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Re: [PATCH] don't generate long nops on i686 (only when -march=[intel])


On Fri, Aug 6, 2010 at 2:50 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Aug 6, 2010 at 12:46 PM, Quentin Neill
> <quentin.neill.gnu@gmail.com> wrote:
>> On Fri, Aug 6, 2010 at 12:58 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>> On Fri, Aug 6, 2010 at 9:59 AM, Quentin Neill
>>> <quentin.neill.gnu@gmail.com> wrote:
>>>> On Fri, Aug 6, 2010 at 11:35 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>>
>>>>> I was wondering if it is necessary and possible to add the negative option
>>>>> to all ISAs?
>>>>
>>>> It is feasible, I looked into this.
>>>>
>>>> One problem with the current setup, there is no way to disambiguate
>>>> between ".nop" and ".nonop" and ".nononop" (etc) because we're using a
>>>> pair of entries in the init table, and then detecting the "no" prefix.
>>>>
>>> [snip]
>>>
>>> One problem is .nosse turns off ALL SSE/SSE2/..., not just
>>> SSE. So what should ".arch .nosse3" disable? Should it also
>>> disable SSSE3, SSE4, AVX, ....
>>
>> Okay, now I see that ".nosse" is one of several outliers:
>>
>> .no87 means &= ?~(8087|287|387|687|FISTTP)
>> .nosse means &= ~(sse|sse2|sse2|ssse3|sse4_1|sse4_2|sse4a|avx)
>> .noavx means &= ~(avx)
>>
>> .87 [is not defined]
>> .sse means |= (mmx|sse)
>> .avx means |= (mmx|sse|sse2|sse3|ssse3|sse4_1|sse4_2|avx)
>>
>> If we normalize all entries so that ".noXXX" would means exactly the
>> opposite of ".XXX" in all cases (answering your question about
>> .nosse3), we'd have to clean these up and/or change their meanings..
>>
>> If not, then I guess we have to go and define what all the ".noXXX"
>> flags mean, which in my opinion means just adding them to the existing
>> infrastructure.
>>
>> Note I'm not really answering your question "if it is necessary". ?Do
>> we have an idea how many people are using these directives?
>>
>
> I have no idea. I don't think we need to do anything for now. We will
> see if anyone asks for them. ?Then we can get some ideas on exactly
> what should be done.
>
> Thanks.

I agree.  I always prefer developing features based on hard feedback
over anticipation, no matter how clear my "crystal ball" seems to be
working.
-- 
Quentin


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