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

[Christophe Leroy]

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