This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: PATCH: PR target/44588: Very inefficient 8bit mod/div
On Tue, Jun 22, 2010 at 11:44 AM, Uros Bizjak <ubizjak@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.
> 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.
--
H.J.