On Thu, Nov 7, 2019 at 11:19 AM Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
>
>
>
> On 11/7/19 1:00 PM, Arnd Bergmann wrote:
> > On Thu, Nov 7, 2019 at 10:46 AM Andrey Ryabinin <aryabinin@xxxxxxxxxxxxx> wrote:
> >> On 11/7/19 12:22 PM, Alexander Potapenko wrote:
> >>> On Thu, Nov 7, 2019 at 10:04 AM Arnd Bergmann <arnd@xxxxxxxx> wrote:
> >>>> On Thu, Nov 7, 2019 at 7:08 AM Sergey Senozhatsky
> >>>> <sergey.senozhatsky.work@xxxxxxxxx> wrote:
> >>>> The normal way to do a volatile access would be
> >>>> READ_ONCE()/WRITE_ONCE(), but that seems stronger than
> >>>> the barrier() here. I'd just stick to adding a barrier.
> >>> I actually like the READ_ONCE idea more, as READ_ONCE is really a
> >>> documented way to prevent the compiler from merging reads, which is
> >>> what we want here.
> >>
> >> I would rather go with -fno-builtin-bcmp or maybe even -fno-builtin if that works.
> >
> > The commit message for 5f074f3e192f ("lib/string.c: implement a basic bcmp")
> > mentions that -fno-builtin-bcmp did not work for LTO when the global bcmp()
> > help was added. I don't know whether the same applies here, but my guess is
> > that it's the same issue.
>
> But we don't do LTO.
I don't think not doing LTO now is a valid argument, as some
distributions may start doing LTO in the future.
(Android already does LTO, by the way)
Regarding this particular case, -fno-builtin-bcmp is insufficient, as
Clang falls back to memcmp() in that case.
Building with -fno-builtin-bcmp -fno-builtin-memcmp does the trick,
but we'd probably better use -fno-builtin just to avoid future
surprises.
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Alexander Potapenko
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