[llvm-dev] Questions about jump threading optimization and what we can do

Karl Rehm via llvm-dev
Mon, 03 Feb 2020 00:24:00 -0000

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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