@@ -0,0 +1,65 @@
+; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+; CHECK-LABEL: test_with_zext
+; CHECK: NoAlias: i8* %a, i8* %b
+
+define void @test_with_zext() {
+ %1 = tail call i8* @malloc(i64 120)
+ %a = getelementptr inbounds i8* %1, i64 8
+ %2 = getelementptr inbounds i8* %1, i64 16
+ %3 = zext i32 3 to i64
+ %b = getelementptr inbounds i8* %2, i64 %3
+ ret void
+}
+
+; CHECK-LABEL: test_with_lshr
+; CHECK: NoAlias: i8* %a, i8* %b
+
+define void @test_with_lshr(i64 %i) {
+ %1 = tail call i8* @malloc(i64 120)
+ %a = getelementptr inbounds i8* %1, i64 8
+ %2 = getelementptr inbounds i8* %1, i64 16
+ %3 = lshr i64 %i, 2
+ %b = getelementptr inbounds i8* %2, i64 %3
+ ret void
+}
+
+; CHECK-LABEL: test_with_a_loop
+; CHECK: NoAlias: i8* %a, i8* %b
+
+define void @test_with_a_loop() {
+ %1 = tail call i8* @malloc(i64 120)
+ %a = getelementptr inbounds i8* %1, i64 8
+ %2 = getelementptr inbounds i8* %1, i64 16
+ br label %for.loop
+
+for.loop:
+ %i = phi i32 [ 0, %0 ], [ %i.next, %for.loop ]
+ %3 = zext i32 %i to i64
+ %b = getelementptr inbounds i8* %2, i64 %3
+ %i.next = add nuw nsw i32 %i, 1
+ %4 = icmp eq i32 %i.next, 10
+ br i1 %4, label %for.loop.exit, label %for.loop
+
+for.loop.exit:
+ ret void
+}
+
+; CHECK-LABEL: test_sign_extension
+; CHECK: PartialAlias: i64* %b.i64, i8* %a
+
+define void @test_sign_extension(i32 %p) {
+ %1 = tail call i8* @malloc(i64 120)
+ %p.64 = zext i32 %p to i64
+ %a = getelementptr inbounds i8* %1, i64 %p.64
+ %p.minus1 = add i32 %p, -1
+ %p.minus1.64 = zext i32 %p.minus1 to i64
+ %b.i8 = getelementptr inbounds i8* %1, i64 %p.minus1.64
+ %b.i64 = bitcast i8* %b.i8 to i64*
+ ret void
+}
+
+; Function Attrs: nounwind
+declare noalias i8* @malloc(i64)
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