This is a patch Raphael wrote a few years back, but which never got
upstreamed. I can vaguely recall evaluating the patch and concluding to
drop it both internally and from the queue of things to upstream. But
the details of how I reached that conclusion are lost.
The core problem is we didn't recognize the fairly complex widening
multiply RTL, so it got costed by recursion which is painful on its
own. To add insult to injury the input operands are TI mode which,
IIRC, also increased the cost of the mulh insn.
The net was a totally bogus cost, it was totally off the charts which in
turn discouraged using multiplies for division-by-constant operations as
can be seen in the BZ.
This patch from Raphael fixes recognition of the mulh RTL and uses the
integer multiply cost out of the costing structure. This results in
sensible costs for the multiply and using a multiply-by-reciprocal
instead of division in more cases.
It's been tested on riscv32-elf and riscv64-elf without regressions. I
haven't bootstrapped it as I had my system disabled yesterday for
"reasons". I'll obviously wait for the pre-commit verdict.
Jeff
PR target/125387
gcc/
* config/riscv/riscv.cc (riscv_rtx_costs): Recognize and properly cost
mulh instructions.
gcc/testsuite
* gcc.target/riscv/pr125387.c: New test.
diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index a2730a90ca8b..99e1e4b3d8d2 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -4677,8 +4677,26 @@ riscv_rtx_costs (rtx x, machine_mode mode, int
outer_code, int opno ATTRIBUTE_UN
return true;
}
gcc_fallthrough ();
- case ASHIFTRT:
case LSHIFTRT:
+ /* mulh[u] pattern */
+ if (GET_CODE (x) == LSHIFTRT
+ && TARGET_MUL
+ && mode == GET_MODE_2XWIDER_MODE (word_mode).require ()
+ && outer_code == TRUNCATE
+ && (GET_CODE (XEXP (x, 0)) == MULT)
+ && (GET_CODE (XEXP (XEXP (x, 0), 0)) == ZERO_EXTEND
+ || GET_CODE (XEXP (XEXP (x, 0), 0)) == SIGN_EXTEND)
+ && (GET_CODE (XEXP (XEXP (x, 0), 1)) == ZERO_EXTEND
+ || GET_CODE (XEXP (XEXP (x, 0), 1)) == SIGN_EXTEND)
+ && (GET_CODE (XEXP (x, 1)) == CONST_INT
+ && INTVAL (XEXP (x, 1)) == BITS_PER_WORD))
+ {
+ *total = COSTS_N_INSNS (tune_param->int_mul[mode == TImode]);
+ return true;
+ }
+ /* Fall through. */
+
+ case ASHIFTRT:
*total = riscv_binary_cost (x, SINGLE_SHIFT_COST,
CONSTANT_P (XEXP (x, 1)) ? 4 : 9);
return false;
diff --git a/gcc/testsuite/gcc.target/riscv/pr125387.c
b/gcc/testsuite/gcc.target/riscv/pr125387.c
new file mode 100644
index 000000000000..c381b7a28e21
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr125387.c
@@ -0,0 +1,9 @@
+/* { dg-do compile { target rv64} } */
+/* { dg-additional-options "-mtune=tt-ascalon-d8 -march=rv64gc" }
+/* { dg-skip-if "" { *-*-* } { "-Os" "-Oz" } } */
+
+long foo1 (long x) { return x / 10000L; }
+long foo2 (long x) { return x / 30; }
+
+/* { dg-final { scan-assembler-times "mulh" 2 } } */
+/* { dg-final { scan-assembler-not "\tdiv\t" } } */
