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sim/cosim-fw/ — system-level lock-step (compiled C vs Spike, free-running slave)

make cosim-fw compiles real C firmware with gcc -O0/-O2/-O3, runs it on the core against the canonical free-running registered BRAM (+dfree), and compares every committed GPR write against Spike instruction-by-instruction. A divergence is a real architectural bug.

Why it exists

The 2026-06 integrator escapes (LMB back-to-back, store→load) lived in real compiled-code patterns on real BRAM — function epilogues, stack spill/restore, back-to-back memory, pointer chases — none of which hand-written asm on the benign default BFM exercised. This harness closes that gap:

  • Compiled C, not asm — -O0/-O2/-O3 give three different codegen shapes per workload (the same axis the P7.1 firmware refactor changed). -O0 especially spills every local to the stack → dense store-then-restore-load traffic.
  • Free-running slave (+dfree) — the canonical Xilinx BRAM Port A (data_rd_q <= mem[addr] every cycle), the slave that exposes the store→load class. Override with SLAVE=+dedge=1 etc.
  • Spike is the golden model — the workloads are pure compute + memory (no MMIO), so Spike's architectural GPR stream is the reference. MMIO aperture timing is the integrator's concern, not the core's.

Proven teeth: with the rc7 store→load fix reverted, callchain (all opts), memops, and others DIVERGE from Spike here — i.e. this harness catches P7.1 organically from compiled C, no hand-written reproducer required.

Run

make -C sim cosim-fw                 # all workloads × -O0/-O2/-O3 on +dfree
SLAVE=+dedge=1 make -C sim cosim-fw  # against the edge-detect slave instead
OPTS="-O2" make -C sim cosim-fw      # one opt level

Requires WSL: riscv64-unknown-elf-gcc (C), Spike (SPIKE_BIN), the arch sim.

Files

File Role
run_cosim_fw.sh build each workload × opt → run core (+dfree, retire trace) → Spike --log-commits → diff (../cosim/cosim_compare.py)
crt0.S bare-metal C runtime (sp, zero .bss, call main, HTIF tohost exit)
link.ld base 0x8000_0000 (matches Spike + arch sim), 16 KiB stack
workloads/ callchain (epilogue spill/restore), memops (struct copy / sub-word), ptrchase (load→load), sortsum (in-place sort + recursion)

Adding a workload

Drop a workloads/<name>.c with an int main(void) that is deterministic and pure compute + memory (no libc, no MMIO, no FP). The lock-step against Spike is the check — no in-program assertions needed.