gcc vax bug in built-in ffs function/displacement indexed addressing

[Felix Deichmann <m4j0rd0m0%gmail.com@localhost>]

Hi,

I fought against SCSI problems on my VS4000/96 and /60 with NetBSD-current:
    http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=51761

It was related to tagged queueing somehow, but finally turned out to be
a gcc bug in the built-in ffs function (used in scsipi_get_tag()) and/or
"byte displacement indexed" addressing mode.

Above PR says it all, but again:

	bit = ffs(periph->periph_freetags[word]);

is compiled as

     15b0:	ea 00 20 48 	ffs $0x0,$0x20,0x50(r10)[r8],r9
     15b4:	aa 50 59
     [...]
     15c0:	d6 58       	incl r8

which is obviously wrong, as r8 seems to be a byte index, but variable
"word" is an index into u_int32_t periph_freetags[].

The complete disassembly of an affected scsipi_base.o is here:
    http://www.netbsd.org/~flxd/scsipi_base_vax_asm.txt

Swapping built-in ffs with a generic implementation successfully worked
around the problem.

It would be great if someone more familiar with gcc vax could have a
look and come up with a fix. Thanks!

Cheers,
Felix