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

Karl Rehm via llvm-dev
2020-02-03T19:18:03Z

Wait, you used the same example as I did. I'm confused then; if ScEv is
having troubles but it still gets optimized away somewhere, what do you
think is doing it?

On Mon, Feb 3, 2020 at 2:16 PM Karl Rehm <klrehm123 at gmail.com> wrote:

> Ah, I see. I think there's something else going on here too, though.
> https://godbolt.org/z/dCqdvv is optimized away but ScEv doesn't know
> whether this loop terminates either.
>
> On Mon, Feb 3, 2020 at 1:36 PM Michael Kruse <llvmdev at
meinersbur.de>
> wrote:
>
>> I am not convinced this is a jump-threading issue, but in the domain
>> of ScalarEvolution. ScEv would need to find out which of the exits are
>> taken or whether it loops infinitely. One can see that it exits after
>> 10000 iterations, but ScalarEvolution cannot tell:
>> https://godbolt.org/z/dCqdvv
>>
>> Michael
>>
>> Am Mo., 3. Feb. 2020 um 11:56 Uhr schrieb Karl Rehm <klrehm123 at
gmail.com
>> >:
>> >
>> > Well ideally it'd do the same thing as before (with two if
statements).
>> Something like:
>> > ret i64 4
>> >
>> > It's not really about needing to be as good as gcc, it's
more that I
>> wonder why the same concept with just two if-else blocks gets optimized
yet
>> more doesn't.
>> > An interesting point to make is that I don't think it's
jump threading
>> optimizing the original example fully: https://godbolt.org/z/rGvHJk
>> >
>> > -Karl
>> >
>> > On Mon, Feb 3, 2020 at 12:29 PM Michael Kruse <llvmdev at
meinersbur.de>
>> wrote:
>> >>
>> >> 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
>> >> >
>> >> > _______________________________________________
>> >> > LLVM Developers mailing list
>> >> > llvm-dev at lists.llvm.org
>> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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