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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <762b62b5-0246-7045-e4cf-7559ab8c5bb2@c-s.fr>
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