llvm dev - Oct 2019 - Replicate Individual O3 optimizations

Hello,
I want to study the individual O3 optimizations. For this I am using
following commands, but unable to replicate O3 behavior.

1. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O1
-Xclang -disable-llvm-passes -emit-llvm -S vecsum.c -o vecsum-noopt.ll

2. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O3
-mllvm -debug-pass=Arguments -emit-llvm -S vecsum.c

3. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/opt
<optimization sequence obtained in step 2> -S vecsum-noopt.ll -S -o
o3-chk.ll

Why the IR obtained by above step i.e individual O3 sequences, is not same
when O3 is passed?

Where I am doing mistake?

Please help.

Thank You
Regards
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20191013/cc43faf7/attachment.html>

hameeza ahmed via llvm-dev <llvm-dev at lists.llvm.org> writes:
> Hello,
> I want to study the individual O3 optimizations. For this I am using
> following commands, but unable to replicate O3 behavior.
>
> 1. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O1
> -Xclang -disable-llvm-passes -emit-llvm -S vecsum.c -o vecsum-noopt.ll
>
> 2. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O3
> -mllvm -debug-pass=Arguments -emit-llvm -S vecsum.c
>
> 3. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/opt
> <optimization sequence obtained in step 2> -S vecsum-noopt.ll -S -o
> o3-chk.ll
>
> Why the IR obtained by above step i.e individual O3 sequences, is not same
> when O3 is passed?
>
> Where I am doing mistake?
I think you need to turn off LLVM optimizations when doing the
-emit-llvm dump.  Something like this:

Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O3 \
  -mllvm -debug-pass=Arguments -Xclang -disable-llvm-optzns -emit-llvm \
  -S vecsum.c

Otherwise you are effectively running the O3 pipeline twice, as clang
will emit LLVM IR after optimization, not before (this confused me too
when I first tried it).

That said, I'm not sure you will get the same IR out of opt as with
clang -O3 even with the above.  For example, clang sets
TargetTransformInfo for the pass pipeline and the detailed information
it uses may or may not be transmitted via the IR it dumps out.  I have
not personally tried to do this kind of thing in a while.

                  -David

On Thu, Oct 17, 2019 at 11:22 AM David Greene via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> hameeza ahmed via llvm-dev <llvm-dev at lists.llvm.org> writes:
>
> > Hello,
> > I want to study the individual O3 optimizations. For this I am using
> > following commands, but unable to replicate O3 behavior.
> >
> > 1. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang
-O1
> > -Xclang -disable-llvm-passes -emit-llvm -S vecsum.c -o vecsum-noopt.ll
> >
> > 2. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang
-O3
> > -mllvm -debug-pass=Arguments -emit-llvm -S vecsum.c
> >
> > 3. Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/opt
> > <optimization sequence obtained in step 2> -S vecsum-noopt.ll -S
-o
> > o3-chk.ll
> >
> > Why the IR obtained by above step i.e individual O3 sequences, is not
> same
> > when O3 is passed?
> >
> > Where I am doing mistake?
>
If you could provide the full reproducer, it could help to debug this.

>
> I think you need to turn off LLVM optimizations when doing the
> -emit-llvm dump.  Something like this:
>
> Documents/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang -O3 \
>   -mllvm -debug-pass=Arguments -Xclang -disable-llvm-optzns -emit-llvm \
>   -S vecsum.c
>
> Otherwise you are effectively running the O3 pipeline twice, as clang
> will emit LLVM IR after optimization, not before (this confused me too
> when I first tried it).
>
This is the common pitfall indeed!
I think they are doing it correctly in step 1 though by including: `-Xclang
-disable-llvm-passes`.


That said, I'm not sure you will get the same IR out of opt as
with
> clang -O3 even with the above.  For example, clang sets
> TargetTransformInfo for the pass pipeline and the detailed information
> it uses may or may not be transmitted via the IR it dumps out.  I have
> not personally tried to do this kind of thing in a while.

I struggled as well to setup TTI and TLI the same way clang does :(
It'd be nice to revisit our PassManagerBuilder setup and the opt
integration to provide reproducibility (maybe could be a starter project
for someone?).

-- 
Mehdi
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20191019/1bd12e26/attachment-0001.html>