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Re: [PATCH]: x86 gas: allow 'rep' prefix on 'bsf' and 'bsr' instructions
>>> On 06.07.12 at 16:09, "H.J. Lu" <hjl.tools@gmail.com> wrote:
> 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" <hjl.tools@gmail.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 <mcgrathr@google.com> 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.
That is exactly my point: It will, even for non-zero inputs,
produce different results depending on whether it is being
run on an old or a new CPU. Hence it's not suitable for
trickery like can be played with the bsf/tzcnt pair.
And just to be clear - the fact that this being possible was
implicitly (but wrongly) suggested by the original description
of the patch here made me almost fall into that trap and try
to use tzcnt as (presumably better performing) replacement
for bsr somewhere else.
Jan