Re: [PATCH v3 4/7] ifcvt/optabs: Allow using a CC comparison for emit_conditional_move.

[Jeff Law <jeffreyalaw@gmail.com>]

On 12/6/2021 11:43 AM, Robin Dapp via Gcc-patches wrote:
> Currently we only ever call emit_conditional_move with the comparison
> (as well as its comparands) we got from the jump.  Thus, backends are
> going to emit a CC comparison for every conditional move that is being
> generated instead of re-using the existing CC.
> This, combined with emitting temporaries for each conditional move,
> causes sky-high costs for conditional moves.
>
> This patch allows to re-use a CC so the costing situation is improved a
> bit.
> ---
>   gcc/expmed.c |   8 +--
>   gcc/expr.c   |  10 ++--
>   gcc/ifcvt.c  |  45 ++++++++++-------
>   gcc/optabs.c | 135 ++++++++++++++++++++++++++++++++++++++-------------
>   gcc/optabs.h |   4 +-
>   gcc/rtl.h    |  11 ++++-
>   6 files changed, 150 insertions(+), 63 deletions(-)
>
>

> @@ -4948,17 +4947,85 @@ emit_conditional_move (rtx target, enum rtx_code code, rtx op0, rtx op1,
>
> +/* Helper for emitting a conditional move.  */
> +
> +static rtx
> +emit_conditional_move_1 (rtx target, rtx comparison,
> +			 rtx op2, rtx op3, machine_mode mode)
> +{
> +  enum insn_code icode;
> +
> +  if (comparison == NULL_RTX || !COMPARISON_P (comparison))
> +    return NULL_RTX;
> +
> +  /* If the two source operands are identical, that's just a move.  */
> +  if (rtx_equal_p (op2, op3))
> +    {
> +      if (!target)
> +	target = gen_reg_rtx (mode);
> +
> +      emit_move_insn (target, op3);
> +      return target;
> +    }
What if the condition has a side effect?  Doesn't this drop the side 
effect by converting the conditional move into a simple move?

That's my only technical concern.  Obviously the patch needs a ChangeLog.

Jeff