Activate DureMark as a benchmark

[jserv]

DureMark was build-only: "rvopt mux" could not lower it because libgcc's soft div/mod return through a "jr t0" runtime JALR, and GCC hoisted the ecall syscall number out of each ecall's own basic block (rvopt resolves it only from in-block constants). Fix both at the source:

• soft.c supplies self-contained __mulsi3/__udivsi3/__umodsi3/__modsi3 (jal/ret only, shift/subtract so they cannot recurse into libgcc). It is linked before libgcc, so its four helpers win and the "jr t0" millicode is never pulled; libgcc trails only as a fallback for any other pure-RV32I helper a future codegen might emit.
• run.c loads a7 inside the ecall asm template with an a7 clobber, so the syscall number materializes in the ecall's own block.
• ITERATIONS 1 -> 500 makes the run a substantial (~15 s) benchmark; the count feeds the checksum, re-pinned to 1069216870 and gated by rv32i-check.

Add benchmark targets: bench-forth times seven compute-bound eForth kernels, bench-rv32i delegates to the duremark sub-make run target (build

• lower + bounded VM run + checksum assert) with a TIME_P prefix so the benchmark times exactly the gated path, and bench runs both. Timing uses /usr/bin/time -p; runs are bounded by timeout. Wire make bench into the CI rv32i job (and install GNU time there).

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Summary by cubic

Activate DureMark as a timed benchmark by making it fully lowerable with rvopt mux and wiring new bench targets into CI. This adds a deterministic checksum gate and measures both eForth kernels and the rvopt‑lowered DureMark path.

• New Features
  • DureMark now runs end to end: added soft.c with RV32I __mulsi3/__udivsi3/__umodsi3/__modsi3 (no libgcc JALR), and run.c loads a7 inside the ecall template. Iterations set to 500; checksum pinned to 1069216870.
  • Makefile: new bench-forth (7 compute kernels) and bench-rv32i (delegates to tests/rv32i/duremark run), plus bench to run both. Uses /usr/bin/time -p; runs are bounded by timeout.
  • rv32i-check now lowers and runs DureMark alongside demo and unopt programs.
  • CI installs time and runs make bench in the RV32I job.

^{Written for commit 6a17d01. Summary will update on new commits.}

[cubic-dev-ai]

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