linux-kernel - Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin

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Date:   Wed, 11 May 2022 22:56:15 +0200
From:   Christophe JAILLET <christophe.jaillet@...adoo.fr>
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>
Subject: Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate
 constant expressions

Le 11/05/2022 à 18:03, Vincent Mailhol a écrit :
> 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.
> 

[...]

> 
> ** Statistics **
> 
> On a allyesconfig, before applying this patch...:
> 
> | $ objdump -d vmlinux.o | grep bsf | wc -l
> | 3607
> 
> ...and after:
> 
> | $ objdump -d vmlinux.o | grep bsf | wc -l
> | 792
> 
> So, roughly 26.7% of the call to ffs() were using constant expression
> and were optimized out.
> 
> 
nitpicking: numbers look odd.
    3607 is the exact same number as in patch 2/2. (ok, could be)
    26.7% is surprising with these numbers. (I guess it is (total_before 
- remaining) / total_before x 100 = (3607-792)/36.07 = 78.0%)

(but patch looks great to me :)

CJ