[llvm] 83914ee - [InstCombine] Remove side effect of replaced constrained intrinsics
Serge Pavlov via llvm-commits
llvm-commits at lists.llvm.org
Thu May 5 10:25:21 PDT 2022
>
> This is the perfect illustration of my point. Consider what happens when
> %result is used.
>
> SimplifyCall returns ConstantFloat(2.0) in your example. Dropping the
> call *without* doing replaceAllUsesWith is a bug.
>
This path is executed for calls that have no uses. In this case call to
replaceAllUsesWith does not make sense.
The resulting transformation in this case may be considered as merging of
two operations:
1. SimplifyCall returns ConstantFloat(2.0) and the call is replaced by this
expression.
2. As this ConstantFloat(2.0) is not used, it is removed.
Both operations are valid and occur in InstCombiner. Their combination must
be valid also. It is just what the discussed code does.
Thanks,
--Serge
On Thu, May 5, 2022 at 11:39 PM Philip Reames <listmail at philipreames.com>
wrote:
>
> On 5/5/22 09:32, Serge Pavlov wrote:
>
> My point is that SimplifyCall is allowed to simplify one side effecting
>> call to another side effecting call.
>>
>
> It also may simplify side effecting call to something that has no side
> effect, if the side effect is actually absent. It takes place for the code:
>
> define float @f_unused_precise() #0 {
> %result = call float @llvm.experimental.constrained.fadd.f32(float 1.0,
> float 1.0, metadata !"round.upward", metadata !"fpexcept.strict") #0
> ret float 1.0
> }
>
> In it experimental.constrained.fadd has side effect, because exception
> behavior is strict, but SimplifyCall can reveal that actually side effect
> is absent and simplifies the call to the constant 2.0. As the result of the
> call is not used the net result is removal of the call.
>
> This is the perfect illustration of my point. Consider what happens when
> %result is used.
>
> SimplifyCall returns ConstantFloat(2.0) in your example. Dropping the
> call *without* doing replaceAllUsesWith is a bug.
>
> Please revert this change. If you want, we can continue discussing on the
> review afterwards, but please revert this change now.
>
>
> Thanks,
> --Serge
>
>
> On Thu, May 5, 2022 at 11:17 PM Philip Reames <listmail at philipreames.com>
> wrote:
>
>>
>> On 5/5/22 09:14, Serge Pavlov wrote:
>>
>> This looks incorrect to me. SimplifyCall returns a Value* for a
>>> simplified expression and you're interpreting that as a boolean meaning
>>> "delete call".
>>>
>>
>> In this case SimplifyCall is called for a call, which is not used and was
>> not removed only because it has declared side effect. Non-null replacement
>> returned by SimplifyCall is used as an indicator that the call may be
>> simplified, so its side effect is absent or may be ignored. Such unused
>> calls appear as a result of other transformations, they are not removed due
>> to the side effect and produce useless instructions in resulting code.
>>
>> My point is that SimplifyCall is allowed to simplify one side effecting
>> call to another side effecting call. As in, you can't make assumptions
>> about *which* simplifications SimplifyCall has made. If it returns a
>> nonnull result, you must use it (or leave the instruction unmodified).
>>
>>
>> I would expect this to be in isInstructionTriviallyDead anyways.
>>>
>>
>> It would be nice but the concern is that simplifying such calls is
>> expensive and isInstructionTriviallyDead is called often enough, so
>> negative impact on compile time is unavoidable. There are many
>> transformations, not only constant folding and their number will
>> increase, so putting all of them into isInstructionTriviallyDead does not
>> look like a good design.
>>
>> Probably it is not a perfect solution, but it seems that we have no other
>> solution at the moment. This problem was discussed in
>> https://reviews.llvm.org/D118426 and in some previous patch reviews. If
>> you have any ideas you could continue that discussion.
>>
>> Thanks,
>> --Serge
>>
>>
>> On Thu, May 5, 2022 at 10:00 PM Philip Reames <listmail at philipreames.com>
>> wrote:
>>
>>>
>>> On 5/4/22 22:03, Serge Pavlov via llvm-commits wrote:
>>> > Author: Serge Pavlov
>>> > Date: 2022-05-05T12:02:42+07:00
>>> > New Revision: 83914ee96fc2d828e1cfb8913f5d156d39150e2c
>>> >
>>> > URL:
>>> https://github.com/llvm/llvm-project/commit/83914ee96fc2d828e1cfb8913f5d156d39150e2c
>>> > DIFF:
>>> https://github.com/llvm/llvm-project/commit/83914ee96fc2d828e1cfb8913f5d156d39150e2c.diff
>>> >
>>> > LOG: [InstCombine] Remove side effect of replaced constrained
>>> intrinsics
>>> >
>>> > If a constrained intrinsic call was replaced by some value, it was not
>>> > removed in some cases. The dangling instruction resulted in useless
>>> > instructions executed in runtime. It happened because constrained
>>> > intrinsics usually have side effect, it is used to model the
>>> interaction
>>> > with floating-point environment. In some cases it is correct behavior
>>> > but often the side effect is actually absent or can be ignored.
>>> >
>>> > This change adds specific treatment of constrained intrinsics so that
>>> > their side effect can be removed if it actually absents.
>>> >
>>> > Differential Revision: https://reviews.llvm.org/D118426
>>> >
>>> > Added:
>>> > llvm/test/Transforms/InstCombine/constrained.ll
>>> >
>>> > Modified:
>>> > llvm/include/llvm/Analysis/InstructionSimplify.h
>>> > llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
>>> >
>>> > Removed:
>>> >
>>> >
>>> >
>>> >
>>> ################################################################################
>>> > diff --git a/llvm/include/llvm/Analysis/InstructionSimplify.h
>>> b/llvm/include/llvm/Analysis/InstructionSimplify.h
>>> > index 8f6ed3a6a1928..612a73551f720 100644
>>> > --- a/llvm/include/llvm/Analysis/InstructionSimplify.h
>>> > +++ b/llvm/include/llvm/Analysis/InstructionSimplify.h
>>> > @@ -299,6 +299,10 @@ Value *SimplifyBinOp(unsigned Opcode, Value *LHS,
>>> Value *RHS, FastMathFlags FMF,
>>> > const SimplifyQuery &Q);
>>> >
>>> > /// Given a callsite, fold the result or return null.
>>> > +///
>>> > +/// \note A call with declared side effect may be simplified into a
>>> value
>>> > +/// without such. It happens if simplification code deduces that side
>>> effect
>>> > +/// is actually absent.
>>> > Value *SimplifyCall(CallBase *Call, const SimplifyQuery &Q);
>>> >
>>> > /// Given an operand for a Freeze, see if we can fold the result.
>>> >
>>> > diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
>>> b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
>>> > index c96919caee2b1..6b2ab24b4b746 100644
>>> > --- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
>>> > +++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
>>> > @@ -1237,6 +1237,16 @@ Instruction
>>> *InstCombinerImpl::visitCallInst(CallInst &CI) {
>>> > return NewCall;
>>> > }
>>> >
>>> > + // Unused constrained FP intrinsic calls may have declared side
>>> effect, which
>>> > + // actually absent. If SimplifyCall returns a replacement for such
>>> call,
>>> > + // assume side effect is absent and the call may be removed.
>>> > + if (CI.use_empty() && isa<ConstrainedFPIntrinsic>(CI)) {
>>> > + if (SimplifyCall(&CI, SQ.getWithInstruction(&CI))) {
>>> > + eraseInstFromFunction(CI);
>>> > + return nullptr;
>>> > + }
>>> > + }
>>> > +
>>>
>>> This looks incorrect to me. SimplifyCall returns a Value* for a
>>> simplified expression and you're interpreting that as a boolean meaning
>>> "delete call".
>>>
>>> I would expect this to be in isInstructionTriviallyDead anyways.
>>>
>>> Please revert unless I'm misreading.
>>>
>>> > Intrinsic::ID IID = II->getIntrinsicID();
>>> > switch (IID) {
>>> > case Intrinsic::objectsize:
>>> >
>>> > diff --git a/llvm/test/Transforms/InstCombine/constrained.ll
>>> b/llvm/test/Transforms/InstCombine/constrained.ll
>>> > new file mode 100644
>>> > index 0000000000000..b5ef71e6edfba
>>> > --- /dev/null
>>> > +++ b/llvm/test/Transforms/InstCombine/constrained.ll
>>> > @@ -0,0 +1,125 @@
>>> > +; NOTE: Assertions have been autogenerated by
>>> utils/update_test_checks.py
>>> > +; RUN: opt -S -instcombine %s | FileCheck %s
>>> > +
>>> > +; Treatment of operation with unused result.
>>> > +
>>> > +; If operation does not raise exceptions, it may be removed even in
>>> strict mode.
>>> > +define float @f_unused_precise() #0 {
>>> > +; CHECK-LABEL: @f_unused_precise(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 1.000000e+00
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fadd.f32(float
>>> 1.0, float 1.0, metadata !"round.upward", metadata !"fpexcept.strict") #0
>>> > + ret float 1.0
>>> > +}
>>> > +
>>> > +; If operation raises exceptions, it cannot be removed in strict mode.
>>> > +define float @f_unused_strict() #0 {
>>> > +; CHECK-LABEL: @f_unused_strict(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: [[RESULT:%.*]] = call float
>>> @llvm.experimental.constrained.fdiv.f32(float 1.000000e+00, float
>>> 3.000000e+00, metadata !"round.tonearest", metadata !"fpexcept.strict")
>>> #[[ATTR0:[0-9]+]]
>>> > +; CHECK-NEXT: ret float 1.000000e+00
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.tonearest", metadata !"fpexcept.strict") #0
>>> > + ret float 1.0
>>> > +}
>>> > +
>>> > +; If operation raises exceptions, it can be removed in non-strict
>>> mode.
>>> > +define float @f_unused_ignore() #0 {
>>> > +; CHECK-LABEL: @f_unused_ignore(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 1.000000e+00
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.towardzero", metadata !"fpexcept.ignore")
>>> #0
>>> > + ret float 1.0
>>> > +}
>>> > +
>>> > +; If operation raises exceptions, it can be removed in non-strict
>>> mode even if rounding mode is dynamic.
>>> > +define float @f_unused_dynamic_ignore() #0 {
>>> > +; CHECK-LABEL: @f_unused_dynamic_ignore(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 1.000000e+00
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.dynamic", metadata !"fpexcept.ignore") #0
>>> > + ret float 1.0
>>> > +}
>>> > +
>>> > +; If operation raises exceptions, it can be removed in "maytrap" mode.
>>> > +define float @f_unused_maytrap() #0 {
>>> > +; CHECK-LABEL: @f_unused_maytrap(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 1.000000e+00
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.tonearest", metadata !"fpexcept.maytrap")
>>> #0
>>> > + ret float 1.0
>>> > +}
>>> > +
>>> > +; Constant evaluation.
>>> > +
>>> > +; If operation does not raise exceptions, it may be folded even in
>>> strict mode.
>>> > +define float @f_eval_precise() #0 {
>>> > +; CHECK-LABEL: @f_eval_precise(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 2.000000e+00
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fadd.f32(float
>>> 1.0, float 1.0, metadata !"round.upward", metadata !"fpexcept.strict") #0
>>> > + ret float %result
>>> > +}
>>> > +
>>> > +; If operation raises exceptions, it cannot be folded in strict mode.
>>> > +define float @f_eval_strict() #0 {
>>> > +; CHECK-LABEL: @f_eval_strict(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: [[RESULT:%.*]] = call float
>>> @llvm.experimental.constrained.fdiv.f32(float 1.000000e+00, float
>>> 3.000000e+00, metadata !"round.upward", metadata !"fpexcept.strict")
>>> #[[ATTR0]]
>>> > +; CHECK-NEXT: ret float [[RESULT]]
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.upward", metadata !"fpexcept.strict") #0
>>> > + ret float %result
>>> > +}
>>> > +
>>> > +; If operation raises exceptions, it can be folded in non-strict mode.
>>> > +define float @f_eval_ignore() #0 {
>>> > +; CHECK-LABEL: @f_eval_ignore(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 0x3FD5555540000000
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.downward", metadata !"fpexcept.ignore") #0
>>> > + ret float %result
>>> > +}
>>> > +
>>> > +; if result is imprecise, it cannot be folded if rounding mode is
>>> dynamic.
>>> > +define float @f_eval_dynamic_ignore() #0 {
>>> > +; CHECK-LABEL: @f_eval_dynamic_ignore(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: [[RESULT:%.*]] = call float
>>> @llvm.experimental.constrained.fdiv.f32(float 1.000000e+00, float
>>> 3.000000e+00, metadata !"round.dynamic", metadata !"fpexcept.ignore")
>>> #[[ATTR0]]
>>> > +; CHECK-NEXT: ret float [[RESULT]]
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.dynamic", metadata !"fpexcept.ignore") #0
>>> > + ret float %result
>>> > +}
>>> > +
>>> > +; If result is imprecise and rounding mode is not dynamic, operation
>>> can be folded in "maytrap" mode.
>>> > +define float @f_eval_maytrap() #0 {
>>> > +; CHECK-LABEL: @f_eval_maytrap(
>>> > +; CHECK-NEXT: entry:
>>> > +; CHECK-NEXT: ret float 0x3FD5555560000000
>>> > +;
>>> > +entry:
>>> > + %result = call float @llvm.experimental.constrained.fdiv.f32(float
>>> 1.0, float 3.0, metadata !"round.tonearest", metadata !"fpexcept.maytrap")
>>> #0
>>> > + ret float %result
>>> > +}
>>> > +
>>> > +
>>> > +declare float @llvm.experimental.constrained.fadd.f32(float, float,
>>> metadata, metadata)
>>> > +declare float @llvm.experimental.constrained.fdiv.f32(float, float,
>>> metadata, metadata)
>>> > +
>>> > +attributes #0 = { strictfp }
>>> >
>>> >
>>> >
>>> > _______________________________________________
>>> > llvm-commits mailing list
>>> > llvm-commits at lists.llvm.org
>>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>
>>
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