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/8e16d73346f8091461319a7dfc4ddd18eedcff13
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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

authored by

Nick Desaulniers and committed by

Linus Torvalds 7 years ago 5f074f3e 7f46774c

+23

2 changed files

expand all

include

linux

string.h

lib

string.c

include/linux/string.h

··· 150 150 #ifndef __HAVE_ARCH_MEMCMP 151 151 extern int memcmp(const void *,const void *,__kernel_size_t); 152 152 #endif 153 + #ifndef __HAVE_ARCH_BCMP 154 + extern int bcmp(const void *,const void *,__kernel_size_t); 155 + #endif 153 156 #ifndef __HAVE_ARCH_MEMCHR 154 157 extern void * memchr(const void *,int,__kernel_size_t); 155 158 #endif

+20

lib/string.c

··· 866 866 EXPORT_SYMBOL(memcmp); 867 867 #endif 868 868 869 + #ifndef __HAVE_ARCH_BCMP 870 + /** 871 + * bcmp - returns 0 if and only if the buffers have identical contents. 872 + * @a: pointer to first buffer. 873 + * @b: pointer to second buffer. 874 + * @len: size of buffers. 875 + * 876 + * The sign or magnitude of a non-zero return value has no particular 877 + * meaning, and architectures may implement their own more efficient bcmp(). So 878 + * while this particular implementation is a simple (tail) call to memcmp, do 879 + * not rely on anything but whether the return value is zero or non-zero. 880 + */ 881 + #undef bcmp 882 + int bcmp(const void *a, const void *b, size_t len) 883 + { 884 + return memcmp(a, b, len); 885 + } 886 + EXPORT_SYMBOL(bcmp); 887 + #endif 888 + 869 889 #ifndef __HAVE_ARCH_MEMSCAN 870 890 /** 871 891 * memscan - Find a character in an area of memory.

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