[llvm] 4069ccc - [InstCombine] fold icmp with sub and bool

Sun May 22 09:13:46 PDT 2022

On Sun, May 22, 2022 at 6:11 PM Nikita Popov <nikita.ppv at gmail.com> wrote:

> On Sun, May 22, 2022 at 5:54 PM Sanjay Patel via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
>>
>> Author: Sanjay Patel
>> Date: 2022-05-22T11:51:07-04:00
>> New Revision: 4069cccf3b4ff4afb743d3d371ead9e2d5491e3a
>>
>> URL:
>> https://github.com/llvm/llvm-project/commit/4069cccf3b4ff4afb743d3d371ead9e2d5491e3a
>> DIFF:
>> https://github.com/llvm/llvm-project/commit/4069cccf3b4ff4afb743d3d371ead9e2d5491e3a.diff
>>
>> LOG: [InstCombine] fold icmp with sub and bool
>>
>> This is the specific pattern seen in #53432, but it can be extended
>> in multiple ways:
>> 1. The 'zext' could be an 'and'
>> 2. The 'sub' could be some other binop with a similar ==0 property (udiv).
>>
>> There might be some way to generalize using knownbits, but that
>> would require checking that the 'bool' value is created with
>> some instruction that can be replaced with new icmp+logic.
>>
>> https://alive2.llvm.org/ce/z/-KCfpa
>>
>
> Are the multi-use cases here motivating? Otherwise, I would have found it
> more intuitive to use a pattern like https://alive2.llvm.org/ce/z/XqbXm4
> that is not specific to sub.
>

Also, isn't this missing a check that the zext is from i1 rather than some
other type?

Nikita

>
>
>> Added:
>>
>>
>> Modified:
>>     llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
>>     llvm/test/Transforms/InstCombine/icmp-range.ll
>>
>> Removed:
>>
>>
>>
>>
>> ################################################################################
>> diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
>> b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
>> index 4f20a0699ec50..e2a6b6b1495b8 100644
>> --- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
>> +++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
>> @@ -5631,6 +5631,29 @@ Instruction
>> *InstCombinerImpl::foldICmpUsingKnownBits(ICmpInst &I) {
>>    return nullptr;
>>  }
>>
>> +/// If one operand of an icmp is effectively a bool (value range of
>> {0,1}),
>> +/// then try to reduce patterns based on that limit.
>> +static Instruction *foldICmpUsingBoolRange(ICmpInst &I,
>> +                                           InstCombiner::BuilderTy
>> &Builder) {
>> +  Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
>> +  const ICmpInst::Predicate Pred = I.getPredicate();
>> +
>> +  Value *X, *Y, *Z;
>> +  if (Pred != ICmpInst::ICMP_ULT || !match(Op0, m_Sub(m_Value(X),
>> m_Value(Y))))
>> +    return nullptr;
>> +
>> +  unsigned ExtraUses = !Op0->hasOneUse() + !Op1->hasOneUse();
>> +
>> +  // Sub must be 0 and bool must be true for "ULT":
>> +  // (sub X, Y) <u (zext i1 Z) --> (X == Y) && Z
>> +  if (match(Op1, m_ZExt(m_Value(Z))) && ExtraUses < 2) {
>> +    Value *EqXY = Builder.CreateICmpEQ(X, Y);
>> +    return BinaryOperator::CreateAnd(EqXY, Z);
>> +  }
>> +
>> +  return nullptr;
>> +}
>> +
>>  llvm::Optional<std::pair<CmpInst::Predicate, Constant *>>
>>  InstCombiner::getFlippedStrictnessPredicateAndConstant(CmpInst::Predicate
>> Pred,
>>                                                         Constant *C) {
>> @@ -6058,6 +6081,9 @@ Instruction
>> *InstCombinerImpl::visitICmpInst(ICmpInst &I) {
>>    if (Instruction *Res = foldICmpWithDominatingICmp(I))
>>      return Res;
>>
>> +  if (Instruction *Res = foldICmpUsingBoolRange(I, Builder))
>> +    return Res;
>> +
>>    if (Instruction *Res = foldICmpUsingKnownBits(I))
>>      return Res;
>>
>>
>> diff  --git a/llvm/test/Transforms/InstCombine/icmp-range.ll
>> b/llvm/test/Transforms/InstCombine/icmp-range.ll
>> index b67356675eba6..a29d4cd81b9d1 100644
>> --- a/llvm/test/Transforms/InstCombine/icmp-range.ll
>> +++ b/llvm/test/Transforms/InstCombine/icmp-range.ll
>> @@ -173,9 +173,8 @@ define i1 @test_two_ranges3(i32* nocapture readonly
>> %arg1, i32* nocapture readon
>>
>>  define i1 @sub_ult_zext(i1 %b, i8 %x, i8 %y) {
>>  ; CHECK-LABEL: @sub_ult_zext(
>> -; CHECK-NEXT:    [[Z:%.*]] = zext i1 [[B:%.*]] to i8
>> -; CHECK-NEXT:    [[S:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
>> -; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[S]], [[Z]]
>> +; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[X:%.*]], [[Y:%.*]]
>> +; CHECK-NEXT:    [[R:%.*]] = and i1 [[TMP1]], [[B:%.*]]
>>  ; CHECK-NEXT:    ret i1 [[R]]
>>  ;
>>    %z = zext i1 %b to i8
>> @@ -188,8 +187,8 @@ define i1 @sub_ult_zext_use1(i1 %b, i8 %x, i8 %y) {
>>  ; CHECK-LABEL: @sub_ult_zext_use1(
>>  ; CHECK-NEXT:    [[Z:%.*]] = zext i1 [[B:%.*]] to i8
>>  ; CHECK-NEXT:    call void @use(i8 [[Z]])
>> -; CHECK-NEXT:    [[S:%.*]] = sub i8 [[X:%.*]], [[Y:%.*]]
>> -; CHECK-NEXT:    [[R:%.*]] = icmp ult i8 [[S]], [[Z]]
>> +; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i8 [[X:%.*]], [[Y:%.*]]
>> +; CHECK-NEXT:    [[R:%.*]] = and i1 [[TMP1]], [[B]]
>>  ; CHECK-NEXT:    ret i1 [[R]]
>>  ;
>>    %z = zext i1 %b to i8
>> @@ -201,10 +200,10 @@ define i1 @sub_ult_zext_use1(i1 %b, i8 %x, i8 %y) {
>>
>>  define <2 x i1> @zext_ugt_sub_use2(<2 x i1> %b, <2 x i8> %x, <2 x i8>
>> %y) {
>>  ; CHECK-LABEL: @zext_ugt_sub_use2(
>> -; CHECK-NEXT:    [[Z:%.*]] = zext <2 x i1> [[B:%.*]] to <2 x i8>
>>  ; CHECK-NEXT:    [[S:%.*]] = sub <2 x i8> [[X:%.*]], [[Y:%.*]]
>>  ; CHECK-NEXT:    call void @use_vec(<2 x i8> [[S]])
>> -; CHECK-NEXT:    [[R:%.*]] = icmp ult <2 x i8> [[S]], [[Z]]
>> +; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq <2 x i8> [[X]], [[Y]]
>> +; CHECK-NEXT:    [[R:%.*]] = and <2 x i1> [[TMP1]], [[B:%.*]]
>>  ; CHECK-NEXT:    ret <2 x i1> [[R]]
>>  ;
>>    %z = zext <2 x i1> %b to <2 x i8>
>> @@ -214,6 +213,8 @@ define <2 x i1> @zext_ugt_sub_use2(<2 x i1> %b, <2 x
>> i8> %x, <2 x i8> %y) {
>>    ret <2 x i1> %r
>>  }
>>
>> +; negative test - too many extra uses
>> +
>>  define i1 @sub_ult_zext_use3(i1 %b, i8 %x, i8 %y) {
>>  ; CHECK-LABEL: @sub_ult_zext_use3(
>>  ; CHECK-NEXT:    [[Z:%.*]] = zext i1 [[B:%.*]] to i8
>> @@ -231,6 +232,8 @@ define i1 @sub_ult_zext_use3(i1 %b, i8 %x, i8 %y) {
>>    ret i1 %r
>>  }
>>
>> +; negative test - wrong predicate
>> +
>>  define i1 @sub_ule_zext(i1 %b, i8 %x, i8 %y) {
>>  ; CHECK-LABEL: @sub_ule_zext(
>>  ; CHECK-NEXT:    [[Z:%.*]] = zext i1 [[B:%.*]] to i8
>>
>>
>>
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>>
>
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