[llvm] r219135 - [BasicAA] Revert "Revert r218714 - Make better use of zext and sign information."
Lang Hames
lhames at gmail.com
Thu Oct 9 11:34:19 PDT 2014
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?
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)
>
>
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