Hi,
This patch pulls the comparisons between if_info->branch_cost and a magic
number representing an instruction count to a common function. While I'm
doing it, I've documented the instructions that the magic numbers relate
to, and updated them where they were inconsistent.
If our measure of the cost of a branch is now in rtx costs units, we can
get to an estimate for the cost of an expression from the number of
instructions by multiplying through by COSTS_N_INSNS (1).
Alternatively, we could actually construct the cheap sequences and
check the sequence. But in these cases we're expecting to if-convert on
almost all targets, the transforms in this patch are almost universally
a good idea, even for targets with a very powerful branch predictor,
eliminating the branch eliminates a basic block boundary so might be
helpful for scheduling, combine, and other RTL optimizers.
Bootstrapped on x86-64 and aarch64 as part of the full sequence.
OK?
Thanks,
James
---
2016-06-21 James Greenhalgh
* ifcvt.c (noce_if_info): New field: max_seq_cost.
(noce_estimate_conversion_profitable_p): New.
(noce_try_store_flag_constants): Use it.
(noce_try_addcc): Likewise.
(noce_try_store_flag_mask): Likewise.
(noce_try_cmove): Likewise.
(noce_try_cmove_arith): Likewise.
(noce_find_if_block): Record targetm.max_noce_ifcvt_seq_cost.
diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index fd29516..0b97114 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -814,6 +814,10 @@ struct noce_if_info
/* Estimated cost of the particular branch instruction. */
unsigned int branch_cost;
+ /* Maximum permissible cost for the unconditional sequence we should
+ generate to replace this branch. */
+ unsigned int max_seq_cost;
+
/* The name of the noce transform that succeeded in if-converting
this structure. Used for debugging. */
const char *transform_name;
@@ -835,6 +839,17 @@ static int noce_try_minmax (struct noce_if_info *);
static int noce_try_abs (struct noce_if_info *);
static int noce_try_sign_mask (struct noce_if_info *);
+/* This function is always called when we would expand a number of "cheap"
+ instructions. Multiply NINSNS by COSTS_N_INSNS (1) to approximate the
+ RTX cost of those cheap instructions. */
+
+inline static bool
+noce_estimate_conversion_profitable_p (struct noce_if_info *if_info,
+ unsigned int ninsns)
+{
+ return if_info->max_seq_cost >= ninsns * COSTS_N_INSNS (1);
+}
+
/* Helper function for noce_try_store_flag*. */
static rtx
@@ -1320,7 +1335,8 @@ noce_try_store_flag_constants (struct noce_if_info *if_info)
&& (REG_P (XEXP (a, 0))
|| (noce_operand_ok (XEXP (a, 0))
&& ! reg_overlap_mentioned_p (if_info->x, XEXP (a, 0
- && if_info->branch_cost >= 2)
+ /* We need one instruction, the ADD of the store flag. */
+ && noce_estimate_conversion_profitable_p (if_info, 1))
{
common = XEXP (a, 0);
a = XEXP (a, 1);
@@ -1393,22 +1409,32 @@ noce_try_store_flag_constants (struct noce_if_info *if_info)
else
gcc_unreachable ();
}
+ /* Is this (cond) ? 2^n : 0? */
else if (ifalse == 0 && exact_log2 (itrue) >= 0
&& (STORE_FLAG_VALUE == 1
- || if_info->branch_cost >= 2))
+ /* We need ASHIFT, IOR. */
+ || noce_estimate_conversion_profitable_p (if_info, 2)))
normalize = 1;
+ /* Is this (cond) ? 0 : 2^n? */
else if (itrue == 0 && exact_log2 (ifalse) >= 0 && can_reverse
- && (STORE_FLAG_VALUE == 1 || if_info->branch_cost >= 2))
+ && (STORE_FLAG_VALUE == 1
+ /* We need ASHIFT, IOR. */
+ || noce_estimate_conversion_profitable_p (if_info, 2)))
{
normalize = 1;
reversep = true;
}
+ /* Is this (cond) ? -1 : x? */
else if (itrue == -1
&& (STORE_FLAG_VALUE == -1
- || if_info->branch_cost >= 2))
+ /* Just an IOR. */
+ || noce_estimate_conversion_profitable_p (if_info, 1)))
normalize = -1;
+ /* Is this (cond) ? x : -1? */
else if (ifalse == -1 && can_reverse
- && (STORE_FLAG_VALUE == -1 || if_info->branch_cost >= 2))
+ && (STORE_FLAG_VALUE == -1
+ /* Just an IOR. */
+ || noce_estimate_conversion_profitable_p (if_info, 1)))
{
normalize = -1;
reversep = true;
@@ -1564,8 +1590,8 @@ noce_try_addcc (struct noce_if_info *if_info)
}
/* If that fails, construct conditional increment or decrement using
- setcc. */
- if (if_info->branch_cost >= 2
+ setcc. We'd only need an ADD/SUB for this. */
+ if (noce_estimate_conversion_profitable_p (if_info, 1)
&& (XEXP (if_info->a, 1) == const1_rtx
|| XEXP (if_info->a, 1) == constm1_rtx))
{
@@ -1621,7 +1647,9 @@ noce_try_store_flag_mask (struct noce_if_info *if_info)
return FALSE;
reversep = 0;
- if ((if_info->branch_cost >= 2
+
+ /* One instruction, AND. */
+ if ((noce_estimate_conversion_profitable_p (if_info,
