Re: ffs() (was: [Linux-NTFS-Dev] SOLVED - Re: PROBLEM: compiling NTFS write support)

[Anton Altaparmakov <aia21@cam.ac.uk>]

On Wed, 2004-05-12 at 13:58, Szakacsits Szabolcs wrote:
> On Wed, 12 May 2004, Anton Altaparmakov wrote:
> > On Wed, 2004-05-12 at 12:56, andrea.fracasso@infoware-srl.com wrote:
> > > > On Wed, 2004-05-12 at 11:14, andrea.fracasso@infoware-srl.com wrote:
> > > >> Hi, I have found a problem compiling te source of kernel 2.6.6, if I
> > > >> enable NTFS write support when i run "make" i get this error:
> > > >>
> > > >> ....
> > > >>   CC      fs/ntfs/inode.o
> > > >>   CC      fs/ntfs/logfile.o
> > > >> {standard input}: Assembler messages:
> > > >> {standard input}:683: Error: suffix or operands invalid for `bsf'
> > > >> make[2]: *** [fs/ntfs/logfile.o] Error 1
> > > >> make[1]: *** [fs/ntfs] Error 2
> > > >> make: *** [fs] Error 2
> > > >>
> > > >> my kernel version is:
> > > >> Linux version 2.6.5-AS1500 (root@ntb-gozzolox) (gcc version 3.3.2
> > > >> 20031022
> > > >> (Red Hat Linux 3.3.2-1)) #3 Thu Apr 15 10:13:11 CEST 2004
> > > 
> > > The binutils ver is:
> > > binutils-2.14.90.0.6-4
> > 
> > This happens because gcc (wrongly!) optimizes a variable into a constant
> > and then ffs() fails to assemble because the bsfl instruction is only
> > allowed with memory operands and not constants.
> 
> IMHO this should be worked around (fixed) in the inlined __asm__ ffs. Does
> it happen only on Opteron or on other platforms as well?

Seems to be opteron only so far.  Obviously gcc on the opteron must be
'better' at optimizing...  I imagine the below would work:

--- bklinux-2.6/include/asm-x86_64/bitops.h.old	2004-05-12
14:22:16.370662976 +0100
+++ bklinux-2.6/include/asm-x86_64/bitops.h	2004-05-12
14:23:17.800324248 +0100
@@ -452,7 +452,7 @@ static inline int sched_find_first_bit(c
  * the libc and compiler builtin ffs routines, therefore
  * differs in spirit from the above ffz (man ffs).
  */
-static __inline__ int ffs(int x)
+static __inline__ int mem_ffs(int x)
 {
 	int r;
 
@@ -462,6 +462,8 @@ static __inline__ int ffs(int x)
 	return r+1;
 }
 
+#define ffs(x) 	(__builtin_constant_p(x) ? generic_ffs(x) : mem_ffs(x))
+
 /**
  * hweightN - returns the hamming weight of a N-bit word
  * @x: the word to weigh

But what is to say that the next gcc version in i386 won't have the same
problem?  We would really need to do this on all arch who use an
instruction which is not capable of working on constants.  asm-i386
would be a prime candidate.

Andrea, could you test the above change instead of the one to logfile.c
as well and let us know if it worked?

Best regards,

	Anton
-- 
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/