Re: [PATCH 2/4] Revert "lib/string.c: implement a basic bcmp"

[Nathan Chancellor <natechancellor@gmail.com>]

On Mon, Aug 17, 2020 at 03:02:10PM -0700, Nick Desaulniers wrote:
> This reverts commit 5f074f3e192f10c9fade898b9b3b8812e3d83342.
> 
> Use `-fno-builtin-bcmp` instead.
> 
> The issue with using `-fno-builtin-*` flags was that they were not
> retained during an LTO link with LLVM.  This was fixed in clang-11 by
> https://reviews.llvm.org/D71193
> (0508c994f0b14144041f2cfd3ba9f9a80f03de08), which is also the minimum
> supported version of clang for LTO.
> 
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  Makefile               |  1 +
>  include/linux/string.h |  3 ---
>  lib/string.c           | 20 --------------------
>  3 files changed, 1 insertion(+), 23 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index 211a1b6f6478..722ff5864275 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -964,6 +964,7 @@ endif
>  # to provide implementations of these routines, then prevent the compiler from
>  # emitting calls to what will be undefined symbols.
>  KBUILD_CFLAGS	+= -fno-builtin-stpcpy
> +KBUILD_CFLAGS	+= -fno-builtin-bcmp

I personally think that this hunk should be its own patch before this
one then have this patch just be the revert, that way there is no
regression across a bisect (if one were to ever occur) and so the revert
is a straight 'git revert', rather than have something else mixed in
that requires reading the actual changelog text.

No objections if you disagree though.

>  # include additional Makefiles when needed
>  include-y			:= scripts/Makefile.extrawarn
> diff --git a/include/linux/string.h b/include/linux/string.h
> index b1f3894a0a3e..f3bdb74bc230 100644
> --- a/include/linux/string.h
> +++ b/include/linux/string.h
> @@ -155,9 +155,6 @@ extern void * memscan(void *,int,__kernel_size_t);
>  #ifndef __HAVE_ARCH_MEMCMP
>  extern int memcmp(const void *,const void *,__kernel_size_t);
>  #endif
> -#ifndef __HAVE_ARCH_BCMP
> -extern int bcmp(const void *,const void *,__kernel_size_t);
> -#endif
>  #ifndef __HAVE_ARCH_MEMCHR
>  extern void * memchr(const void *,int,__kernel_size_t);
>  #endif
> diff --git a/lib/string.c b/lib/string.c
> index 6012c385fb31..69328b8353e1 100644
> --- a/lib/string.c
> +++ b/lib/string.c
> @@ -922,26 +922,6 @@ __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
> -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.
> -- 
> 2.28.0.220.ged08abb693-goog
> 

Cheers,
Nathan