Juzhe-Zhong <juzhe.zh...@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. */
