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Date: Fri, 18 Oct 2019 12:00:22 -0700
From: Nathan Chancellor <natechancellor@gmail.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
Subject: Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a
 reference to bcmp
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Cc: Nick Desaulniers <ndesaulniers@google.com>,
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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        This function may be withdrawn in a future version.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