From: Kyrylo Tkachov <[email protected]>
average_cost subtracts its unsigned operands before converting the result
to signed int. When THEN_COST is less than ELSE_COST, this relies on an
implementation-defined conversion. profile_probability::apply also rounds
by adding half the denominator, which is not correct for negative inputs.
Use the cheaper arm as the base and scale the nonnegative cost difference
by the probability of executing the more expensive arm. Cast the
expensive arm cost to gcov_type before subtracting the cheaper arm cost.
This keeps the result between the two arm costs and gives the same estimate
when the arms and probability are reversed.
Add an x86 test with the same costs and probabilities expressed using
reversed CFG arms. Both forms must make the same profitability decision.
This symmetry is also needed when the PR125557 diamond conversion chooses
either CFG arm as its primary arm.
Bootstrapped and regression tested on aarch64-none-linux-gnu and
x86_64-pc-linux-gnu.
Ok for trunk?
Thanks,
Kyrill
gcc/ChangeLog:
PR tree-optimization/125557
* ifcvt.cc (average_cost): Scale a nonnegative cost difference using
the probability of the more expensive arm.
gcc/testsuite/ChangeLog:
PR tree-optimization/125557
* gcc.target/i386/ifcvt-average-cost-1.c: New test.
Signed-off-by: Kyrylo Tkachov <[email protected]>
---
gcc/ifcvt.cc | 7 ++++-
.../gcc.target/i386/ifcvt-average-cost-1.c | 31 +++++++++++++++++++
2 files changed, 37 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/gcc.target/i386/ifcvt-average-cost-1.c
diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
index d75f1a8ed26..03d9a4c6ab3 100644
--- a/gcc/ifcvt.cc
+++ b/gcc/ifcvt.cc
@@ -4311,7 +4311,12 @@ bb_ok_for_noce_convert_multiple_sets (basic_block
test_bb, unsigned *cost)
static unsigned
average_cost (unsigned then_cost, unsigned else_cost, edge e)
{
- return else_cost + e->probability.apply ((signed) (then_cost - else_cost));
+ if (then_cost < else_cost)
+ return then_cost
+ + e->probability.invert ().apply ((gcov_type) else_cost - then_cost);
+
+ return else_cost
+ + e->probability.apply ((gcov_type) then_cost - else_cost);
}
/* Given a simple IF-THEN-JOIN or IF-THEN-ELSE-JOIN block, attempt to convert
diff --git a/gcc/testsuite/gcc.target/i386/ifcvt-average-cost-1.c
b/gcc/testsuite/gcc.target/i386/ifcvt-average-cost-1.c
new file mode 100644
index 00000000000..2e70cde06be
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/ifcvt-average-cost-1.c
@@ -0,0 +1,31 @@
+/* { dg-do compile } */
+/* { dg-require-effective-target lp64 } */
+/* { dg-options "-O2 -mtune=generic
--param=max-rtl-if-conversion-unpredictable-cost=0 -fdump-rtl-ce1" } */
+
+/* These functions describe the same branch probabilities and arm costs with
+ the arms reversed. */
+long
+then_cheaper (long c, long a, long b)
+{
+ long x;
+ if (__builtin_expect_with_probability (c != 0, 0, 0.90))
+ x = a ^ b;
+ else
+ x = b * 3 + 1;
+ return x;
+}
+
+long
+then_costlier (long c, long a, long b)
+{
+ long x;
+ if (__builtin_expect_with_probability (c == 0, 1, 0.90))
+ x = b * 3 + 1;
+ else
+ x = a ^ b;
+ return x;
+}
+
+/* { dg-final { scan-rtl-dump-not "if-conversion succeeded through
noce_try_cmove_arith" "ce1" } } */
+/* { dg-final { scan-assembler-not {\tcmov} } } */
+/* { dg-final { scan-assembler-times {\tj(e|ne)\t} 2 } } */
--
2.50.1 (Apple Git-155)
