[llvm-dev] Replicate Individual O3 optimizations

Mehdi AMINI via llvm-dev llvm-dev at lists.llvm.org
Sat Oct 19 10:48:58 PDT 2019

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

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?).

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