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

[Uros Bizjak]

On Tue, 2010-06-22 at 11:27 -0700, H.J. Lu wrote:

> >> This patch adds 8bit divmov pattern for x86. X86 8bit divide
> >> instructions return result in AX with
> >>
> >> AL <- Quotient
> >> AH <- Remainder
> >>
> >> This patch models it and properly extends quotient. Tested
> >> on Intel64 with -m64 and -m32.  There are no regressions.
> >> OK for trunk?
> >>
> >> BTW, there is only one divb used in subreg_get_info in
> >> gcc compilers. The old code is
> >>
> >>         movzbl  mode_size(%r13), %edi
> >>         movzbl  mode_size(%r14), %esi
> >>         xorl    %edx, %edx
> >>         movl    %edi, %eax
> >>         divw    %si
> >>         testw   %dx, %dx
> >>         jne     .L1194
> >>
> >> The new one is
> >>
> >>         movzbl  mode_size(%r13), %edi
> >>         movl    %edi, %eax
> >>         divb    mode_size(%r14)
> >>         movzbl  %ah, %eax
> >>         testb   %al, %al
> >>         jne     .L1194
> >>
> >
> > Hm, something is not combined correctly, I'd say "testb %ah, %ah" is
> > optimal in the second case.
> >
> 
> Here is another update adjusted for mov pattern changes in i386.md.
> 
> 8bit result is stored in
> 
> AL <- Quotient
> AH <- Remainder
> 
> 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 ?

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.

Uros.