https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111139
Bug ID: 111139
Summary: RISC-V: improve scalar constants cost model
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: vineetg at gcc dot gnu.org
CC: jeffreyalaw at gmail dot com, kito.cheng at gmail dot com,
palmer at dabbelt dot com
Target Milestone: ---
The current const cost determination in riscv_rtx_costs () and its children
such as riscv_const_insns () needs improvements.
1. There's some likely inaccuracies with various heuristics.
2. Those heuristics are distributed in a bunch of places and better be
consolidated.
3. We need to make const cost cpu/uarch tunable as hardware widgets like macro
fusions could ammortize multi-insn const costs.
Some of the heuristics to cleanup/revisit:
1a. riscv_rtx_cost() returns 1 insn even if riscv_const_insns () returns > 1.
case CONST:
if ((cost = riscv_const_insns (x)) > 0)
{
if (cost == 1 && outer_code == SET)
*total = COSTS_N_INSNS (1);
else if (outer_code == SET || GET_MODE (x) == VOIDmode)
*total = COSTS_N_INSNS (1);
}
1b. riscv_const_insns () caps the const cost to 4 even if it higher with intent
to force const pool. RV backend in general no longer favors const pools for
large constants since 2e886eef7f2b5a ("RISC-V: Produce better code with complex
constants [PR95632] [PR106602]"). This heuristic needs to be revisited.
1c. riscv_split_integer_cost () adds 2 to initial cost computed.
