[PATCH] x86-64: Make bcmp an alias of __memcmpeq

[H.J. Lu]

On Sun, Feb 6, 2022 at 11:34 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Sun, Feb 6, 2022 at 1:26 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > ---
> >  sysdeps/x86_64/memcmp.S                | 2 +-
> >  sysdeps/x86_64/multiarch/memcmp-sse2.S | 6 ++++--
> >  sysdeps/x86_64/multiarch/memcmp.c      | 2 --
> >  sysdeps/x86_64/multiarch/memcmpeq.c    | 2 ++
> >  4 files changed, 7 insertions(+), 5 deletions(-)
> >
> > diff --git a/sysdeps/x86_64/memcmp.S b/sysdeps/x86_64/memcmp.S
> > index e02a53ea1e..2768d45f10 100644
> > --- a/sysdeps/x86_64/memcmp.S
> > +++ b/sysdeps/x86_64/memcmp.S
> > @@ -405,8 +405,8 @@ END(memcmp)
> >
> >  #ifdef USE_AS_MEMCMPEQ
> >  libc_hidden_def (memcmp)
> > -#else
> >  # undef bcmp
> >  weak_alias (memcmp, bcmp)
> > +#else
> >  libc_hidden_builtin_def (memcmp)
> >  #endif
> > diff --git a/sysdeps/x86_64/multiarch/memcmp-sse2.S b/sysdeps/x86_64/multiarch/memcmp-sse2.S
> > index e10555638d..d825c13ede 100644
> > --- a/sysdeps/x86_64/multiarch/memcmp-sse2.S
> > +++ b/sysdeps/x86_64/multiarch/memcmp-sse2.S
> > @@ -29,8 +29,10 @@
> >  #  define libc_hidden_def(ignored)
> >  # endif
> >
> > -# undef weak_alias
> > -# define weak_alias(ignored1, ignored2)
> > +# ifdef USE_MULTIARCH
> > +#  undef weak_alias
> > +#  define weak_alias(ignored1, ignored2)
> > +# endif
> >
> >  # undef strong_alias
> >  # define strong_alias(ignored1, ignored2)
> > diff --git a/sysdeps/x86_64/multiarch/memcmp.c b/sysdeps/x86_64/multiarch/memcmp.c
> > index 7757d1ec4e..bae29d8dad 100644
> > --- a/sysdeps/x86_64/multiarch/memcmp.c
> > +++ b/sysdeps/x86_64/multiarch/memcmp.c
> > @@ -27,8 +27,6 @@
> >  # include "ifunc-memcmp.h"
> >
> >  libc_ifunc_redirected (__redirect_memcmp, memcmp, IFUNC_SELECTOR ());
> > -# undef bcmp
> > -weak_alias (memcmp, bcmp)
> >
> >  # ifdef SHARED
> >  __hidden_ver1 (memcmp, __GI_memcmp, __redirect_memcmp)
> > diff --git a/sysdeps/x86_64/multiarch/memcmpeq.c b/sysdeps/x86_64/multiarch/memcmpeq.c
> > index aa1c8ad4de..45cb6bad5b 100644
> > --- a/sysdeps/x86_64/multiarch/memcmpeq.c
> > +++ b/sysdeps/x86_64/multiarch/memcmpeq.c
> > @@ -27,6 +27,8 @@
> >  # include "ifunc-memcmpeq.h"
> >
> >  libc_ifunc_redirected (__redirect___memcmpeq, __memcmpeq, IFUNC_SELECTOR ());
> > +# undef bcmp
> > +weak_alias (__memcmpeq, bcmp)
> >
> >  # ifdef SHARED
> >  __hidden_ver1 (__memcmpeq, __GI___memcmpeq, __redirect___memcmpeq)
> > --
> > 2.34.1
> >
>
> Are we sure this is safe? One of the original rationales for adding
> '__memcmpeq' as
> a new interface instead of just optimizing 'bcmp' was that users may
> be relying on
> the fact that 'bcmp' has historically returned the LT/GT of the comparison.

The bcmp man page has:

NAME
       bcmp - compare byte sequences

SYNOPSIS
       #include <strings.h>

       int bcmp(const void *s1, const void *s2, size_t n);

DESCRIPTION
       The bcmp() function compares the two byte sequences s1 and s2 of length
       n each.  If they are equal, and in particular if n is zero, bcmp()  re‐
       turns 0.  Otherwise, it returns a nonzero result.

RETURN VALUE
       The  bcmp()  function returns 0 if the byte sequences are equal, other‐
       wise a nonzero result is returned.

> As well, if we are going to change 'bcmp' should we do it for all
> architectures so that
> its consistent?

Only x86-64 has __memcmpeq which isn't an alias of memcmp.

-- 
H.J.