Thanks for your reply. But I am afraid your pointer does not make it
clearer to me. The excerpt of "define_insn "<u>mulsidi3_64bit_hilo" "
I specified is from an older version of GCC which I need to stick to,
so I need to understand why and when to use "unspec" as I am extending
a GCC backend based on the MIPS one.
On Wed, Sep 3, 2014 at 3:03 AM, Andrew Haley <aph@xxxxxxxxxx> wrote:
> On 03/09/14 02:31, Cherry Vanc wrote:
>> I cannot seem to pin down the scope of the keyword "unspec" in the
>> machine description files. From the MIPS backend files, I see the word
>> "unspec" being used in two ways :
>>
>> 1. names of constants (UNSPEC_LOAD_LOW, UNSPEC_LOAD_HIGH, .. so on) :
>> I understand that these are just names, but Why UNSPEC_ preceding each
>> name ?
>> 2. some define_insn patterns have the "unspec" rtl expression. What
>> does "unspec" precisely mean here - unspecified operation ? How does
>> that work in the RTL generation pass ? E.g. :
>>
>> (define_insn "<u>mulsidi3_64bit_hilo"
>> [(set (match_operand:TI 0 "register_operand" "=x")
>> (unspec:TI
>> [(mult:DI
>> (any_extend:DI (match_operand:SI 1 "register_operand" "d"))
>> (any_extend:DI (match_operand:SI 2 "register_operand" "d")))]
>> UNSPEC_SET_HILO))]
>> "TARGET_64BIT && !TARGET_FIX_R4000"
>> "mult<u>\t%1,%2"
>> [(set_attr "type" "imul")
>> (set_attr "mode" "SI")])
>
> Look at
>
> (define_expand "<u>mulsidi3_64bit_split"
> [(set (match_operand:DI 0 "register_operand")
> (mult:DI (any_extend:DI (match_operand:SI 1 "register_operand"))
> (any_extend:DI (match_operand:SI 2 "register_operand"))))
> (clobber (match_operand:DI 3 "register_operand"))]
> ""
> {
> rtx hilo;
>
> hilo = gen_rtx_REG (TImode, MD_REG_FIRST);
> emit_insn (gen_<u>mulsidi3_64bit_hilo (hilo, operands[1], operands[2]));
>
> ...
>
> Andrew.
>
