Why does combine turn zero_extract into lshiftrf?

[Jeff Law]

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