[V3] internal-fn: Support undefined rtx for uninitialized SSA_NAME

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.

Bootstrap and Regression on x86 passed.

gcc/ChangeLog:

	* internal-fn.cc (expand_fn_using_insn): Support undefined rtx value.
	* 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      | 14 +++++++++++++-
 gcc/optabs.h       | 14 +++++++++++++-
 3 files changed, 30 insertions(+), 2 deletions(-)

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.
>
> Bootstrap and Regression on x86 passed.
>
> gcc/ChangeLog:
>
> 	* internal-fn.cc (expand_fn_using_insn): Support undefined rtx value.
> 	* 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      | 14 +++++++++++++-
>  gcc/optabs.h       | 14 +++++++++++++-
>  3 files changed, 30 insertions(+), 2 deletions(-)
>
> 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..75b1d54ad7c 100644
> --- a/gcc/optabs.cc
> +++ b/gcc/optabs.cc
> @@ -8102,6 +8102,17 @@ maybe_legitimize_operand (enum insn_code icode, unsigned int opno,
>  	  goto input;
>  	}
>        break;
> +
> +    case EXPAND_UNDEFINED_INPUT:
> +      /* 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.  */

I don't think we should assume that default SSA definitions are the
only source of uninitialised RTL operands.  How about:

/* See if the predicate accepts a SCRATCH rtx, which in this context
   indicates an undefined value.  Use an uninitialized register if not.  */

> +      if (!insn_operand_matches (icode, opno, op->value))
> +	{
> +	  op->value = gen_reg_rtx (op->mode);
> +	  goto input;
> +	}
> +      return true;
>      }
>    return insn_operand_matches (icode, opno, op->value);
>  }
> @@ -8140,7 +8151,8 @@ can_reuse_operands_p (enum insn_code icode,
>    switch (op1->type)
>      {
>      case EXPAND_OUTPUT:
> -      /* Outputs must remain distinct.  */
> +    case EXPAND_UNDEFINED_INPUT:
> +      /* Outputs and undefined intputs must remain distinct.  */
>        return false;
>  
>      case EXPAND_FIXED:
> diff --git a/gcc/optabs.h b/gcc/optabs.h
> index c80b7f4dc1b..6faebf7cb63 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_INPUT
>  };
>  
>  /* 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.  */

How about:

/* Make OP describe an undefined input operand of mode MODE.  MODE cannot
   be null.  */

Preapproved with those changes, thanks.  No need for another review.

Richard

> +
> +inline void
> +create_undefined_input_operand (class expand_operand *op, machine_mode mode)
> +{
> +  create_expand_operand (op, EXPAND_UNDEFINED_INPUT, 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.  */