[llvm-dev] Difference between clang -Oz and opt -Oz

[Fernando Magno Quintao Pereira via llvm-dev]

Dear Fang-rui,

    Thank you very much for the detailed answers. That was very nice!

Happy New Year,

Fernando

On Tue, Dec 29, 2020 at 6:58 PM Fāng-ruì Sòng <maskray at google.com> wrote:
>
> On Tue, Dec 29, 2020 at 12:02 PM Fernando Magno Quintao Pereira via
> llvm-dev <llvm-dev at lists.llvm.org> wrote:
> >
> > Dear LLVMers,
> >
> > Why does opt -Oz and clang -Oz produce different codes? And could
> > someone point me to some documentation/discussion that explains the
> > differences between them?
> >
> > Regards,
> >
> > Fernando
> >
> > ------------------
> >
> > Example
> > =======
> >
> > This program below ends up with 74 instructions with opt -Oz, and 22
> > instructions with clang -Oz (LLVM 10.0). Opt unrolls the loop 2x,
> > whereas clang doesn't:
> >
> > void prefix_sum(int *src, int *dst, int N) {
> >   if (0 < N) {
> >     int i = 0;
> >     do {
> >       int tmp = 0;
> >       int j = 0;
> >       if (j < i) {
> >         do  {
> >           tmp += src[j];
> >           j++;
> >         } while (j < i);
> >         dst[i] = tmp;
> >       }
> >       i++;
> >     } while (i < N);
> >   }
> > }
> >
> > I can see some optimizations with clang, that might not run with opt when I do:
> >
> > echo 'int;' | clang -xc -Oz - -o /dev/null -\#\#\#
> >
> > Would that be the reason for the difference?
>
> clang -fno-legacy-pass-manager -S -emit-llvm -Oz
> populates the pipeline with PassBuilder::buildPerModuleDefaultPipeline
>
> If you use
> clang -fno-legacy-pass-manager -S -emit-llvm -Oz -Xclang -disable-llvm-passes
> opt -S -passes='default<Oz>'
> the behavior will be quite similar, though the variable naming will be
> different.
>
> ---
>
> If you invoke
> clang -fno-legacy-pass-manager -S -emit-llvm -O0 -Xclang -disable-llvm-passes
> followed by
> opt -S -passes='default<Oz>' calls
>
> clang -O0 adds optnone and (for most functions) noinline function attributes.
>   (llvm::PassInstrumentation::runBeforePass<llvm::Function, ...> skips
> passes if the function has the optnone attribute.)
> clang -Oz adds optsize and minsize function attributes.
>
> noinline/optsize/minsize can affect optimization passes' heuristics.
>
>
> You can drop optnone with
> clang -fno-legacy-pass-manager -S -emit-llvm -O0 -Xclang
> -disable-O0-optnone -Xclang -disable-llvm-passes
> however, noinline is still there. You can use -O1
> clang -fno-legacy-pass-manager -S -emit-llvm -O1 -Xclang -disable-llvm-passes
> however, there is still a llvm.loop.unroll.disable metadata difference.
> clang -fno-legacy-pass-manager -S -emit-llvm -O2 -Xclang -disable-llvm-passes