linux-kernel - Re: [PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin

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Message-ID: <CAMZ6Rq+Nm9wSajUFP7PyctB50t5ANpe9LhRmO3GJvmgkuXzEOA@mail.gmail.com>
Date:   Thu, 12 May 2022 13:29:47 +0900
From:   Vincent MAILHOL <mailhol.vincent@...adoo.fr>
To:     Joe Perches <joe@...ches.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Tom Rix <trix@...hat.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, David Howells <dhowells@...hat.com>,
        Jan Beulich <JBeulich@...e.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>
Subject: Re: [PATCH v4 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate
 constant expressions

On Thu. 12 May 2022 at 12:02, Joe Perches <joe@...ches.com> wrote:
> On Thu, 2022-05-12 at 10:18 +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.
> []
> > -static __always_inline int ffs(int x)
> > +static __always_inline int variable_ffs(int x)
> >  {
> >       int r;
> >
> > @@ -310,6 +299,19 @@ 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) : variable_ffs(x))
>
> How about not defining another function and using parentheses around
> the function definition to avoid the macro expansion like:
>
> #define ffs(x) (__builtin_constant_p(x) ? __builtin_ffs(x) : ffs(x))
>
> and
>
> static __always_inline int (ffs)(int x)
> {
>         etc...
> }

Sorry, but I don’t really like this approach.

Main issue I see is that this code will emit a -Wshadow warning.

And using parentheses around the function definition just seems an
obscure hack to me. The variable_foo() gives me less headache. Was
this pattern ever used anywhere else in the kernel?


Yours sincerely,
Vincent Mailhol