On Wed, Mar 13, 2019 at 11:02 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > A recent optimization in Clang (r355672) lowers comparisons of the > return value of memcmp against zero to comparisons of the return value > of bcmp against zero. This helps some platforms that implement bcmp > more efficiently than memcmp. glibc simply aliases bcmp to memcmp, but > an optimized implementation is in the works. > > This results in linkage failures for all targets with Clang due to the > undefined symbol. For now, just implement bcmp as a tailcail to memcmp > to unbreak the build. This routine can be further optimized in the > future. > > Other ideas discussed: > * A weak alias was discussed, but breaks for architectures that define > their own implementations of memcmp since aliases to declarations are > not permitted (only definitions). Arch-specific memcmp implementations > typically declare memcmp in C headers, but implement them in assembly. > * -ffreestanding also is used sporadically throughout the kernel. > * -fno-builtin-bcmp doesn't work when doing LTO. > > Link: https://bugs.llvm.org/show_bug.cgi?id=41035 > Link: https://code.woboq.org/userspace/glibc/string/memcmp.c.html#bcmp > Link: https://github.com/llvm/llvm-project/commit/8e16d73346f8091461319a7dfc4ddd18eedcff13 > Link: https://github.com/ClangBuiltLinux/linux/issues/416 > Cc: stable@xxxxxxxxxxxxxxx > Reported-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > Reported-by: Adhemerval Zanella <adhemerval.zanella@xxxxxxxxxx> > Suggested-by: Arnd Bergmann <arnd@xxxxxxxx> > Suggested-by: James Y Knight <jyknight@xxxxxxxxxx> > Suggested-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx> > Suggested-by: Nathan Chancellor <natechancellor@xxxxxxxxx> > Suggested-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > lib/string.c | 15 +++++++++++++++ ah, looks like I should provide a declaration in include/linux/string.h, too, in case others within the kernel would like to use bcmp. WIll send a v2. > 1 file changed, 15 insertions(+) > > diff --git a/lib/string.c b/lib/string.c > index 38e4ca08e757..5b2377d7143f 100644 > --- a/lib/string.c > +++ b/lib/string.c > @@ -866,6 +866,21 @@ __visible int memcmp(const void *cs, const void *ct, size_t count) > EXPORT_SYMBOL(memcmp); > #endif > > +#ifndef __HAVE_ARCH_BCMP > +/** > + * bcmp - Like memcmp but the return code simply indicates a non-match. > + * @cs: One area of memory. > + * @ct: Another area of memory. > + * @count: The size of the areas. > + */ > +#undef bcmp > +int bcmp(const void *cs, const void *ct, size_t count) > +{ > + return memcmp(cs, ct, count); > +} > +EXPORT_SYMBOL(bcmp); > +#endif > + > #ifndef __HAVE_ARCH_MEMSCAN > /** > * memscan - Find a character in an area of memory. > -- > 2.21.0.360.g471c308f928-goog > -- Thanks, ~Nick Desaulniers
