Re: [PATCH v4] lib/string.c: implement a basic bcmp

Andrew Morton
2019-03-21T02:11:39Z

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