Commit 5f074f3e authored Apr 05, 2019 by Nick Desaulniers Committed by Linus Torvalds Apr 05, 2019

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>

parent 7f46774c

include/linux/string.h

+3 −0

Original line number	Diff line number	Diff line
		@@ -150,6 +150,9 @@ extern void * memscan(void *,int,__kernel_size_t);
		#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

lib/string.c

+20 −0

Original line number	Diff line number	Diff line
		@@ -866,6 +866,26 @@ __visible int memcmp(const void cs, const void ct, size_t count)
		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.

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