llvm dev - Apr 2020 - Branch is not optimized because of right shift

Hi John,

I hadn't seen alive2, amazing program, thank you! Keep up the good work.
> I'm not sure that I've captured what you mean here
To be more specific for everyone:
- First of all, I hope it's clear that in the original (C) code, the region
- 0x8 > 1000 branch should
be eliminated. That is because it is inside a block that has ensured that 8
< region < 12. But for some reason,
it is not eliminated. I'm trying to find that reason. :)
- I understand that in the -O2 .ll output, we take %0 and do it a right
shift. That means that we don't have
any range info about %0 or %0 >>= 1, so we can't eliminate the branch.
What
I don't understand
is why we chose to reuse %0 and do it a right shift and instead we didn't
have a phi there.
- Finally, I saw that putting nuw in the addition with -8 eliminates the
branch. This makes sense since,
we know probably know from the code above that %0 >>= 1 is less than 12.
Although, I don't understand
why the right shift was translated to an add with -16.

I hope this made some sense. You may ignore the last 2 and focus on the
first, i.e. what optimization
should have been done but it's not and what we can do about it.

A clearer version of the .ll: https://godbolt.org/z/2t4RU5

Best,
Stefanos

p.s. John I had sent you an e-mail regarding loop rotation some time ago.
I'm not sure if it was intentional
on your part to be skipped, if so, sorry to bother you. :) But there's the
alternative that it was just lost.

Στις Κυρ, 5 Απρ 2020 στις 7:12 π.μ., ο/η John Regehr via llvm-dev <
llvm-dev at lists.llvm.org> έγραψε:
> I'm not sure that I've captured what you mean here, but you can
explore
> these sorts of things using Alive2:
>
>    http://volta.cs.utah.edu:8080/z/b7Yj7j
>
> John
>
>
>
> On 4/4/20 6:29 PM, Stefanos Baziotis via llvm-dev wrote:
> > Hi everyone,
> >
> > In a twitch chat someone redirected me to an example that is not
> > optimized: https://godbolt.org/z/BL-4jL
> >
> > I included the original source code and this is after -O2. We both
> > thought that the -8 branch could be optimized out. I added a nuw in
the
> > subtraction and it actually does it.
> >
> > Any thoughts on why that doesn't happen already?
> >
> > Best,
> > Stefanos Baziotis
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Hi,
> On Apr 5, 2020, at 14:08, Stefanos Baziotis via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> To be more specific for everyone:
> - First of all, I hope it's clear that in the original (C) code, the
region - 0x8 > 1000 branch should
> be eliminated. That is because it is inside a block that has ensured that 8
< region < 12. But for some reason,
> it is not eliminated. I'm trying to find that reason. :)
> - I understand that in the -O2 .ll output, we take %0 and do it a right
shift. That means that we don't have
> any range info about %0 or %0 >>= 1, so we can't eliminate the
branch. What I don't understand
> is why we chose to reuse %0 and do it a right shift and instead we
didn't have a phi there.
> - Finally, I saw that putting nuw in the addition with -8 eliminates the
branch. This makes sense since,
> we know probably know from the code above that %0 >>= 1 is less than
12. Although, I don't understand
> why the right shift was translated to an add with -16.
> 
> I hope this made some sense. You may ignore the last 2 and focus on the
first, i.e. what optimization
> should have been done but it's not and what we can do about it.
> 
> A clearer version of the .ll: https://godbolt.org/z/2t4RU5
I think the IR in both of your examples makes things harder for the compiler
than expected from the original C source.

With the IR in your original example (https://godbolt.org/z/BL-4jL), I think the
problem is that the branch condition is '%0 - 16 < 12’, which allows us
to limit the range of `%0 - 16` easily, but in the branch only %0 is used.
Sinking the lshr too early made the analysis harder.

In the second example (https://godbolt.org/z/2t4RU5), things are hard to
simplify because we need to use the information from the condition of one select
to simplify the earlier select.

The version in https://godbolt.org/z/_ipKhb  is probably the easiest for
analysis (basically the original C source code built with `clang -O0 -S
-emit-llvm`, followed by running `opt -mem2reg`). There’s a patch under review
that adds support for conditional range propagation
(https://reviews.llvm.org/D76611) and with the patch it can be simplified to the
code below by running `opt  -ipsccp -simplifycfg -instcombine`

define i32 @test(i32 %0) {
  %.off = add i32 %0, -16
  %2 = icmp ult i32 %.off, 12
  %spec.select = select i1 %2, i32 66, i32 45
  ret i32 %spec.select
}


The reason it does not yet work in the default pipeline is that the branch
condition will be combined to use an AND before the conditional propagation and
D76611 does not support that yet. But once it lands that should be an easy
extension.

The problems with your two versions could be addressed as well of course for
that special case relatively easily I think, but the challenge is to fix it in a
general and efficient (compile-time wise) way. I hope the conditional
propagation should cover such cases soon though.

Cheers,
Florian

Hi,
> I think the IR in both of your examples makes things harder for the
compiler than expected from the original C source.

Note that both versions are from clang with -O2. The first is with version
9.0 and the second is with the trunk.
> but in the branch only %0 is used. Sinking the lshr too early made the
analysis harder.

Yes, exactly! That's what I figured too.
> The version in https://godbolt.org/z/_ipKhb  is probably the easiest for
analysis (basically the original C source code built with `clang -O0 -S
-emit-llvm`, followed by running `opt -mem2reg`). There’s a patch under
review that adds support for conditional range propagation (
https://reviews.llvm.org/D76611) and with the patch it can be simplified to
the code below by running `opt  -ipsccp -simplifycfg
-instcombine`
>
> define i32 @test(i32 %0) {
>   %.off = add i32 %0, -16
>   %2 = icmp ult i32 %.off, 12
>   %spec.select = select i1 %2, i32 66, i32 45
>   ret i32 %spec.select
> }
>
>
> The reason it does not yet work in the default pipeline is that the
branch condition will be combined to use an AND before the conditional
propagation and D76611 does not support that yet. But once it lands that
should be an easy extension.
>
> The problems with your two versions could be addressed as well of course
for that special case relatively easily I think, but the challenge is to
fix it in a general and efficient (compile-time wise) way. I hope the
conditional propagation should cover such cases soon though.

Ok, I checked the patch, best of luck! I see how it does the
transformation, it's interesting. I think that
https://reviews.llvm.org/rGe9963034314edf49a12ea5e29f694d8f9f52734a could
maybe help in such things.
Any idea about how the compiler could remove the lshr and use a add -16?

Best,
Stefanos Baziotis


Στις Κυρ, 5 Απρ 2020 στις 10:06 μ.μ., ο/η Florian Hahn <
florian_hahn at apple.com> έγραψε:
> Hi,
>
> > On Apr 5, 2020, at 14:08, Stefanos Baziotis via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> > To be more specific for everyone:
> > - First of all, I hope it's clear that in the original (C) code,
the
> region - 0x8 > 1000 branch should
> > be eliminated. That is because it is inside a block that has ensured
> that 8 < region < 12. But for some reason,
> > it is not eliminated. I'm trying to find that reason. :)
> > - I understand that in the -O2 .ll output, we take %0 and do it a
right
> shift. That means that we don't have
> > any range info about %0 or %0 >>= 1, so we can't eliminate
the branch.
> What I don't understand
> > is why we chose to reuse %0 and do it a right shift and instead we
> didn't have a phi there.
> > - Finally, I saw that putting nuw in the addition with -8 eliminates
the
> branch. This makes sense since,
> > we know probably know from the code above that %0 >>= 1 is less
than 12.
> Although, I don't understand
> > why the right shift was translated to an add with -16.
> >
> > I hope this made some sense. You may ignore the last 2 and focus on
the
> first, i.e. what optimization
> > should have been done but it's not and what we can do about it.
> >
> > A clearer version of the .ll: https://godbolt.org/z/2t4RU5
>
> I think the IR in both of your examples makes things harder for the
> compiler than expected from the original C source.
>
> With the IR in your original example (https://godbolt.org/z/BL-4jL), I
> think the problem is that the branch condition is '%0 - 16 < 12’,
which
> allows us to limit the range of `%0 - 16` easily, but in the branch only %0
> is used. Sinking the lshr too early made the analysis harder.
>
> In the second example (https://godbolt.org/z/2t4RU5), things are hard to
> simplify because we need to use the information from the condition of one
> select to simplify the earlier select.
>
> The version in https://godbolt.org/z/_ipKhb  is probably the easiest for
> analysis (basically the original C source code built with `clang -O0 -S
> -emit-llvm`, followed by running `opt -mem2reg`). There’s a patch under
> review that adds support for conditional range propagation (
> https://reviews.llvm.org/D76611) and with the patch it can be simplified
> to the code below by running `opt  -ipsccp -simplifycfg -instcombine`
>
> define i32 @test(i32 %0) {
>   %.off = add i32 %0, -16
>   %2 = icmp ult i32 %.off, 12
>   %spec.select = select i1 %2, i32 66, i32 45
>   ret i32 %spec.select
> }
>
>
> The reason it does not yet work in the default pipeline is that the branch
> condition will be combined to use an AND before the conditional propagation
> and D76611 does not support that yet. But once it lands that should be an
> easy extension.
>
> The problems with your two versions could be addressed as well of course
> for that special case relatively easily I think, but the challenge is to
> fix it in a general and efficient (compile-time wise) way. I hope the
> conditional propagation should cover such cases soon though.
>
> Cheers,
> Florian
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