Re: [2.6.6-BK] x86_64 has buggy ffs() implementation

[Anton Altaparmakov]

On Wed, 12 May 2004, H. Peter Anvin wrote:
> Followup to:  <1084369416.16624.53.camel@xxxxxxxxxxxxxxxxx>
> By author:    Anton Altaparmakov <aia21@xxxxxxxxx>
> 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 ... */

If you are going to play with that why not just use generic_ffs() instead
of doing it by hand?

Best regards,

	Anton

> 	} else {
> 		__asm__("bsfl %1,%0\n\t"
> 			"cmovzl %2,%0"
> 			: "=r" (r) : "rm" (x), "r" (-1));
> 		return r+1;
> 	}
> }

-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Unix Support, Computing Service, University of Cambridge, CB2 3QH, UK
Linux NTFS maintainer / IRC: #ntfs on irc.freenode.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/
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