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
