Re: [PATCH 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions

[Nick Desaulniers]

On Tue, May 10, 2022 at 7:26 AM Vincent Mailhol
<mailhol.vincent@xxxxxxxxxx> wrote:
>
> For x86_64, the current ffs() implementation does not produce
> optimized code when called with a constant expression. On the
> contrary, the __builtin_ffs() function of both GCC and clang is able
> to simplify the expression into a single instruction.
>
> * Example *
>
> Let's consider two dummy functions foo() and bar() as below:
>
> | #include <linux/bitops.h>
> | #define CONST 0x01000000
> |
> | unsigned int foo(void)
> | {
> |       return ffs(CONST);
> | }
> |
> | unsigned int bar(void)
> | {
> |       return __builtin_ffs(CONST);
> | }
>
> GCC would produce below assembly code:
>
> | 0000000000000000 <foo>:
> |    0: b8 ff ff ff ff          mov    $0xffffffff,%eax
> |    5: 0f bc c7                bsf    %edi,%eax
> |    8: 83 c0 01                add    $0x1,%eax
> |    b: c3                      ret
> |    c: 0f 1f 40 00             nopl   0x0(%rax)
> |
> | 0000000000000010 <bar>:
> |   10: b8 19 00 00 00          mov    $0x19,%eax
> |   15: c3                      ret
>
> And clang would produce:
>
> | 0000000000000000 <foo>:
> |    0: 55                      push   %rbp
> |    1: 48 89 e5                mov    %rsp,%rbp
> |    4: b8 ff ff ff ff          mov    $0xffffffff,%eax
> |    9: 0f bc 05 00 00 00 00    bsf    0x0(%rip),%eax        # 10 <foo+0x10>
> |   10: ff c0                   inc    %eax
> |   12: 5d                      pop    %rbp
> |   13: c3                      ret
> |   14: 66 2e 0f 1f 84 00 00    cs nopw 0x0(%rax,%rax,1)
> |   1b: 00 00 00
> |   1e: 66 90                   xchg   %ax,%ax
> |
> | 0000000000000020 <bar>:
> |   20: 55                      push   %rbp
> |   21: 48 89 e5                mov    %rsp,%rbp
> |   24: b8 19 00 00 00          mov    $0x19,%eax
> |   29: 5d                      pop    %rbp
> |   2a: c3                      ret

Right, we need to allocate registers to move the inputs into the asm
block, and the results back out. Inline asm is analogous to a call
with a custom calling convention, where we don't look into the body of
the inline asm.

Does -fomit-frame-pointer clean make these snippets clearer, or did
you not build with -O2?  Consider using those flags if so, since we
generally prefer the ORC unwinder on x86, not the frame pointer
unwinder.  If the compilers are forcing a frame pointer when using the
builtins once optimizations are enabled, that's a problem (that we've
seen in the past with the builtins for reading eflags with clang; now
fixed).

>
> For both examples, we clearly see the benefit of using __builtin_ffs()
> instead of the kernel's asm implementation for constant expressions.
>
> However, for non constant expressions, the ffs() asm version of the
> kernel remains better for x86_64 because, contrary to GCC, it doesn't
> emit the CMOV assembly instruction, c.f. [1] (noticeably, clang is
> able optimize out the CMOV call).
>
> This patch uses the __builtin_constant_p() to select between the
> kernel's ffs() and the __builtin_ffs() depending on whether the
> argument is constant or not.
>
>
> As a side benefit, this patch also removes below -Wshadow warning:
>
> | ./arch/x86/include/asm/bitops.h:283:28: warning: declaration of 'ffs' shadows a built-in function [-Wshadow]
> |   283 | static __always_inline int ffs(int x)

Nice! :)

>
> [1] commit ca3d30cc02f7 ("x86_64, asm: Optimise fls(), ffs() and fls64()")
> http://lkml.kernel.org/r/20111213145654.14362.39868.stgit@xxxxxxxxxxxxxxxxxxxxxx

+ David, author of ca3d30cc02f7.  I was wondering if this applied to
more than just x86, but I see now that some architectures just include
include/asm-generic/bitops/builtin-ffs.h into their
arch/*/include/asm/bitops.h. It's only when we want to beat the
compiler for non-ICE expressions.

Patch LGTM; just minor comments on commit message, naming, and formatting.

>
>
> Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/bitops.h | 29 +++++++++++++++++------------
>  1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
> index a288ecd230ab..535a7a358c14 100644
> --- a/arch/x86/include/asm/bitops.h
> +++ b/arch/x86/include/asm/bitops.h
> @@ -269,18 +269,7 @@ static __always_inline unsigned long __fls(unsigned long word)
>  #undef ADDR
>
>  #ifdef __KERNEL__
> -/**
> - * ffs - find first set bit in word
> - * @x: the word to search
> - *
> - * This is defined the same way as the libc and compiler builtin ffs
> - * routines, therefore differs in spirit from the other bitops.
> - *
> - * ffs(value) returns 0 if value is 0 or the position of the first
> - * set bit if value is nonzero. The first (least significant) bit
> - * is at position 1.
> - */
> -static __always_inline int ffs(int x)
> +static __always_inline int __ffs_asm(int x)

How about variable_ffs rather than __ffs_asm? Let's try to stick with
the convention used by test_bit?

>  {
>         int r;
>
> @@ -310,6 +299,22 @@ static __always_inline int ffs(int x)
>         return r + 1;
>  }
>
> +/**
> + * ffs - find first set bit in word
> + * @x: the word to search
> + *
> + * This is defined the same way as the libc and compiler builtin ffs
> + * routines, therefore differs in spirit from the other bitops.
> + *
> + * ffs(value) returns 0 if value is 0 or the position of the first
> + * set bit if value is nonzero. The first (least significant) bit
> + * is at position 1.
> + */
> +#define ffs(x)                                 \
> +        (__builtin_constant_p(x) ?             \
> +         __builtin_ffs(x) :                    \
> +         __ffs_asm(x))
> +

I think this whole #define can fit on one line? If not, perhaps the
BCP can start on the initial line?  Otherwise it looks like the
then/else clauses are indented by 1 tab followed by 1 space. Consider
just using tabs.

>  /**
>   * fls - find last set bit in word
>   * @x: the word to search
> --
> 2.35.1
>


--
Thanks,
~Nick Desaulniers