Re: Questions about match_scratch in machine description files

[Richard Sandiford <rsandifo at redhat dot com>]

Fred Fish <fnf at public dot ninemoons dot com> writes:
> Another thing I don't understand, again using mips.md as an example,
> is that some templates use match_scratch but don't appear to use any
> of the temporary operands.  For example:
> 
>   (define_insn "mulsi3_mult3"
>     [(set (match_operand:SI 0 "register_operand" "=d,l")
>           (mult:SI (match_operand:SI 1 "register_operand" "d,d")
>                    (match_operand:SI 2 "register_operand" "d,d")))
>      (clobber (match_scratch:SI 3 "=h,h"))
>      (clobber (match_scratch:SI 4 "=l,X"))
>      (clobber (match_scratch:SI 5 "=a,a"))]
>     "GENERATE_MULT3_SI
>      || TARGET_MAD"
>     "*
>   {
>     if (which_alternative == 1)
>       return \"mult\\t%1,%2\";
>     if (TARGET_MAD
>         || TARGET_MIPS5400
>         || TARGET_MIPS5500
>         || ISA_MIPS32
>         || ISA_MIPS32R2
>         || ISA_MIPS64)
>       return \"mul\\t%0,%1,%2\";
>     return \"mult\\t%0,%1,%2\";
>   }"
>     [(set_attr "type"     "imul")
>      (set_attr "mode"     "SI")])
> 
> In the above, I don't see any use of %3, %4, or %5, so why are those
> match_scratch expressions there?

The idea is that 'mult r1,r2' stores the useful result in lo
('l' constraint) but also changes the value of hi ('h' constraint)
and the very dubious "hilo" register ('a').

Same thing for the three-operand mult expect that the useful
result is stored in a GPR and all of lo, hi and "hilo" are
clobbered.

Richard