linux-kernel - Re: [PATCH v3] lib: optimize cpumask_next

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Date:   Thu, 26 Oct 2017 14:58:00 +0200
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     courbet@...gle.com
Cc:     arnd@...db.de, linux@...musvillemoes.dk, akpm@...ux-foundation.org,
        mawilcox@...rosoft.com, ynorov@...iumnetworks.com,
        mingo@...nel.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] lib: optimize cpumask_next_and()

>  - Refactored _find_next_common_bit into _find_next_bit., as suggested
>    by Yury Norov. This has no adverse effects on the performance side,
>    as the compiler successfully inlines the code.

1)
Gentoo ships 5.4.0 which doesn't inline this code on x86_64 defconfig
(which has OPTIMIZE_INLINING).


ffffffff813556c0 <find_next_bit>:
ffffffff813556c0:       55                      push   rbp
ffffffff813556c1:       48 89 d1                mov    rcx,rdx
ffffffff813556c4:       45 31 c0                xor    r8d,r8d
ffffffff813556c7:       48 89 f2                mov    rdx,rsi
ffffffff813556ca:       31 f6                   xor    esi,esi
ffffffff813556cc:       48 89 e5                mov    rbp,rsp
ffffffff813556cf:       e8 7c ff ff ff          call
ffffffff81355650 <_find_next_bit>
ffffffff813556d4:       5d                      pop    rbp
ffffffff813556d5:       c3                      ret

2)
Making "and" operation to be centerpiece of this code is kind of meh
find_next_or_bit() will be hard to implement.