[PATCH v2] RISC-V: Add LMUL-aware RVV cost model for the Spacemit-X60 core

[Nikola Ratkovac]

This patch introduces a vector cost model for the Spacemit-X60 core,
using dynamic LMUL scaling with the -madjust-lmul-cost flag.

Compared to the previous patch, I dropped the local 'vector_lmul'
attribute and the corresponding LMUL-aware cost logic in spacemit-x60.md.
Instead, Spacemit-X60 tuning now enables -madjust-lmul-cost implicitly,
and riscv_sched_adjust_cost is updated so that the adjustment applies to
spacemit_x60 in addition to the generic out-of-order model.

The stress tests I previously used to tune individual instruction costs
(with the LMUL-aware logic implemented directly in spacemit-x60.md)
now show a regression in performance. The most likely cause is the implicit
-madjust-lmul-cost scaling, since some instructions performed better
with non-power-of-two scaling (or with no LMUL scaling at all), so the
uniform ×(1,2,4,8) adjustment affects performance.

Updated performance results:

| Benchmark        | Metric | Trunk            | Vector Cost Model | Δ (%)   |
|------------------|--------|------------------|-------------------|---------|
| SciMark2-C       | cycles | 311,450,555,453  | 313,278,899,107   | +0.56%  |
|------------------|--------|------------------|-------------------|---------|
| tramp3d-v4       | cycles | 23,788,980,247   | 21,073,526,428    | -12.89% |
|------------------|--------|------------------|-------------------|---------|
| Freebench/neural | cycles | 471,707,641      | 435,842,612       | -8.23%  |
|------------------|--------|------------------|-------------------|---------|

Benchmarks were run from the LLVM test-suite
(MultiSource/Benchmarks) using:

taskset -c 0 perf stat -r 10 ./...

SciMark2-C (ANSI C), FreeBench/neural, and tramp3d-v4
were used as representative numerical workloads.

For tramp3d-v4, the workload parameters (--cartvis 1.0 0.0, --rhomin 1e-8,
-n 20) increase floating-point intensity and dependency pressure, placing
greater stress on the scheduler.

2026-02-13  Nikola Ratkovac  <Nikola.Ratkovac@rt-rk.com>

gcc/ChangeLog:

        * config/riscv/spacemit-x60.md: Add primary vector pipeline model
        for Spacemit-X60.
        (spacemit_x60_dummy): Rename from spacemi6_x60_dummy.
        * config/riscv/riscv.cc (riscv_sched_adjust_cost): Support spacemit_x60.
        (riscv_override_options_internal): Enable TARGET_ADJUST_LMUL_COST
        for spacemit_x60.



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