On Wed, Mar 13, 2019 at 11:02 AM Nick Desaulniers ndesaulniers@google.com wrote:
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 Cc: stable@vger.kernel.org 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 Signed-off-by: Nick Desaulniers ndesaulniers@google.com
lib/string.c | 15 +++++++++++++++
ah, looks like I should provide a declaration in include/linux/string.h, too, in case others within the kernel would like to use bcmp. WIll send a v2.
1 file changed, 15 insertions(+)
diff --git a/lib/string.c b/lib/string.c index 38e4ca08e757..5b2377d7143f 100644 --- a/lib/string.c +++ b/lib/string.c @@ -866,6 +866,21 @@ __visible int memcmp(const void *cs, const void *ct, size_t count) EXPORT_SYMBOL(memcmp); #endif
+#ifndef __HAVE_ARCH_BCMP +/**
- bcmp - Like memcmp but the return code simply indicates a non-match.
- @cs: One area of memory.
- @ct: Another area of memory.
- @count: The size of the areas.
- */
+#undef bcmp +int bcmp(const void *cs, const void *ct, size_t count) +{
return memcmp(cs, ct, count);+} +EXPORT_SYMBOL(bcmp); +#endif
#ifndef __HAVE_ARCH_MEMSCAN /**
- memscan - Find a character in an area of memory.
-- 2.21.0.360.g471c308f928-goog