On Tue, 08 Nov 2022 11:54:34 PST (-0800), philipp.toms...@vrull.eu wrote:
The strength-reduction implementation in expmed.c will assess the
profitability of using shift-and-add using a RTL expression that wraps
a MULT (with a power-of-2) in a PLUS. Unless the RISC-V rtx_costs
function recognizes this as expressing a sh[123]add instruction, we
will return an inflated cost---thus defeating the optimization.
This change adds the necessary idiom recognition to provide an
accurate cost for this for of expressing sh[123]add.
Instead on expanding to
li a5,200
mulw a0,a5,a0
with this change, the expression 'a * 200' is sythesized as:
sh2add a0,a0,a0 // *5 = a + 4 * a
sh2add a0,a0,a0 // *5 = a + 4 * a
slli a0,a0,3 // *8
That's more instructions, but multiplication is generally expensive. At
some point I remember the SiFive cores getting very fast integer
multipliers, but I don't see that reflected in the cost model anywhere
so maybe I'm just wrong? Andrew or Kito might remember...
If the mul-based sequences are still faster on the SiFive cores then we
should probably find a way to keep emitting them, which may just be a
matter of adjusting those multiply costs. Moving to the shift-based
sequences seems reasonable for a generic target, though.
Either way, it probably warrants a test case to make sure we don't
regress in the future.
gcc/ChangeLog:
* config/riscv/riscv.c (riscv_rtx_costs): Recognize shNadd,
if expressed as a plus and multiplication with a power-of-2.
---
gcc/config/riscv/riscv.cc | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/gcc/config/riscv/riscv.cc b/gcc/config/riscv/riscv.cc
index ab6c745c722..0b2c4b3599d 100644
--- a/gcc/config/riscv/riscv.cc
+++ b/gcc/config/riscv/riscv.cc
@@ -2451,6 +2451,19 @@ riscv_rtx_costs (rtx x, machine_mode mode, int
outer_code, int opno ATTRIBUTE_UN
*total = COSTS_N_INSNS (1);
return true;
}
+ /* Before strength-reduction, the shNadd can be expressed as the addition
+ of a multiplication with a power-of-two. If this case is not handled,
+ the strength-reduction in expmed.c will calculate an inflated cost. */
+ if (TARGET_ZBA
+ && mode == word_mode
+ && GET_CODE (XEXP (x, 0)) == MULT
+ && REG_P (XEXP (XEXP (x, 0), 0))
+ && CONST_INT_P (XEXP (XEXP (x, 0), 1))
+ && IN_RANGE (pow2p_hwi (INTVAL (XEXP (XEXP (x, 0), 1))), 1, 3))
IIUC the fall-through is biting us here and this matches power-of-2 +1
and power-of-2 -1. That looks to be the case for the one below, though,
so not sure if I'm just missing something?
+ {
+ *total = COSTS_N_INSNS (1);
+ return true;
+ }
/* shNadd.uw pattern for zba.
[(set (match_operand:DI 0 "register_operand" "=r")
(plus:DI
