On Wed, 13 Mar 2019 14:13:31 -0700 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.
I guess we should backport this into -stable so that older kernels can be built with newer Clang.
...
--- a/lib/string.c +++ b/lib/string.c @@ -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
What is the undef for?
+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.
-- 2.21.0.360.g471c308f928-goog