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
