Re: Unable to generate reloads

[Lars Brinkhoff <lars dot spam at nocrew dot org>]

Lars Brinkhoff <lars@nocrew.org> writes:
> In my back end, there is an instruction that can load a value from
> memory (or another register) and then conditionally jump depending on
> how the value compares with zero:
> 
> (define_insn "*move_and_skipsi"
>   [(set (pc)
> 	(if_then_else (match_operator 0 "comparison_operator"
> 		       [(match_operand:SI 2 "reg_or_mem_operand" "rm")
> 		        (const_int 0)])
> 	 (label_ref (match_operand 3 "" ""))
> 	 (pc)))
>    (set (match_operand:SI 1 "register_operand" "=r")
> 	(match_dup 2))]
>   ...)
> 
> A few files in the testsuite generate the "Unable to generate reloads
> for:" error for this instruction.  Example:
> 
> testsuite/gcc.c-torture/compile/920625-1.c:279: Unable to generate reloads for:
> (jump_insn 45 4975 5103 (parallel[ 
>             (set (pc)
>                 (if_then_else (ge (reg:SI 1217)
>                         (const_int 0 [0x0]))
>                     (label_ref 48)
>                     (pc)))
>             (set (reg/v:SI 30)
>                 (reg:SI 1217))
>         ] ) 68 {*move_and_skipsi} (nil)
>     (expr_list:REG_DEAD (reg:SI 1217)
>         (expr_list:REG_BR_PROB (const_int 8900 [0x22c4])
>             (nil))))

It seems that the explanation to this is in reload.c:find_reloads:

  /* JUMP_INSNs and CALL_INSNs are not allowed to have any output reloads;
     neither are insns that SET cc0.  Insns that use CC0 are not allowed
     to have any input reloads.  */
  if (GET_CODE (insn) == JUMP_INSN || GET_CODE (insn) == CALL_INSN)
    no_output_reloads = 1;

If the register being loaded (pseudo 30 above) didn't get a hard
register, the instruction fails later in find_reloads:

	      /* Alternative loses if it requires a type of reload not
		 permitted for this insn.  We can always reload SCRATCH
		 and objects with a REG_UNUSED note.  */
	      else if (GET_CODE (operand) != SCRATCH
		       && modified[i] != RELOAD_READ && no_output_reloads
		       && ! find_reg_note (insn, REG_UNUSED, operand))
		bad = 1;

Does this mean that I must add an alternative to operand 1 to store
into a memory location?