Hi,
this patch corrects a problem where our cost estimation in if-conversion is
broken when the 'else' block is missing: we then take the cost of 'then' block,
which leads gcc willing to replace a conditionally-executed block with cost 80
with an unconditionally-executed block with cost 72.
Existing heuristic is clearly discontinuous: if the 'else' block wasn't missing
but instead had cost 1, we'd take the minimum of 1 and 80 and match replacement
cost against that.
A more sensible approach is to take average expected time of the if-then-else
(i.e. add costs of 'then' and 'else' scaled by their probabilities), e.g. in
the above example take 0.5*80 = 40 if we expect the block to be executed about
every other time.
Martin Jambor (thanks!) kindly benchmarked this for me on SPEC 2006 and 2017
(sans 521.wrf_r and 527.cam4_r). There is a 2% improvement for 403.gcc and
3% for 433.milc, both on Skylake.
Considering also changes in 1..2% range, there might be a 1.5% regression on
526.blender_r on Zen 2, and 1.5% improvements for 520.omnetpp_r, 538.imagick_r
and 549.fotonik3d_r on Skylake.
In the PR Richi notes we sometimes won't get high-quality probability data
for such edges on RTL, but that shouldn't be a reason not to replace a broken
heuristic with a less broken one.
Bootstrapped on x86-64, ok for trunk? For backports?
Thanks.
Alexander
* ifcvt.c (average_cost): New static function. Use it...
(noce_process_if_block): ... here.
diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index e0c9522057a..283bb47f51a 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -3358,6 +3358,16 @@ bb_ok_for_noce_convert_multiple_sets (basic_block
test_bb)
return count > 1 && count <= param;
}
+/* Compute average of two given costs weighted by relative probabilities
+ of respective basic blocks in an IF-THEN-ELSE. E is the IF-THEN edge.
+ With P as the probability to take the IF-THEN branch, return
+ P * THEN_COST + (1 - P) * ELSE_COST. */
+static unsigned
+average_cost (unsigned then_cost, unsigned else_cost, edge e)
+{
+ return else_cost + e->probability.apply ((int) then_cost - else_cost);
+}
+
/* Given a simple IF-THEN-JOIN or IF-THEN-ELSE-JOIN block, attempt to convert
it without using conditional execution. Return TRUE if we were successful
at converting the block. */
@@ -3413,10 +3423,9 @@ noce_process_if_block (struct noce_if_info *if_info)
&if_info->else_simple))
return false;
- if (else_bb == NULL)
- if_info->original_cost += then_cost;
- else if (speed_p)
- if_info->original_cost += MIN (then_cost, else_cost);
+ if (speed_p)
+ if_info->original_cost += average_cost (then_cost, else_cost,
+ find_edge (test_bb, then_bb));
else
if_info->original_cost += then_cost + else_cost;
