Re: powerpc: Discard ffs() function and use builtin_ffs instead

[Christophe Leroy]

Le 13/05/2016 Ã 08:53, Christophe Leroy a Ãcrit :
> Le 13/05/2016 Ã 08:16, Michael Ellerman a Ãcrit :
> > 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