[llvm] r218714 - [BasicAA] Make better use of zext and sign information

Thu Oct 2 22:08:01 PDT 2014

----- Original Message -----
> From: "Lang Hames" <lhames at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>
> Sent: Thursday, October 2, 2014 8:44:36 PM
> Subject: Re: [llvm] r218714 - [BasicAA] Make better use of zext and sign information
> 
> 
> Hi Hal,
> 
> 
> I've reverted this in 218944 to get the bots green again. I'm still
> working on a reduced test case, but I expect to have something for
> you tomorrow.

Thanks!

 -Hal

> 
> 
> Cheers,
> Lang.
> 
> 
> 
> 
> On Tue, Sep 30, 2014 at 3:43 PM, Hal Finkel < hfinkel at anl.gov >
> wrote:
> 
> 
> 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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> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory