With the i386 backend no longer double-counting the cost of a SET, the
default implementation default_max_noce_ifcvt_seq_cost now provides too
high a bound for if conversion, allowing very costly substitutions.
The following patch fixes this by making an i386 variant of the hook,
but first - James, do you recall how you arrived at the COSTS_N_INSNS(3)
magic number? What happens on arm if you lower that in the default hook?
The change in ifcvt.c seems to have no bearing on the PR, I just noticed
we were missing a cost check in one place.
Lightly tested so far, will bootstrap & test on x86_64-linux.
Bernd
PR rtl-optimization/78634
* config/i386/i386.c (ix86_max_noce_ifcvt_seq_cost): New function.
(TARGET_MAX_NOCE_IFCVT_SEQ_COST): Define.
* ifcvt.c (noce_try_cmove): Add missing cost check.
Index: gcc/config/i386/i386.c
===================================================================
--- gcc/config/i386/i386.c (revision 242958)
+++ gcc/config/i386/i386.c (working copy)
@@ -50301,6 +50301,28 @@ ix86_spill_class (reg_class_t rclass, ma
return NO_REGS;
}
+/* Implement TARGET_MAX_NOCE_IFCVT_SEQ_COST. Like the default implementation,
+ but returns a lower bound. */
+
+static unsigned int
+ix86_max_noce_ifcvt_seq_cost (edge e)
+{
+ bool predictable_p = predictable_edge_p (e);
+
+ enum compiler_param param
+ = (predictable_p
+ ? PARAM_MAX_RTL_IF_CONVERSION_PREDICTABLE_COST
+ : PARAM_MAX_RTL_IF_CONVERSION_UNPREDICTABLE_COST);
+
+ /* If we have a parameter set, use that, otherwise take a guess using
+ BRANCH_COST. */
+ if (global_options_set.x_param_values[param])
+ return PARAM_VALUE (param);
+ else
+ return BRANCH_COST (true, predictable_p) * COSTS_N_INSNS (2);
+}
+
+
/* Implement targetm.vectorize.init_cost. */
static void *
@@ -51615,6 +51637,8 @@ ix86_run_selftests (void)
#undef TARGET_EXPAND_DIVMOD_LIBFUNC
#define TARGET_EXPAND_DIVMOD_LIBFUNC ix86_expand_divmod_libfunc
+#undef TARGET_MAX_NOCE_IFCVT_SEQ_COST
+#define TARGET_MAX_NOCE_IFCVT_SEQ_COST ix86_max_noce_ifcvt_seq_cost
#if CHECKING_P
#undef TARGET_RUN_TARGET_SELFTESTS
#define TARGET_RUN_TARGET_SELFTESTS selftest::ix86_run_selftests
Index: gcc/ifcvt.c
===================================================================
--- gcc/ifcvt.c (revision 242958)
+++ gcc/ifcvt.c (working copy)
@@ -1826,7 +1826,7 @@ noce_try_cmove (struct noce_if_info *if_
noce_emit_move_insn (if_info->x, target);
seq = end_ifcvt_sequence (if_info);
- if (!seq)
+ if (!seq || !noce_conversion_profitable_p (seq, if_info))
return FALSE;
emit_insn_before_setloc (seq, if_info->jump,
