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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <154f41707c58acdac26c3300c5b429f381c45708.camel@perches.com>
Date:   Wed, 11 May 2022 20:02:28 -0700
From:   Joe Perches <joe@...ches.com>
To:     Vincent Mailhol <mailhol.vincent@...adoo.fr>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     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, 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...
}