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Re: [PATCH]: x86 gas: allow 'rep' prefix on 'bsf' and 'bsr' instructions
On Fri, Jul 6, 2012 at 6:47 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 06.07.12 at 15:06, "H.J. Lu" <email@example.com> wrote:
>> On Fri, Jul 6, 2012 at 2:52 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>> On 21.06.12 at 23:36, Roland McGrath <firstname.lastname@example.org> wrote:
>>>> 'rep; bsf ...'/'rep; bsr ...' are encoded the same as 'tzcnt ...'/'lzcnt
>>> While tzcnt really is an extension of bsf, lzcnt is not one of bsr,
>>> so I don't really follow why it's useful to allow the prefix there.
>> It can be used to encode lzcnt with the older assembler,
>> similar to pause.
> You didn't get my point: rep bsr, for non-zero input, is _not_
> equivalent to bsr (this equivalence exists only for bsf/tzcnt,
> no matter what the Intel SDM says). Hence allowing this
> combination is potentially misleading, and not useful to gcc
> (which was the supposed original goal of the change).
lzcnt is executed as bsr on the older processors.