[llvm-dev] Dead store elimination in the backend for -ftrivial-auto-var-init

[Alexander Potapenko via llvm-dev]

On Thu, Aug 1, 2019 at 6:09 PM JF Bastien <jfbastien at apple.com> wrote:
>
> Hi Alexander,
>
> The code doesn’t compile. Could you send a godbolt.org link that shows the issue?
Sorry about that, here's the link: https://godbolt.org/z/-PinQP

Lines 4 to 8 are initializing |acpar|.
If I'm understanding correctly, the store to 8(%rsp) at line 7 can be
removed because of the store at line 35.
Same for store to 24(%rsp), which is later overwritten at lines 11 and 16.
> Thanks,
>
> JF
>
> > On Aug 1, 2019, at 7:21 AM, Alexander Potapenko <glider at google.com> wrote:
> >
> > Hi folks,
> >
> > When compiling the attached example with -ftrivial-auto-var-init=zero:
> >
> > $ clang   -no-integrated-as  -mno-sse -m64 -mstack-alignment=8    -O2
> > -ftrivial-auto-var-init=zero
> > -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang
> > -g  -o ipt.ll -c ipt.i -w -S -emit-llvm
> >
> > , Clang generates an initialization memset() call for |acpar| in the IR:
> >
> >    %0 = bitcast %struct.xt_action_param* %acpar to i8*, !dbg !27
> >    call void @llvm.memset.p0i8.i64(i8* nonnull align 8 %0, i8 0, i64
> > 40, i1 false), !dbg !28
> >
> > Clang only splits memsets into series of stores under certain
> > conditions, so it's hard to remove redundant stores on the IR level
> > (even with the recent DSE patches: https://reviews.llvm.org/D61879).
> > In the resulting assembly, however, the memset is lowered into a
> > series of MOVQ instructions (see the attached ipt.s file), of which at
> > least the stores to 24(%rsp) and 16(%rsp) are redundant.
> >
> > Given that at MCInst level we already know if a memset is split into a
> > sequence of stores, can we do a better job by making the backend
> > delete these redundant stores for us?
> >
> > Alex
> >
> > --
> > Alexander Potapenko
> > Software Engineer
> >
> > Google Germany GmbH
> > Erika-Mann-Straße, 33
> > 80636 München
> >
> > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> > Registergericht und -nummer: Hamburg, HRB 86891
> > Sitz der Gesellschaft: Hamburg
> > <ipt.i><ipt.s>
>


-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg