llvm dev - Apr 2020 - Does anyone know a workaround for this branching bug I hit?

I believe you all can access the test file and the diff in my bug report
https://bugs.llvm.org/show_bug.cgi?id=45690

In short, I'm experimenting with optimization and wrote branchless code -O2
generates a jump instruction causing my loop to be twice as slow. I changed
the jne+movl to cmove and my loop seems to take half as long.

I'm a little worried that this will interfere with other optimizations I
make in the same function (bug report is a small reproduce). Is there a
temporary workaround I can use? I'm thinking no and that I would have to
suck it up. But I seen stranger things so I thought I'd ask. I guess for
now I'll experiment allowing the slow jumps and I'll hand change it once
I'm done optimizing that function
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You can try "-mllvm -x86-cmov-converter=false". That should keep it as
a
cmov, but might give you performance issues elsewhere.

~Craig


On Mon, Apr 27, 2020 at 12:14 AM Levo DeLellis via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> I believe you all can access the test file and the diff in my bug report
> https://bugs.llvm.org/show_bug.cgi?id=45690
>
> In short, I'm experimenting with optimization and wrote branchless code
> -O2 generates a jump instruction causing my loop to be twice as slow. I
> changed the jne+movl to cmove and my loop seems to take half as long.
>
> I'm a little worried that this will interfere with other optimizations
I
> make in the same function (bug report is a small reproduce). Is there a
> temporary workaround I can use? I'm thinking no and that I would have
to
> suck it up. But I seen stranger things so I thought I'd ask. I guess
for
> now I'll experiment allowing the slow jumps and I'll hand change it
once
> I'm done optimizing that function
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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