From: Nick Desaulniers ndesaulniers@google.com Subject: lib/string.c: implement a basic bcmp
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/8e16d73346f8091461319a7dfc4ddd18... Link: https://github.com/ClangBuiltLinux/linux/issues/416 Link: http://lkml.kernel.org/r/20190313211335.165605-1-ndesaulniers@google.com Signed-off-by: Nick Desaulniers ndesaulniers@google.com Reported-by: Nathan Chancellor natechancellor@gmail.com Reported-by: Adhemerval Zanella adhemerval.zanella@linaro.org Suggested-by: Arnd Bergmann arnd@arndb.de Suggested-by: James Y Knight jyknight@google.com Suggested-by: Masahiro Yamada yamada.masahiro@socionext.com Suggested-by: Nathan Chancellor natechancellor@gmail.com Suggested-by: Rasmus Villemoes linux@rasmusvillemoes.dk Acked-by: Steven Rostedt (VMware) rostedt@goodmis.org Reviewed-by: Nathan Chancellor natechancellor@gmail.com Tested-by: Nathan Chancellor natechancellor@gmail.com Reviewed-by: Masahiro Yamada yamada.masahiro@socionext.com Reviewed-by: Andy Shevchenko andriy.shevchenko@linux.intel.com Cc: David Laight David.Laight@ACULAB.COM Cc: Rasmus Villemoes linux@rasmusvillemoes.dk Cc: Namhyung Kim namhyung@kernel.org Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: Alexander Shishkin alexander.shishkin@linux.intel.com Cc: Dan Williams dan.j.williams@intel.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
include/linux/string.h | 3 +++ lib/string.c | 20 ++++++++++++++++++++ 2 files changed, 23 insertions(+)
--- a/include/linux/string.h~lib-stringc-implement-a-basic-bcmp +++ a/include/linux/string.h @@ -150,6 +150,9 @@ extern void * memscan(void *,int,__kerne #ifndef __HAVE_ARCH_MEMCMP extern int memcmp(const void *,const void *,__kernel_size_t); #endif +#ifndef __HAVE_ARCH_BCMP +extern int bcmp(const void *,const void *,__kernel_size_t); +#endif #ifndef __HAVE_ARCH_MEMCHR extern void * memchr(const void *,int,__kernel_size_t); #endif --- a/lib/string.c~lib-stringc-implement-a-basic-bcmp +++ a/lib/string.c @@ -866,6 +866,26 @@ __visible int memcmp(const void *cs, con EXPORT_SYMBOL(memcmp); #endif
+#ifndef __HAVE_ARCH_BCMP +/** + * bcmp - returns 0 if and only if the buffers have identical contents. + * @a: pointer to first buffer. + * @b: pointer to second buffer. + * @len: size of buffers. + * + * The sign or magnitude of a non-zero return value has no particular + * meaning, and architectures may implement their own more efficient bcmp(). So + * while this particular implementation is a simple (tail) call to memcmp, do + * not rely on anything but whether the return value is zero or non-zero. + */ +#undef bcmp +int bcmp(const void *a, const void *b, size_t len) +{ + return memcmp(a, b, len); +} +EXPORT_SYMBOL(bcmp); +#endif + #ifndef __HAVE_ARCH_MEMSCAN /** * memscan - Find a character in an area of memory. _