Re: Register constraints + and =

[Ian Lance Taylor <iant at google dot com>]

"Paulo J. Matos" <paulo@matos-sorge.com> writes:

> Expand generates:
>
> (define_insn_and_split "movmem_long"
>   [(set (match_operand:QI 2 "register_operand" "d,c") (const_int 0))
>    (set (mem:BLK (match_operand:QI 0 "register_operand" "d,c"))
>         (mem:BLK (match_operand:QI 1 "register_operand" "x,c")))
>    (set (match_dup 0) (plus:QI (match_dup 0) (match_dup 2)))
>    (set (match_dup 1) (plus:QI (match_dup 1) (match_dup 2)))
>    (clobber (match_scratch:QI 3 "w,w"))]
>   "!TARGET_NO_BLOCK_COPY"
>   "#"
>   "&& reload_completed"
>   [(const_int 0)]
> {
>   if((which_alternative == 0 && REGNO(operands[2]) == RAH))
>      || which_alternative == 1)
>   {
>     emit_move_insn(operands[3], operands[0]);
>     emit_move_insn(operands[0], operands[2]);
>     emit_move_insn(operands[2], operands[3]);
>   }
>   emit_insn(gen_bc2());
>   DONE;
> })
>
> From what I understand + is for input/output operands, = for output
> only operands. Since in the above rule (a parallel) all operands are
> written to and read to, does this mean all their constraints should
> start with +? Or this only applies per set within each parallel (which
> doesn't seem to make much sense)?

I agree that there is something wrong here.  I agree that as written
the constraints for operands 0, 1, and 2 should have a '+'.

That said, a '+' constraint is most useful for a pattern that expands
into multiple instructions.  I think this would be better written along
the lines of

  (set (match_operand:QI 2 "register_operand" "=d,c") (const_int 0))
  (set (mem:BLK (match_operand:QI 3 "register_operand" "0")
                (match_operand:QI 4 "register_operand" "1")))
  (set (match_operand:QI 0 "register_operand" "=d,c")
       (plus:QI (match_dup 3)
                (match_operand:QI 5 "register_operand" "2"))))
  (set (match_operand:QI 1 "register_operand" "=x,c")
       (plus:QI (match_dup 4) (match_dup 5)))
  (clobber (match_scratch:QI 3 "=w,w"))

Also it looks like it might be possible to add a third alternative such
that that alternative does not require the match_scratch.

Ian