From: Nathan Chancellor <natechancellor@gmail.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Nick Desaulniers <ndesaulniers@google.com>,
LKML <linux-kernel@vger.kernel.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Paul Mackerras <paulus@samba.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp
Date: Fri, 18 Oct 2019 12:00:22 -0700 [thread overview]
Message-ID: <20191018190022.GA1292@ubuntu-m2-xlarge-x86> (raw)
In-Reply-To: <20191014191141.GK28442@gate.crashing.org>
On Mon, Oct 14, 2019 at 02:11:41PM -0500, Segher Boessenkool wrote:
> On Mon, Oct 14, 2019 at 08:56:12AM -0700, Nick Desaulniers wrote:
> > On Mon, Oct 14, 2019 at 2:35 AM Segher Boessenkool
> > <segher@kernel.crashing.org> wrote:
> > >
> > > On Sun, Oct 13, 2019 at 07:51:01PM -0700, Nathan Chancellor wrote:
> > > > r374662 gives LLVM the ability to convert certain loops into a reference
> > > > to bcmp as an optimization; this breaks prom_init_check.sh:
> > >
> > > When/why does LLVM think this is okay? This function has been removed
> > > from POSIX over a decade ago (and before that it always was marked as
> > > legacy).
> >
> > Segher, do you have links for any of the above? If so, that would be
> > helpful to me.
>
> Sure!
>
> https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xsh_chap03.html
>
> Older versions are harder to find online, unfortunately. But there is
>
> https://kernel.org/pub/linux/docs/man-pages/man-pages-posix/
>
> in which man3p/bcmp.3p says:
>
> FUTURE DIRECTIONS
> This function may be withdrawn in a future version.
>
> Finally, the Linux man pages say (man bcmp):
>
> CONFORMING TO
> 4.3BSD. This function is deprecated (marked as LEGACY in
> POSIX.1-2001): use memcmp(3) in new programs. POSIX.1-2008 removes the
> specification of bcmp().
>
>
> > I'm arguing against certain transforms that assume that
> > one library function is faster than another, when such claims are
> > based on measurements from one stdlib implementation.
>
> Wow. The difference between memcmp and bcmp is trivial (just the return
> value is different, and that costs hardly anything to add). And memcmp
> is guaranteed to exist since C89/C90 at least.
>
> > The rationale for why it was added was that memcmp takes a measurable
> > amount of time in Google's fleet, and most calls to memcmp don't care
> > about the position of the mismatch; bcmp is lower overhead (or at
> > least for our libc implementation, not sure about others).
>
> You just have to do the read of the last words you compare as big-endian,
> and then you can just subtract the two words, convert that to "int" (which
> is very inconvenient to do, but hardly expensive), and there you go.
>
> Or on x86 use the bswap insn, or something like it.
>
> Or, if you use GCC, it has __builtin_memcmp but also __builtin_memcmp_eq,
> and those are automatically used, too.
>
>
> Segher
Just as an FYI, there was some more discussion around the availablity
and use of bcmp in this LLVM bug which spawned
commit 5f074f3e192f ("lib/string.c: implement a basic bcmp").
https://bugs.llvm.org/show_bug.cgi?id=41035#c13
I believe this is the proper solution but I am fine with whatever works,
I just want our CI to be green without any out of tree patches again...
Cheers,
Nathan
next prev parent reply other threads:[~2019-10-18 19:02 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-11 18:20 [PATCH v3 0/3] LLVM/Clang fixes for pseries_defconfig Nathan Chancellor
2019-09-11 18:20 ` [PATCH v3 1/3] powerpc: Don't add -mabi= flags when building with Clang Nathan Chancellor
2019-09-11 18:20 ` [PATCH v3 2/3] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-09-11 20:55 ` Nick Desaulniers
2019-09-11 18:20 ` [PATCH v3 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp Nathan Chancellor
2019-09-11 21:01 ` Nick Desaulniers
2019-09-12 5:43 ` Nathan Chancellor
2019-09-12 17:30 ` Nick Desaulniers
2019-10-14 2:50 ` [PATCH v4 0/3] LLVM/Clang fixes for pseries_defconfig Nathan Chancellor
2019-10-14 2:50 ` [PATCH v4 1/3] powerpc: Don't add -mabi= flags when building with Clang Nathan Chancellor
2019-10-14 2:51 ` [PATCH v4 2/3] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-10-14 2:51 ` [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp Nathan Chancellor
2019-10-14 9:35 ` Segher Boessenkool
2019-10-14 15:56 ` Nick Desaulniers
2019-10-14 19:11 ` Segher Boessenkool
2019-10-18 19:00 ` Nathan Chancellor [this message]
2019-10-18 20:02 ` Segher Boessenkool
2019-10-22 5:15 ` Nathan Chancellor
2019-10-22 8:57 ` Segher Boessenkool
2019-10-30 4:12 ` Nathan Chancellor
2019-10-14 16:03 ` [PATCH v4 0/3] LLVM/Clang fixes for pseries_defconfig Nick Desaulniers
2019-11-19 4:57 ` [PATCH v5 0/3] LLVM/Clang fixes for a few defconfigs Nathan Chancellor
2019-11-19 4:57 ` [PATCH v5 1/3] powerpc: Don't add -mabi= flags when building with Clang Nathan Chancellor
2019-11-26 1:13 ` Michael Ellerman
2019-11-19 4:57 ` [PATCH v5 2/3] powerpc: Avoid clang warnings around setjmp and longjmp Nathan Chancellor
2019-11-19 4:57 ` [PATCH v5 3/3] powerpc/prom_init: Use -ffreestanding to avoid a reference to bcmp Nathan Chancellor
2019-11-25 20:15 ` [PATCH v5 0/3] LLVM/Clang fixes for a few defconfigs Nick Desaulniers
2019-11-28 4:59 ` Michael Ellerman
2019-11-28 7:45 ` Nathan Chancellor
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