Re: Why does combine turn zero_extract into lshiftrf?

[Jeff Law <law at redhat dot com>]

On 06/23/10 16:44, H.J. Lu wrote:
> On Tue, Jun 22, 2010 at 11:05 AM, Uros Bizjak<ubizjak@gmail.com>  wrote:
>    
> > On Mon, 2010-06-21 at 12:33 -0700, H.J. Lu wrote:
> >      
> > > Hi,
> > >
> > > 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.
> >
> > Uros.
> >
> >
> >      
> For some reason, combine tries to turn
>
> (insn 11 10 13 2 umod-6.c:4 (set (reg:QI 67)
>          (subreg:QI (zero_extract:SI (reg:HI 69)
>                  (const_int 8 [0x8])
>                  (const_int 8 [0x8])) 0)) 91 {*movqi_extzv_2_rex64}
> (expr_list:REG_DEAD (reg:HI 69)
>          (nil)))
>
> (insn 13 11 14 2 umod-6.c:4 (set (reg:CCZ 17 flags)
>          (compare:CCZ (reg:QI 67)
>              (const_int 0 [0]))) 0 {*cmpqi_ccno_1} (expr_list:REG_DEAD
> (reg:QI 67)
>          (nil)))
>
> into
>
> (set (reg:CCZ 17 flags)
>      (compare:CCZ (subreg:QI (lshiftrt:SI (subreg:SI (reg:HI 69) 0)
>                  (const_int 8 [0x8])) 0)
>          (const_int 0 [0])))
>    
I believe that's standard form within combine.c

jeff