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[ Upstream commit5f074f3e19] 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:8e16d73346Link: 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> Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.