llvm dev - Feb 2020 - Questions about jump threading optimization and what we can do

Holy crap, I completely missed that. I'm sorry! That's my fault.

On Sun, Feb 2, 2020 at 12:15 PM Johannes Doerfert <jdoerfert at anl.gov>
wrote:
> On 01/30, Karl Rehm via llvm-dev wrote:
> > Since the bug report here: https://bugs.llvm.org/show_bug.cgi?id=44679
> I've
> > been thinking about cases like it, such as:
https://godbolt.org/z/Fwq8mn
> >
> > I wonder what we can do about this in a general sense. As far as I can
> > tell, the jump threading algorithm is *really* conservative, which is
one
> > reason this isn't working as well as I'd hope; however, we
don't want to
> > produce irreducible control flow that the other passes would work less
> > effectively on. Thoughts?
>
> I'm confused. In your godbold link you run it with O0 which disables
> almost all transformations. If we take the IR, remove the optnone
> attribute, run mem2reg and jump-threading we get what I think is
> reasonable:
>   https://godbolt.org/z/u3fcTZ
>
> Please correct me if I misunderstand anything here.
>
> Cheers,
>   Johannes
>
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Here's a better example. https://godbolt.org/z/fpTyFS
I don't know what exactly you would need to change in JumpThreading to
improve this, but I'm open to ideas.

On Sun, Feb 2, 2020 at 12:45 PM Karl Rehm <klrehm123 at gmail.com> wrote:
> Holy crap, I completely missed that. I'm sorry! That's my fault.
>
> On Sun, Feb 2, 2020 at 12:15 PM Johannes Doerfert <jdoerfert at
anl.gov>
> wrote:
>
>> On 01/30, Karl Rehm via llvm-dev wrote:
>> > Since the bug report here:
https://bugs.llvm.org/show_bug.cgi?id=44679
>> I've
>> > been thinking about cases like it, such as:
>> https://godbolt.org/z/Fwq8mn
>> >
>> > I wonder what we can do about this in a general sense. As far as I
can
>> > tell, the jump threading algorithm is *really* conservative, which
is
>> one
>> > reason this isn't working as well as I'd hope; however, we
don't want to
>> > produce irreducible control flow that the other passes would work
less
>> > effectively on. Thoughts?
>>
>> I'm confused. In your godbold link you run it with O0 which
disables
>> almost all transformations. If we take the IR, remove the optnone
>> attribute, run mem2reg and jump-threading we get what I think is
>> reasonable:
>>   https://godbolt.org/z/u3fcTZ
>>
>> Please correct me if I misunderstand anything here.
>>
>> Cheers,
>>   Johannes
>>
>
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How does the code you would like to have look like? I don't see a
relevant difference compared to gcc:
https://godbolt.org/z/F-oah4
(clang unnecessarily introduces another temporary register, but that
seems unrelated)

Michael

Am So., 2. Feb. 2020 um 18:24 Uhr schrieb Karl Rehm via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> Here's a better example. https://godbolt.org/z/fpTyFS
> I don't know what exactly you would need to change in JumpThreading to
improve this, but I'm open to ideas.
>
> On Sun, Feb 2, 2020 at 12:45 PM Karl Rehm <klrehm123 at gmail.com>
wrote:
>>
>> Holy crap, I completely missed that. I'm sorry! That's my
fault.
>>
>> On Sun, Feb 2, 2020 at 12:15 PM Johannes Doerfert <jdoerfert at
anl.gov> wrote:
>>>
>>> On 01/30, Karl Rehm via llvm-dev wrote:
>>> > Since the bug report here:
https://bugs.llvm.org/show_bug.cgi?id=44679 I've
>>> > been thinking about cases like it, such as:
https://godbolt.org/z/Fwq8mn
>>> >
>>> > I wonder what we can do about this in a general sense. As far
as I can
>>> > tell, the jump threading algorithm is *really* conservative,
which is one
>>> > reason this isn't working as well as I'd hope;
however, we don't want to
>>> > produce irreducible control flow that the other passes would
work less
>>> > effectively on. Thoughts?
>>>
>>> I'm confused. In your godbold link you run it with O0 which
disables
>>> almost all transformations. If we take the IR, remove the optnone
>>> attribute, run mem2reg and jump-threading we get what I think is
>>> reasonable:
>>>   https://godbolt.org/z/u3fcTZ
>>>
>>> Please correct me if I misunderstand anything here.
>>>
>>> Cheers,
>>>   Johannes
>
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