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SubjectRe: [PATCH v1 1/2] x86/asm/bitops: Replace __fls() by its generic builtin implementation
On Sun, Nov 6, 2022 at 1:51 AM Vincent Mailhol
<mailhol.vincent@wanadoo.fr> wrote:
>
> Below snippet:
>
> #include <linux/bitops.h>
>
> unsigned int foo(unsigned long word)
> {
> return __fls(word);
> }
>
> produces this on GCC 12.1.0:
>
> 0000000000000000 <foo>:
> 0: f3 0f 1e fa endbr64
> 4: e8 00 00 00 00 call 9 <foo+0x9>
> 9: 53 push %rbx
> a: 48 89 fb mov %rdi,%rbx
> d: e8 00 00 00 00 call 12 <foo+0x12>
> 12: 48 0f bd c3 bsr %rbx,%rax
> 16: 5b pop %rbx
> 17: 31 ff xor %edi,%edi
> 19: e9 00 00 00 00 jmp 1e <foo+0x1e>
>
> and that on clang 14.0.6:
>
> 0000000000000000 <foo>:
> 0: f3 0f 1e fa endbr64
> 4: e8 00 00 00 00 call 9 <foo+0x9>
> 9: 53 push %rbx
> a: 50 push %rax
> b: 48 89 fb mov %rdi,%rbx
> e: e8 00 00 00 00 call 13 <foo+0x13>
> 13: 48 89 1c 24 mov %rbx,(%rsp)
> 17: 48 0f bd 04 24 bsr (%rsp),%rax
> 1c: 48 83 c4 08 add $0x8,%rsp
> 20: 5b pop %rbx
> 21: c3 ret
>
> The implementation from <asm-generic/bitops/builtin-__fls.h> [1]
> produces the exact same code on GCC and below code on clang:
>
> 0000000000000000 <foo>:
> 0: f3 0f 1e fa endbr64
> 4: e8 00 00 00 00 call 9 <foo+0x9>
> 9: 53 push %rbx
> a: 48 89 fb mov %rdi,%rbx
> d: e8 00 00 00 00 call 12 <foo+0x12>
> 12: 48 0f bd c3 bsr %rbx,%rax
> 16: 5b pop %rbx
> 17: c3 ret
>
> The builtin implementation is better for two reasons:
>
> 1/ it saves two instructions on clang (a push and a stack pointer
> decrement) because of a useless tentative to save rax.
>
> 2/ when used on constant expressions, the compiler is only able to
> fold the builtin version (c.f. [2]).
>
> For those two reasons, replace the assembly implementation by its
> builtin counterpart.
>
> [1] https://elixir.bootlin.com/linux/v6.0/source/include/asm-generic/bitops/builtin-__fls.h
>
> [2] commit 146034fed6ee ("x86/asm/bitops: Use __builtin_ffs() to evaluate constant expressions")
>
> CC: Borislav Petkov <bp@suse.de>
> CC: Nick Desaulniers <ndesaulniers@google.com>
> CC: Yury Norov <yury.norov@gmail.com>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>

LGTM; thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
> arch/x86/include/asm/bitops.h | 14 +-------------
> 1 file changed, 1 insertion(+), 13 deletions(-)
>
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index 2edf68475fec..a31453d7686d 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -285,19 +285,7 @@ static __always_inline unsigned long variable_ffz(unsigned long word)
> (unsigned long)__builtin_ctzl(~word) : \
> variable_ffz(word))
>
> -/*
> - * __fls: find last set bit in word
> - * @word: The word to search
> - *
> - * Undefined if no set bit exists, so code should check against 0 first.
> - */
> -static __always_inline unsigned long __fls(unsigned long word)
> -{
> - asm("bsr %1,%0"
> - : "=r" (word)
> - : "rm" (word));
> - return word;
> -}
> +#include <asm-generic/bitops/builtin-__fls.h>
>
> #undef ADDR
>
> --
> 2.37.4
>


--
Thanks,
~Nick Desaulniers

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