How to split mulsi3 pattern

[Mohamed Shafi]

2009/11/10 Richard Henderson <rth@redhat.com>:
> On 11/10/2009 05:48 AM, Mohamed Shafi wrote:
>>
>> (define_insn "mulsi3"
>>  [(set (match_operand:SI 0 "register_operand"           "=&d")
>>       (mult:SI (match_operand:SI 1 "register_operand"  "%d")
>>               (match_operand:SI 2 "register_operand" "d")))]
>
> Note that "%" is only useful if the constraints for the two operands are
> different (e.g. only one operand accepts an immediate input).  When they're
> identical, you simply waste cpu cycles asking reload to try the operands in
> the other order.
>
>>  [(set (match_dup 0)
>>        (ashift:SI
>>         (plus:SI (mult:HI (unspec:HI [(match_dup 2)] UNSPEC_REG_LOW)
>>                           (unspec:HI [(match_dup 1)] UNSPEC_REG_HIGH))
>>                  (mult:HI (unspec:HI [(match_dup 2)] UNSPEC_REG_HIGH)
>>                           (unspec:HI [(match_dup 1)] UNSPEC_REG_LOW)))
>>         (const_int 16)))
>>   (set (match_dup 0)
>>        (plus:SI (match_dup 0)
>>                 (mult:HI (unspec:HI [(match_dup 2)] UNSPEC_REG_LOW)
>>                          (unspec:HI [(match_dup 1)] UNSPEC_REG_LOW))))]
>
> Well for one, your modes don't match.  You actually want your unspecs and
> MULTs to be SImode.
>
> You could probably usefully model the second insn as
>
> (define_insn "mulsi3_part2"
>  [(set (match_operand:SI 0 "register_operand" "=d")
>        (plus:SI
>          (mult:SI (zero_extend:SI
>                     (match_operand:HI 1 "register_operand" "d"))
>                   (zero_extend:SI
>                     (match_operand:HI 2 "register_operand" "d")))
>          (match_operand:SI 3 "register_operand" "0")))]
>  ""
>  ...)

So i need to change the mode of the register from SI to HI after
reloading. Is that allowed?

Regards,
Shafi