[llvm] r221876 - Revert r219432 - "Revert "[BasicAA] Revert "Revert r218714 - Make better use of zext and sign information."""

[Quentin Colombet via llvm-commits]

> On Aug 14, 2015, at 6:40 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> Does http://reviews.llvm.org/D11847 fix this?

Yes, that fixes it.

When do you think this patch will land (it hasn’t been reviewed so I do not feel comfortable to just commit).

Thanks,
-Quentin

> 
> -Hal
> 
> ----- Original Message -----
>> From: "Quentin Colombet via llvm-commits" <llvm-commits at lists.llvm.org>
>> To: "llvm-commits" <llvm-commits at lists.llvm.org>
>> Sent: Friday, August 14, 2015 8:34:46 PM
>> Subject: Re: [llvm] r221876 - Revert r219432 - "Revert "[BasicAA] Revert "Revert	r218714 - Make better use of zext
>> and sign information."""
>> 
>> 
>> +new llvm list
>> 
>> 
>> 
>> 
>> On Aug 14, 2015, at 6:33 PM, Quentin Colombet < qcolombet at apple.com >
>> wrote:
>> 
>> 
>> 
>> Hi Hal,
>> 
>> Sorry to come back to this old commit, but I believe it miscompiles
>> some code:
>> https://llvm.org/bugs/show_bug.cgi?id=24468
>> 
>> 
>> Could you advice on a proper fix or revert?
>> 
>> 
>> Thanks,
>> -Quentin
>> 
>> 
>> 
>> 
>> On Nov 13, 2014, at 1:16 AM, Hal Finkel < hfinkel at anl.gov > wrote:
>> 
>> Author: hfinkel
>> Date: Thu Nov 13 03:16:54 2014
>> New Revision: 221876
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=221876&view=rev
>> Log:
>> Revert r219432 - "Revert "[BasicAA] Revert "Revert r218714 - Make
>> better use of zext and sign information."""
>> 
>> Let's try this again...
>> 
>> This reverts r219432, plus a bug fix.
>> 
>> Description of the bug in r219432 (by Nick):
>> 
>> The bug was using AllPositive to break out of the loop; if the loop
>> break
>> condition i != e is changed to i != e && AllPositive then the
>> test_modulo_analysis_with_global test I've added will fail as the
>> Modulo will
>> be calculated incorrectly (as the last loop iteration is skipped, so
>> Modulo
>> isn't updated with its Scale).
>> 
>> Nick also adds this comment:
>> 
>> ComputeSignBit is safe to use in loops as it takes into account phi
>> nodes, and
>> the == EK_ZeroEx check is safe in loops as, no matter how the
>> variable changes
>> between iterations, zero-extensions will always guarantee a zero sign
>> bit. The
>> isValueEqualInPotentialCycles check is therefore definitely not
>> needed as all
>> the variable analysis holds no matter how the variables change
>> between loop
>> iterations.
>> 
>> And this patch also adds another enhancement to GetLinearExpression -
>> basically
>> to convert ConstantInts to Offsets (see test_const_eval and
>> test_const_eval_scaled for the situations this improves).
>> 
>> Original commit message:
>> 
>> 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=221876&r1=221875&r2=221876&view=diff
>> ==============================================================================
>> --- llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp (original)
>> +++ llvm/trunk/lib/Analysis/BasicAliasAnalysis.cpp Thu Nov 13
>> 03:16:54 2014
>> @@ -207,6 +207,14 @@ static Value *GetLinearExpression(Value
>> return V;
>> }
>> 
>> + if (ConstantInt *Const = dyn_cast<ConstantInt>(V)) {
>> + // if it's a constant, just convert it to an offset
>> + // and remove the variable.
>> + Offset += Const->getValue();
>> + assert(Scale == 0 && "Constant values don't have a scale");
>> + return V;
>> + }
>> +
>> if (BinaryOperator *BOp = dyn_cast<BinaryOperator>(V)) {
>> if (ConstantInt *RHSC = dyn_cast<ConstantInt>(BOp->getOperand(1))) {
>> switch (BOp->getOpcode()) {
>> @@ -254,7 +262,10 @@ 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;
>> }
>> @@ -1051,12 +1062,45 @@ BasicAliasAnalysis::aliasGEP(const GEPOp
>> }
>> }
>> 
>> - // Try to distinguish something like &A[i][1] against &A[42][0].
>> - // 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)
>> - Modulo |= (uint64_t)GEP1VariableIndices[i].Scale;
>> + bool AllPositive = true;
>> + for (unsigned i = 0, e = GEP1VariableIndices.size(); i != e; ++i) {
>> +
>> + // Try to distinguish something like &A[i][1] against &A[42][0].
>> + // Grab the least significant bit set in any of the scales. We
>> + // don't need std::abs here (even if the scale's negative) as we'll
>> + // be ^'ing Modulo with itself later.
>> + Modulo |= (uint64_t) GEP1VariableIndices[i].Scale;
>> +
>> + if (AllPositive) {
>> + // If the Value could change between cycles, then any reasoning
>> about
>> + // the Value this cycle may not hold in the next cycle. We'll just
>> + // give up if we can't determine conditions that hold for every
>> cycle:
>> + const Value *V = GEP1VariableIndices[i].V;
>> +
>> + 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);
>> + }
>> + }
>> +
>> Modulo = Modulo ^ (Modulo & (Modulo - 1));
>> 
>> // We can compute the difference between the two addresses
>> @@ -1066,6 +1110,12 @@ 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=221876&r1=221875&r2=221876&view=diff
>> ==============================================================================
>> --- llvm/trunk/test/Analysis/BasicAA/phi-aa.ll (original)
>> +++ llvm/trunk/test/Analysis/BasicAA/phi-aa.ll Thu Nov 13 03:16:54
>> 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=221876&view=auto
>> ==============================================================================
>> --- llvm/trunk/test/Analysis/BasicAA/zext.ll (added)
>> +++ llvm/trunk/test/Analysis/BasicAA/zext.ll Thu Nov 13 03:16:54 2014
>> @@ -0,0 +1,209 @@
>> +; 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(i8* %mem) {
>> + br label %for.loop
>> +
>> +for.loop:
>> + %i = phi i32 [ 0, %0 ], [ %i.plus1, %for.loop ]
>> + %a = getelementptr inbounds i8* %mem, i64 8
>> + %a.plus1 = getelementptr inbounds i8* %mem, i64 16
>> + %i.64 = zext i32 %i to i64
>> + %b = getelementptr inbounds i8* %a.plus1, i64 %i.64
>> + %i.plus1 = add nuw nsw i32 %i, 1
>> + %cmp = icmp eq i32 %i.plus1, 10
>> + br i1 %cmp, label %for.loop.exit, label %for.loop
>> +
>> +for.loop.exit:
>> + ret void
>> +}
>> +
>> +; CHECK-LABEL: test_with_varying_base_pointer_in_loop
>> +; CHECK: NoAlias: i8* %a, i8* %b
>> +
>> +define void @test_with_varying_base_pointer_in_loop(i8* %mem.orig) {
>> + br label %for.loop
>> +
>> +for.loop:
>> + %mem = phi i8* [ %mem.orig, %0 ], [ %mem.plus1, %for.loop ]
>> + %i = phi i32 [ 0, %0 ], [ %i.plus1, %for.loop ]
>> + %a = getelementptr inbounds i8* %mem, i64 8
>> + %a.plus1 = getelementptr inbounds i8* %mem, i64 16
>> + %i.64 = zext i32 %i to i64
>> + %b = getelementptr inbounds i8* %a.plus1, i64 %i.64
>> + %i.plus1 = add nuw nsw i32 %i, 1
>> + %mem.plus1 = getelementptr inbounds i8* %mem, i64 8
>> + %cmp = icmp eq i32 %i.plus1, 10
>> + br i1 %cmp, 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
>> +}
>> +
>> + at b = global i32 0, align 4
>> + at d = global i32 0, align 4
>> +
>> +; CHECK-LABEL: test_spec2006
>> +; CHECK: PartialAlias: i32** %x, i32** %y
>> +
>> +define void @test_spec2006() {
>> + %h = alloca [1 x [2 x i32*]], align 16
>> + %d.val = load i32* @d, align 4
>> + %d.promoted = sext i32 %d.val to i64
>> + %1 = icmp slt i32 %d.val, 2
>> + br i1 %1, label %.lr.ph, label %3
>> +
>> +.lr.ph: ; preds = %0
>> + br label %2
>> +
>> +; <label>:2 ; preds = %.lr.ph, %2
>> + %i = phi i32 [ %d.val, %.lr.ph ], [ %i.plus1, %2 ]
>> + %i.promoted = sext i32 %i to i64
>> + %x = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64
>> %d.promoted, i64 %i.promoted
>> + %i.plus1 = add nsw i32 %i, 1
>> + %cmp = icmp slt i32 %i.plus1, 2
>> + br i1 %cmp, label %2, label %3
>> +
>> +; <label>:3 ; preds = %._crit_edge, %0
>> + %y = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64 0, i64
>> 1
>> + ret void
>> +}
>> +
>> +; CHECK-LABEL: test_modulo_analysis_easy_case
>> +; CHECK: NoAlias: i32** %x, i32** %y
>> +
>> +define void @test_modulo_analysis_easy_case(i64 %i) {
>> + %h = alloca [1 x [2 x i32*]], align 16
>> + %x = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64 %i,
>> i64 0
>> + %y = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64 0, i64
>> 1
>> + ret void
>> +}
>> +
>> +; CHECK-LABEL: test_modulo_analysis_in_loop
>> +; CHECK: NoAlias: i32** %x, i32** %y
>> +
>> +define void @test_modulo_analysis_in_loop() {
>> + %h = alloca [1 x [2 x i32*]], align 16
>> + br label %for.loop
>> +
>> +for.loop:
>> + %i = phi i32 [ 0, %0 ], [ %i.plus1, %for.loop ]
>> + %i.promoted = sext i32 %i to i64
>> + %x = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64
>> %i.promoted, i64 0
>> + %y = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64 0, i64
>> 1
>> + %i.plus1 = add nsw i32 %i, 1
>> + %cmp = icmp slt i32 %i.plus1, 2
>> + br i1 %cmp, label %for.loop, label %for.loop.exit
>> +
>> +for.loop.exit:
>> + ret void
>> +}
>> +
>> +; CHECK-LABEL: test_modulo_analysis_with_global
>> +; CHECK: PartialAlias: i32** %x, i32** %y
>> +
>> +define void @test_modulo_analysis_with_global() {
>> + %h = alloca [1 x [2 x i32*]], align 16
>> + %b = load i32* @b, align 4
>> + %b.promoted = sext i32 %b to i64
>> + br label %for.loop
>> +
>> +for.loop:
>> + %i = phi i32 [ 0, %0 ], [ %i.plus1, %for.loop ]
>> + %i.promoted = sext i32 %i to i64
>> + %x = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64
>> %i.promoted, i64 %b.promoted
>> + %y = getelementptr inbounds [1 x [2 x i32*]]* %h, i64 0, i64 0, i64
>> 1
>> + %i.plus1 = add nsw i32 %i, 1
>> + %cmp = icmp slt i32 %i.plus1, 2
>> + br i1 %cmp, label %for.loop, label %for.loop.exit
>> +
>> +for.loop.exit:
>> + ret void
>> +}
>> +
>> +; CHECK-LABEL: test_const_eval
>> +; CHECK: NoAlias: i8* %a, i8* %b
>> +define void @test_const_eval(i8* %ptr, i64 %offset) {
>> + %a = getelementptr inbounds i8* %ptr, i64 %offset
>> + %a.dup = getelementptr inbounds i8* %ptr, i64 %offset
>> + %three = zext i32 3 to i64
>> + %b = getelementptr inbounds i8* %a.dup, i64 %three
>> + ret void
>> +}
>> +
>> +; CHECK-LABEL: test_const_eval_scaled
>> +; CHECK: MustAlias: i8* %a, i8* %b
>> +define void @test_const_eval_scaled(i8* %ptr) {
>> + %three = zext i32 3 to i64
>> + %six = mul i64 %three, 2
>> + %a = getelementptr inbounds i8* %ptr, i64 %six
>> + %b = getelementptr inbounds i8* %ptr, i64 6
>> + 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