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

Karl Rehm via llvm-dev
2020-02-03T17:55:46Z

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
> >
> > _______________________________________________
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> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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