lib/string.c: implement a basic bcmp
[ Upstream commit 5f074f3e192f10c9fade898b9b3b8812e3d83342 ] 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.comSigned-off-by: NNick Desaulniers <ndesaulniers@google.com> Reported-by: NNathan Chancellor <natechancellor@gmail.com> Reported-by: NAdhemerval Zanella <adhemerval.zanella@linaro.org> Suggested-by: NArnd Bergmann <arnd@arndb.de> Suggested-by: NJames Y Knight <jyknight@google.com> Suggested-by: NMasahiro Yamada <yamada.masahiro@socionext.com> Suggested-by: NNathan Chancellor <natechancellor@gmail.com> Suggested-by: NRasmus Villemoes <linux@rasmusvillemoes.dk> Acked-by: NSteven Rostedt (VMware) <rostedt@goodmis.org> Reviewed-by: NNathan Chancellor <natechancellor@gmail.com> Tested-by: NNathan Chancellor <natechancellor@gmail.com> Reviewed-by: NMasahiro Yamada <yamada.masahiro@socionext.com> Reviewed-by: NAndy 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: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org> Signed-off-by: NSasha Levin <sashal@kernel.org>
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