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Re: Paradoxical subreg reload issue


Le 02/05/2012 16:41, Ian Lance Taylor a écrit :
> Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com> writes:
> 
>> I have an issue (gcc 4.6.3, private bacakend) when reloading operands of
>> this insn:
>> (set (subreg:SI (reg:QI 21 [ iftmp.1 ]) 0)
>>      (lshiftrt:SI (reg/v:SI 24 [ w ]) (const_int 31 [0x1f]))
>>
>> The register 21 is reloaded into
>> (reg:QI 0 r0 [orig:21 iftmp.1 ] [21]), which is a HI-wide hw register.
>> Since it is a BIG_ENDIAN target, the SI subreg regno is then -1.
>>
>> Note that word_mode is SImode, whereas the class r0 belongs to is
>> HI-wide. I don't know if this matters when reloading.
>>
>> I have no idea how to debug this, if it is a backend or a reload bug.
>> Any idea?
> 
> Where did that insn come from?  It looks like it really wants to be
> 
> (set (reg:QI 21)
>      (truncate:QI (lshiftrt:SI (reg:SI 24) (const_int 31))))
> 

It comes from the combine pass, which merged the following insns:

(insn 20 19 21 5
(set (reg:SI 27)
  (lshiftrt:SI (reg/v:SI 24 [w])
            (const_int 31 [0x1f]))) {*lshrsi3_split}
     (nil))

(insn 21 20 22 5
(set (reg:QI 21 [ iftmp.1 ])
     (subreg:QI (reg:SI 27) 3)) {movqi}
     (expr_list:REG_DEAD (reg:SI 27)

------
Here is the combiner output:

Trying 20 -> 21:
Successfully matched this instruction:
(set (subreg:SI (reg:QI 21 [ iftmp.1 ]) 0)
    (lshiftrt:SI (reg/v:SI 24 [ w ])
        (const_int 31 [0x1f])))
deferring deletion of insn with uid = 20.
modifying insn i3    21 r21:QI#0=r24:SI 0>>0x1f
deferring rescan insn with uid = 21.


Thanks,
Aurélien


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