[llvm] r219135 - [BasicAA] Revert "Revert r218714 - Make better use of zext and sign information."

Thu Oct 9 13:00:17 PDT 2014

----- Original Message -----
> From: "Hal Finkel" <hfinkel at anl.gov>
> To: "James Molloy" <james at jamesmolloy.co.uk>
> Cc: "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>, "Nicholas White" <n.j.white at gmail.com>
> Sent: Thursday, October 9, 2014 2:55:16 PM
> Subject: Re: [llvm] r219135 - [BasicAA] Revert "Revert r218714 - Make better	use of zext and sign information."
> 
> ----- Original Message -----
> > From: "James Molloy" <james at jamesmolloy.co.uk>
> > To: "Hal Finkel" <hfinkel at anl.gov>
> > Cc: "Lang Hames" <lhames at gmail.com>, "Commit Messages and Patches
> > for LLVM" <llvm-commits at cs.uiuc.edu>, "Nicholas
> > White" <n.j.white at gmail.com>
> > Sent: Thursday, October 9, 2014 2:52:08 PM
> > Subject: Re: [llvm] r219135 - [BasicAA] Revert "Revert r218714 -
> > Make better use of zext and sign information."
> > 
> > 
> > Hi,
> > 
> > 
> > I've just narrowed down a build failure on povray in SPEC2k6 to
> > this
> > too.
> > 
> > 
> > Could you please revert?
> 
> Yes, will do now.

r219432.

 -Hal

> 
>  -Hal
> 
> >I can try and get you a testcase in the
> > morning (SPEC2k6 testcases need to be anonymised which takes time
> > :(
> > )
> > 
> > 
> > Cheers,
> > 
> > 
> > James
> > 
> > 
> > On 9 October 2014 19:45, Hal Finkel < hfinkel at anl.gov > wrote:
> > 
> > 
> > ----- 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 >, "Nicholas White" <
> > > n.j.white at gmail.com
> > > >
> > > Sent: Thursday, October 9, 2014 1:34:19 PM
> > > Subject: Re: [llvm] r219135 - [BasicAA] Revert "Revert r218714 -
> > > Make better use of zext and sign information."
> > > 
> > > 
> > > Hi Hal, Nick,
> > > 
> > > 
> > > It looks like this broke SPEC2k6/464.h264ref on x86-64 (and
> > > possibly
> > > ARM as well - I'm still investigating there). Do you guys have
> > > access to SPEC2k6? Have you seen any failures locally?
> > 
> > I do, but I don't have it running nightly (likely should). Feel
> > free
> > to revert while we figure it out.
> > 
> > Thanks again,
> > Hal
> > 
> > 
> > 
> > > 
> > > 
> > > Cheers,
> > > Lang.
> > > 
> > > 
> > > 
> > > 
> > > On Mon, Oct 6, 2014 at 11:38 AM, Hal Finkel < hfinkel at anl.gov >
> > > wrote:
> > > 
> > > 
> > > Author: hfinkel
> > > Date: Mon Oct 6 13:37:59 2014
> > > New Revision: 219135
> > > 
> > > URL: http://llvm.org/viewvc/llvm-project?rev=219135&view=rev
> > > Log:
> > > [BasicAA] Revert "Revert r218714 - Make better use of zext and
> > > sign
> > > information."
> > > 
> > > This reverts r218944, which reverted r218714, plus a bug fix.
> > > 
> > > Description of the bug in r218714 (by Nick)
> > > 
> > > The original patch forgot to check if the Scale in
> > > VariableGEPIndex
> > > flipped the
> > > sign of the variable. The BasicAA pass iterates over the
> > > instructions
> > > in the
> > > order they appear in the function, and so
> > > BasicAliasAnalysis::aliasGEP is
> > > called with the variable it first comes across as parameter GEP1.
> > > Adding a
> > > %reorder label puts the definition of %a after %b so aliasGEP is
> > > called with %b
> > > as the first parameter and %a as the second. aliasGEP later
> > > calculates that %a
> > > == %b + 1 - %idxprom where %idxprom >= 0 (if %a was passed as the
> > > first
> > > parameter it would calculate %b == %a - 1 + %idxprom where
> > > %idxprom
> > > >= 0) -
> > > ignoring that %idxprom is scaled by -1 here lead the patch to
> > > incorrectly
> > > conclude that %a > %b.
> > > 
> > > Revised patch by Nick White, thanks! Thanks to Lang to isolating
> > > the
> > > bug.
> > > Slightly modified by me to add an early exit from the loop and
> > > avoid
> > > unnecessary, but expensive, function calls.
> > > 
> > > Original commit message:
> > > 
> > > 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=219135&r1=219134&r2=219135&view=diff
> > > ==============================================================================
> > > --- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp (original)
> > > +++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp Mon Oct 6
> > > 13:37:59
> > > 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 +1058,38 @@ 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 && AllPositive; ++i) {
> > > + const Value *V = GEP1VariableIndices[i].V;
> > > Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
> > > +
> > > + bool SignKnownZero, SignKnownOne;
> > > + ComputeSignBit(
> > > + const_cast<Value *>(V),
> > > + SignKnownZero, SignKnownOne,
> > > + DL, 0, AT, nullptr, DT);
> > > +
> > > + // Zero-extension widens the variable, and so forces the sign
> > > + // bit to zero.
> > > + bool IsZExt = GEP1VariableIndices[i].Extension == EK_ZeroExt;
> > > + SignKnownZero |= IsZExt;
> > > + SignKnownOne &= !IsZExt;
> > > +
> > > + // If the variable begins with a zero then we know it's
> > > + // positive, regardless of whether the value is signed or
> > > + // unsigned.
> > > + int64_t Scale = GEP1VariableIndices[i].Scale;
> > > + AllPositive &=
> > > + (SignKnownZero && Scale >= 0) ||
> > > + (SignKnownOne && Scale < 0);
> > > +
> > > + // If the Value is currently positive but could change in a
> > > cycle,
> > > + // then we can't guarantee it'll always br positive.
> > > + if (AllPositive && !isValueEqualInPotentialCycles(V, V))
> > > + AllPositive = false;
> > > + }
> > > +
> > > Modulo = Modulo ^ (Modulo & (Modulo - 1));
> > > 
> > > // We can compute the difference between the two addresses
> > > @@ -1066,6 +1099,12 @@ 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=219135&r1=219134&r2=219135&view=diff
> > > ==============================================================================
> > > --- llvm/trunk/test/Analysis/BasicAA/phi-aa.ll (original)
> > > +++ llvm/trunk/test/Analysis/BasicAA/phi-aa.ll Mon Oct 6 13:37:59
> > > 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=219135&view=auto
> > > ==============================================================================
> > > --- llvm/trunk/test/Analysis/BasicAA/zext.ll (added)
> > > +++ llvm/trunk/test/Analysis/BasicAA/zext.ll Mon Oct 6 13:37:59
> > > 2014
> > > @@ -0,0 +1,87 @@
> > > +; 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
> > > +}
> > > +
> > > +; CHECK-LABEL: test_fe_tools
> > > +; CHECK: PartialAlias: i32* %a, i32* %b
> > > +
> > > +define void @test_fe_tools([8 x i32]* %values) {
> > > + br label %reorder
> > > +
> > > +for.loop:
> > > + %i = phi i32 [ 0, %reorder ], [ %i.next, %for.loop ]
> > > + %idxprom = zext i32 %i to i64
> > > + %b = getelementptr inbounds [8 x i32]* %values, i64 0, i64
> > > %idxprom
> > > + %i.next = add nuw nsw i32 %i, 1
> > > + %1 = icmp eq i32 %i.next, 10
> > > + br i1 %1, label %for.loop.exit, label %for.loop
> > > +
> > > +reorder:
> > > + %a = getelementptr inbounds [8 x i32]* %values, i64 0, i64 1
> > > + br label %for.loop
> > > +
> > > +for.loop.exit:
> > > + ret void
> > > +}
> > > +
> > > +; Function Attrs: nounwind
> > > +declare noalias i8* @malloc(i64)
> > > 
> > > 
> > > _______________________________________________
> > > llvm-commits mailing list
> > > llvm-commits at cs.uiuc.edu
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> > > 
> > > 
> > 
> > --
> > Hal Finkel
> > Assistant Computational Scientist
> > Leadership Computing Facility
> > Argonne National Laboratory
> > 
> > 
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> > 
> > 
> 
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 

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