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

From: H. Peter Anvin
Date: Wed May 12 2004 - 15:34:30 EST

Followup to: <1084369416.16624.53.camel@xxxxxxxxxxxxxxxxx>
By author: Anton Altaparmakov <aia21@xxxxxxxxx>
In newsgroup:
> 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 ... */
} else {
__asm__("bsfl %1,%0\n\t"
"cmovzl %2,%0"
: "=r" (r) : "rm" (x), "r" (-1));
return r+1;

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