Messages in this thread | | | From | Vincent MAILHOL <> | Date | Fri, 12 Aug 2022 20:55:11 +0900 | Subject | Re: [RESEND PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate constant expressions |
| |
Hi Borislav,
On Thu. 11 Aug 2022 at 23:59, Borislav Petkov <bp@alien8.de> wrote: > 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.
Thank you for the review! I addressed all your comments and sent a v5: https://lore.kernel.org/all/20220812114438.1574-1-mailhol.vincent@wanadoo.fr/
Yours sincerely, Vincent Mailhol
| |