The latency of LA464 and LA664 division instructions depends on the
input. When I updated the costs in r14-6642, I unintentionally set the
division costs to the best-case latency (when the first operand is 0).
Per a recent discussion [1] we should use "something sensible" instead
of it.
Use the average of the minimum and maximum latency observed instead.
This enables multiplication to reciprocal sequence reduction and speeds
up the following test case for about 30%:
int
main (void)
{
unsigned long stat = 0xdeadbeef;
for (int i = 0; i < 100000000; i++)
stat = (stat * stat + stat * 114514 + 1919810) % 1000000007;
asm(""::"r"(stat));
}
[1]: https://gcc.gnu.org/pipermail/gcc-patches/2024-March/648348.html
gcc/ChangeLog:
* config/loongarch/loongarch-def.cc
(loongarch_rtx_cost_data::loongarch_rtx_cost_data): Increase
default division cost to the average of the best case and worst
case senarios observed.
gcc/testsuite/ChangeLog:
* gcc.target/loongarch/div-const-reduction.c: New test.
---
Bootstrapped and regtested on loongarch64-linux-gnu. Ok for trunk?
gcc/config/loongarch/loongarch-def.cc | 8 ++++----
gcc/testsuite/gcc.target/loongarch/div-const-reduction.c | 9 +++++++++
2 files changed, 13 insertions(+), 4 deletions(-)
create mode 100644 gcc/testsuite/gcc.target/loongarch/div-const-reduction.c
diff --git a/gcc/config/loongarch/loongarch-def.cc
b/gcc/config/loongarch/loongarch-def.cc
index e8c129ce643..93e72a520d5 100644
--- a/gcc/config/loongarch/loongarch-def.cc
+++ b/gcc/config/loongarch/loongarch-def.cc
@@ -95,12 +95,12 @@ loongarch_rtx_cost_data::loongarch_rtx_cost_data ()
: fp_add (COSTS_N_INSNS (5)),
fp_mult_sf (COSTS_N_INSNS (5)),
fp_mult_df (COSTS_N_INSNS (5)),
- fp_div_sf (COSTS_N_INSNS (8)),
- fp_div_df (COSTS_N_INSNS (8)),
+ fp_div_sf (COSTS_N_INSNS (12)),
+ fp_div_df (COSTS_N_INSNS (15)),
int_mult_si (COSTS_N_INSNS (4)),
int_mult_di (COSTS_N_INSNS (4)),
- int_div_si (COSTS_N_INSNS (5)),
- int_div_di (COSTS_N_INSNS (5)),
+ int_div_si (COSTS_N_INSNS (14)),
+ int_div_di (COSTS_N_INSNS (22)),
movcf2gr (COSTS_N_INSNS (7)),
movgr2cf (COSTS_N_INSNS (15)),
branch_cost (6),
diff --git a/gcc/testsuite/gcc.target/loongarch/div-const-reduction.c
b/gcc/testsuite/gcc.target/loongarch/div-const-reduction.c
new file mode 100644
index 00000000000..0ee86410dd7
--- /dev/null
+++ b/gcc/testsuite/gcc.target/loongarch/div-const-reduction.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -mtune=la464" } */
+/* { dg-final { scan-assembler-not "div\.\[dw\]" } } */
+
+int
+test (int a)
+{
+ return a % 1000000007;
+}
--
2.44.0
