Re: PATCH: PR target/44588: Very inefficient 8bit mod/div

["H.J. Lu" <hjl dot tools at gmail dot com>]

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.


-- 
H.J.