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

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Date:	Fri, 13 May 2016 10:29:35 +0200
From:	Gabriel Paubert <paubert@...m.es>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Christophe Leroy <christophe.leroy@....fr>,
	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

On Fri, May 13, 2016 at 04:16:57PM +1000, Michael Ellerman wrote:
> 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?
> 
> And which gcc version first added the builtin version?

It already existed in gcc-2.95, which you do not want to use to compile
anything today but I have in a corner of a chroot environment to maintain
~1997 vintage embedded stuff, running a 2.2.12 kernel!

Hopefully this clears up your concerns :-)

    Cheers,
    Gabriel