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

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Date:	Fri, 13 May 2016 08:53:55 +0200
From:	Christophe Leroy <christophe.leroy@....fr>
To:	Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Scott Wood <oss@...error.net>
Cc:	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 08:16, 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

Christophe