Re: [PATCH V2] internal-fn: Support undefined rtx for uninitialized SSA_NAME

[Richard Sandiford <richard.sandiford@arm.com>]

Juzhe-Zhong <juzhe.zhong@rivai.ai> writes:
> According to PR: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110751
>
> As Richard and Richi suggested, we recognize uninitialized SSA_NAME and convert it
> into SCRATCH rtx if the target predicate allows SCRATCH.
>
> It can help to reduce redundant data move instructions of targets like RISC-V.
>
> gcc/ChangeLog:
>
> 	* internal-fn.cc (expand_fn_using_insn): Support undefined rtx.
> 	* optabs.cc (maybe_legitimize_operand): Ditto.
> 	(can_reuse_operands_p): Ditto.
> 	* optabs.h (enum expand_operand_type): Ditto.
> 	(create_undefined_input_operand): Ditto.
>
> ---
>  gcc/internal-fn.cc |  4 ++++
>  gcc/optabs.cc      | 16 ++++++++++++++++
>  gcc/optabs.h       | 14 +++++++++++++-
>  3 files changed, 33 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/internal-fn.cc b/gcc/internal-fn.cc
> index 0fd34359247..61d5a9e4772 100644
> --- a/gcc/internal-fn.cc
> +++ b/gcc/internal-fn.cc
> @@ -247,6 +247,10 @@ expand_fn_using_insn (gcall *stmt, insn_code icode, unsigned int noutputs,
>  	create_convert_operand_from (&ops[opno], rhs_rtx,
>  				     TYPE_MODE (rhs_type),
>  				     TYPE_UNSIGNED (rhs_type));
> +      else if (TREE_CODE (rhs) == SSA_NAME
> +	       && SSA_NAME_IS_DEFAULT_DEF (rhs)
> +	       && VAR_P (SSA_NAME_VAR (rhs)))
> +	create_undefined_input_operand (&ops[opno], TYPE_MODE (rhs_type));
>        else
>  	create_input_operand (&ops[opno], rhs_rtx, TYPE_MODE (rhs_type));
>        opno += 1;
> diff --git a/gcc/optabs.cc b/gcc/optabs.cc
> index 32ff379ffc3..d8c771547a3 100644
> --- a/gcc/optabs.cc
> +++ b/gcc/optabs.cc
> @@ -8102,6 +8102,21 @@ maybe_legitimize_operand (enum insn_code icode, unsigned int opno,
>  	  goto input;
>  	}
>        break;
> +
> +    case EXPAND_UNDEFINED:
> +      {
> +	mode = insn_data[(int) icode].operand[opno].mode;
> +	rtx scratch = gen_rtx_SCRATCH (mode);

A scratch of the right mode should already be available in op->value,
since it was created by create_undefined_input_operand.

If that doesn't work for some reason, then it would be better for
create_undefined_input_operand to pass NULL_RTX as the "value"
argument to create_expand_operand.

> +	/* For SCRATCH rtx which is converted from uninitialized
> +	   SSA, we convert it as fresh pseudo when target doesn't
> +	   allow scratch rtx in predicate. Otherwise, return true.  */
> +	if (!insn_operand_matches (icode, opno, scratch))
> +	  {
> +	    op->value = gen_reg_rtx (mode);

The mode should come from op->mode.

> +	    goto input;
> +	  }
> +	return true;
> +      }
>      }
>    return insn_operand_matches (icode, opno, op->value);
>  }
> @@ -8147,6 +8162,7 @@ can_reuse_operands_p (enum insn_code icode,
>      case EXPAND_INPUT:
>      case EXPAND_ADDRESS:
>      case EXPAND_INTEGER:
> +    case EXPAND_UNDEFINED:
>        return true;

I think this should be in the "return false" block instead.

>  
>      case EXPAND_CONVERT_TO:
> diff --git a/gcc/optabs.h b/gcc/optabs.h
> index c80b7f4dc1b..4eb1f9ee09a 100644
> --- a/gcc/optabs.h
> +++ b/gcc/optabs.h
> @@ -37,7 +37,8 @@ enum expand_operand_type {
>    EXPAND_CONVERT_TO,
>    EXPAND_CONVERT_FROM,
>    EXPAND_ADDRESS,
> -  EXPAND_INTEGER
> +  EXPAND_INTEGER,
> +  EXPAND_UNDEFINED

Sorry, this was my bad suggestion.  I should have suggested
EXPAND_UNDEFINED_INPUT, to match the name of the function.

Thanks,
Richard

>  };
>  
>  /* Information about an operand for instruction expansion.  */
> @@ -117,6 +118,17 @@ create_input_operand (class expand_operand *op, rtx value,
>    create_expand_operand (op, EXPAND_INPUT, value, mode, false);
>  }
>  
> +/* Make OP describe an undefined input operand for uninitialized
> +   SSA.  It's the scratch operand with mode MODE; MODE cannot be
> +   VOIDmode.  */
> +
> +inline void
> +create_undefined_input_operand (class expand_operand *op, machine_mode mode)
> +{
> +  create_expand_operand (op, EXPAND_UNDEFINED, gen_rtx_SCRATCH (mode), mode,
> +			 false);
> +}
> +
>  /* Like create_input_operand, except that VALUE must first be converted
>     to mode MODE.  UNSIGNED_P says whether VALUE is unsigned.  */