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Message-ID: <324f245e-15e2-a1f2-bc8a-46255a1a859f@wanadoo.fr>
Date:   Wed, 11 May 2022 22:56:15 +0200
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Subject: Re: [PATCH v2 1/2] x86/asm/bitops: ffs: use __builtin_ffs to evaluate
 constant expressions
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To:     Vincent Mailhol <mailhol.vincent@wanadoo.fr>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Tom Rix <trix@redhat.com>, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev, David Howells <dhowells@redhat.com>,
        Jan Beulich <JBeulich@suse.com>
References: <20220511160319.1045812-1-mailhol.vincent@wanadoo.fr>
 <20220511160319.1045812-2-mailhol.vincent@wanadoo.fr>
From:   Christophe JAILLET <christophe.jaillet@wanadoo.fr>
In-Reply-To: <20220511160319.1045812-2-mailhol.vincent@wanadoo.fr>
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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