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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Tue, 27 Oct 2020 15:38:06 -0700
Message-ID: <CAKwvOdmcuSce_R_4=BmewPm_3t75_dkDan7YMXF_cb39mQEDyw@mail.gmail.com>
Subject: Re: [PATCH] bpf: don't rely on GCC __attribute__((optimize)) to
 disable GCSE
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Network Development <netdev@vger.kernel.org>,
        bpf <bpf@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        Randy Dunlap <rdunlap@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Kees Cook <keescook@chromium.org>
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On Tue, Oct 27, 2020 at 3:23 PM Ard Biesheuvel <ardb@kernel.org> wrote:
>
> On Tue, 27 Oct 2020 at 23:03, Nick Desaulniers <ndesaulniers@google.com> wrote:
> >
> > On Tue, Oct 27, 2020 at 2:50 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > >
> > > On Tue, 27 Oct 2020 at 22:20, Nick Desaulniers <ndesaulniers@google.com> wrote:
> > > >
> > > > On Tue, Oct 27, 2020 at 1:57 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> > > > >
> > > > > diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> > > > > index 6e390d58a9f8..ac3fa37a84f9 100644
> > > > > --- a/include/linux/compiler_types.h
> > > > > +++ b/include/linux/compiler_types.h
> > > > > @@ -247,10 +247,6 @@ struct ftrace_likely_data {
> > > > >  #define asm_inline asm
> > > > >  #endif
> > > > >
> > > > > -#ifndef __no_fgcse
> > > > > -# define __no_fgcse
> > > > > -#endif
> > > > > -
> > > > Finally, this is going to disable GCSE for the whole translation unit,
> > > > which may be overkill.   Previously it was isolated to one function
> > > > definition.  You could lower the definition of the preprocessor define
> > > > into kernel/bpf/core.c to keep its use isolated as far as possible.
> > > >
> > >
> > > Which preprocessor define?
> >
> > __no_fgcse
> >
>
> But we can't use that, that is the whole point of this patch.

Ah, right because the attribute drops other command line flags...ok,
then -fno-gcse the whole translation unit it is then.

Still need to avoid new warnings with clang though.
-- 
Thanks,
~Nick Desaulniers