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SubjectRe: [2.6.6-BK] x86_64 has buggy ffs() implementation
On Wed, May 12, 2004 at 08:31:56PM +0000, H. Peter Anvin wrote:
> Followup to: <1084369416.16624.53.camel@imp.csi.cam.ac.uk>
> By author: Anton Altaparmakov <aia21@cam.ac.uk>
> In newsgroup: linux.dev.kernel
> >
> > Hi Andi, Andrew, Linus,
> >
> > x86_64 has incorrect include/asm-x86_64/bitops.h::ffs() implementation.
> > It uses "g" instead of "rm" in the insline assembled bsfl instruction.
> > (This was spotted by Yuri Per.)
> >
> > bsfl does not accept constant values but only memory ones. On i386 the
> > correct "rm" is used.
> >
> > This causes NTFS build to fail as gcc optimizes a variable into a
> > constant and ffs() then fails to assemble.
> >
>
> Of course, this is a good reason to do a __builtin_constant_p()
> wrapper that gcc can optimize:
>
> static __inline__ __attribute_const__ int ffs(int x)
> {
> if ( __builtin_constant_p(x) ) {
> unsigned int y = (unsigned int)x;
> if ( y >= 0x80000000 )
> return 32;
> else if ( y >= 0x40000000 )
> return 31;
> else if /* ... you get the idea ... */

Either I'm asleep or you are emulating bsrl, not bsfl. It
should rather be:

if ( y & 0x00000001) return 1;
if ( y & 0x00000002) return 2;
if ( y & 0x00000004) return 3;
...
if ( y & 0x80000000) return 32;
return 0;

No need for the else clauses either because of the return.
But maybe even __builtin_ffs(y) would work in this case.

Gabriel
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