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Re: PATCH: PR target/44588: Very inefficient 8bit mod/div
On Wed, Jun 23, 2010 at 10:34 AM, Paolo Bonzini <bonzini@gnu.org> wrote:
> On 06/23/2010 04:26 PM, H.J. Lu wrote:
>>
>> On Tue, Jun 22, 2010 at 11:11 PM, Uros Bizjak<ubizjak@gmail.com> ?wrote:
>>>
>>> On Tue, Jun 22, 2010 at 8:57 PM, H.J. Lu<hjl.tools@gmail.com> ?wrote:
>>>
>>>>>> If we use AX for quotient in 8bit divmod pattern, we have to make
>>>>>> sure that AX is valid for quotient. ?We have to extend AL with UNSPEC
>>>>>> since AH isn't the part of quotient,. ?Instead, I use AL for quotient
>>>>>> and
>>>>>> use UNSPEC_MOVQI_EXTZH to extract remainder from AL. Quotient
>>>>>> access can be optimized very nicely. If remainder is used, we may have
>>>>>> an extract move for UNSPEC_MOVQI_EXTZH. I think this is a reasonable
>>>>>> comprise.
>>>>>
>>>>> Why we need to reinvent movqi_extzv_2 ?
>>>>
>>>> Because UNSPEC_MOVQI_EXTZH extracts AH from AL, not from
>>>> AX, EAX, RAX.
>>>
>>> This is wrong, you can't extract AH out from AL...
>>
>> UNSPEC_MOVQI_EXTZH ?is only used on AL from
>> 8bit div where AH has remainder. Should I give it a different
>> name, like UNSPEC_MOVQI_EXTZH_FROM_8BITDIV?
>>
>>>>
>>>>> I guess that<u>divqi3 has to be implemented as multiple-set divmod
>>>>> pattern using strict_low_part subregs to exactly describe in which
>>>>> subreg quotient and remainder go.
>>>>>
>>>>
>>>> I tried it and couldn't get it to work without adding a new
>>>> constraint for the upper 8bit registers.
>>>
>>> "Q" with %h modifier ?
>>>
>>
>> Although there are AX, AL, AH, ......, the rest of compiler only
>> knows one hard register, AX_REG, which is register 0. We can't
>> really access upper 8bit register, ?like AH, ?as a real register. We
>> can only access them via sign_extract and zero_extract. ?As far
>> as the rest of gcc is concerned, there are no upper 8bit registers.
>> I don't think we can describe where 8bit div remainder is to the
>> rest of compiler.
>
> Well, sure you can:
>
> (set (reg:HI r)
> ? (ior:HI (lshift:HI
> ? ? ? ? ? ?(zero_extend:HI (umod:QI ...) (const_int 8)))
> ? ? ? ? ? (zero_extend:HI (udiv:QI ...) (const_int 8))))
>
> (set (reg:QI s) ;; remainder
> ? (zero_extract (reg:HI r) (const_int 8) (const_int 8)))
>
> (set (reg:QI t) ;; quotient
> ? (subreg (reg:HI r) 0))
>
> The question is only whether fwprop, combine and regalloc are smart enough
> to reason about it.
>
That is what I meant by "We can only access them via sign_extract
and zero_extract." The rest of gcc will see the same hard register
number for both lower and upper 8bit registers. They aren't prepared
to deal with it.
--
H.J.