linux-kernel - Re: powerpc: Discard ffs() function and use builtin

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Date:   Fri, 16 Sep 2016 12:01:39 +0200
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: powerpc: Discard ffs() function and use builtin_ffs instead



Le 13/05/2016 à 08:53, Christophe Leroy a écrit :
>
>
> Le 13/05/2016 à 08:16, Michael Ellerman a écrit :
>> On Thu, 2016-12-05 at 15:32:22 UTC, Christophe Leroy wrote:
>>> With the ffs() function as defined in arch/powerpc/include/asm/bitops.h
>>> GCC will not optimise the code in case of constant parameter, as shown
>>> by the small exemple below.
>>>
>>> int ffs_test(void)
>>> {
>>>     return 4 << ffs(31);
>>> }
>>>
>>> c0012334 <ffs_test>:
>>> c0012334:       39 20 00 01     li      r9,1
>>> c0012338:       38 60 00 04     li      r3,4
>>> c001233c:       7d 29 00 34     cntlzw  r9,r9
>>> c0012340:       21 29 00 20     subfic  r9,r9,32
>>> c0012344:       7c 63 48 30     slw     r3,r3,r9
>>> c0012348:       4e 80 00 20     blr
>>>
>>> With this patch, the same function will compile as follows:
>>>
>>> c0012334 <ffs_test>:
>>> c0012334:       38 60 00 08     li      r3,8
>>> c0012338:       4e 80 00 20     blr
>>
>> But what code does it generate when it's not a constant?
>
> The generated code is the same with and without the patch when not a 
> constant:
>
> int ffs_test2(int x)
> {
>     return ffs(x);
> }
>
> c001233c <ffs_test2>:
> c001233c:       7d 23 00 d0     neg     r9,r3
> c0012340:       7d 23 18 38     and     r3,r9,r3
> c0012344:       7c 63 00 34     cntlzw  r3,r3
> c0012348:       20 63 00 20     subfic  r3,r3,32
> c001234c:       4e 80 00 20     blr
>
>>
>> And which gcc version first added the builtin version?
> Don't know, but __builtin_ffs() is already used in 
> arch/powerpc/include/asm/page_32.h
>

Hi Michael,

Any change to get it into 4.9 ?

Christophe