Karl Rehm via llvm-dev
2020-Feb-02 17:45 UTC
[llvm-dev] 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 >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200202/34db416d/attachment.html>
Karl Rehm via llvm-dev
2020-Feb-03 00:24 UTC
[llvm-dev] Questions about jump threading optimization and what we can do
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 >> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200202/b859d259/attachment.html>
Michael Kruse via llvm-dev
2020-Feb-03 17:28 UTC
[llvm-dev] Questions about jump threading optimization and what we can do
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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