Messages in this thread | | | Date | Thu, 11 Aug 2022 16:59:33 +0200 | From | Borislav Petkov <> | Subject | Re: [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions |
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On Sun, Jul 24, 2022 at 12:15:20AM +0900, Vincent Mailhol 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
Those code examples you can simply indent with two spaces.
> In both examples, we clearly see the benefit of using __builtin_ffs()
Who's "we"?
Please use passive voice in your commit message: no "we" or "I", etc, and describe your changes in imperative mood.
> 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
Avoid having "This patch" or "This commit" in the commit message. It is tautologically useless.
Also, do
$ git grep 'This patch' Documentation/process
for more details.
> kernel's ffs() and the __builtin_ffs() depending on whether the > argument is constant or not.
In general, you don't have to say what the patch does - that should be visible from the diff. The more important part is the *why*. And that you do.
Rest looks ok.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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