Sorry for the slow reply, was off for a few days.
I think the new code ought to happen earlier in emit_move_insn, before:
if (CONSTANT_P (y))
{
That way, all the canonicalisation happens on the mode we actually
want the move to have.
"Yangfei (Felix)" <felix.y...@huawei.com> writes:
> diff --git a/gcc/expr.c b/gcc/expr.c
> index dfbeae71518..4442fb83367 100644
> --- a/gcc/expr.c
> +++ b/gcc/expr.c
> @@ -3852,6 +3852,62 @@ emit_move_insn (rtx x, rtx y)
>
> gcc_assert (mode != BLKmode);
>
> + rtx x_inner = NULL_RTX;
> + rtx y_inner = NULL_RTX;
> + machine_mode x_inner_mode, y_inner_mode;
> +
> + if (SUBREG_P (x)
> + && REG_P (SUBREG_REG (x))
> + && known_eq (SUBREG_BYTE (x), 0))
> + {
> + x_inner = SUBREG_REG (x);
> + x_inner_mode = GET_MODE (x_inner);
> + }
> + if (SUBREG_P (y)
> + && REG_P (SUBREG_REG (y))
> + && known_eq (SUBREG_BYTE (y), 0))
> + {
> + y_inner = SUBREG_REG (y);
> + y_inner_mode = GET_MODE (y_inner);
> + }
The later code is only interested in SUBREG_REGs that are the same size
as "mode", so I think it would make sense to check that in the "if"s
above instead of checking SUBREG_BYTE. (SUBREG_BYTE is always zero
when the modes are the same size, but the reverse is not true.)
It might also be better to avoid [xy]_inner_mode and just use GET_MODE
where necessary.
It would be good to have a block comment above the code to explain
what we're doing.
> + if (x_inner != NULL_RTX
> + && y_inner != NULL_RTX
> + && x_inner_mode == y_inner_mode
> + && known_eq (GET_MODE_SIZE (x_inner_mode), GET_MODE_SIZE (mode))
> + && ! targetm.can_change_mode_class (x_inner_mode, mode, ALL_REGS))
> + {
> + x = x_inner;
> + y = y_inner;
> + }
> + else if (x_inner != NULL_RTX && CONSTANT_P (y)
Formatting nit: one subcondition per line when the condition spans
multiple lines.
> + && known_eq (GET_MODE_SIZE (x_inner_mode), GET_MODE_SIZE (mode))
> + && ! targetm.can_change_mode_class (x_inner_mode, mode, ALL_REGS)
> + && targetm.legitimate_constant_p (x_inner_mode, y))
This call isn't valid, since the mode has to match the rtx. ("y" still
has mode "mode" at this point.) I think instead we should just do:
&& (y_inner = simplify_subreg (GET_MODE (x_inner), y, mode, 0))
to convert the constant, and use it if the result is nonnull.
The existing CONSTANT_P emit_move_insn code will handle cases
in which the new constant isn't legitimate.
> +
> + {
> + x = x_inner;
> + }
> + else if (x_inner != NULL_RTX && MEM_P (y)
> + && known_eq (GET_MODE_SIZE (x_inner_mode), GET_MODE_SIZE (mode))
> + && ! targetm.can_change_mode_class (x_inner_mode, mode, ALL_REGS)
> + && (! targetm.slow_unaligned_access (x_inner_mode, MEM_ALIGN (y))
> + || MEM_ALIGN (y) >= GET_MODE_ALIGNMENT (x_inner_mode)))
What is the last condition protecting against? Seems worth a comment.
Thanks,
Richard
