Perf/dma tail wide memset - #896
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Routes the guest's strong `memset` symbol through a bounded DMA ecall, the same shape as the memcpy stub #874 added, and proves each chunk with a new 20-column DMA_SET table. memset is cheaper than memcpy rather than a copy of it: there is no source to read, so a row emits one MEMW write and no read (half the memory traffic per byte), and every byte written is the same constant, so one `fill` column replaces memcpy's eight value lanes. `fill_wide` is `fill` on eight-byte rows and zero on one-byte tail rows, which lets one write tuple serve both widths. `fill <= 255` is proven on the first row; the executor rejects wider values and the guest stub masks a1, mirroring how the byte-count bound is handled. Measured on real mainnet block 25368371 (50,781,394 cycles baseline): #874 memcpy alone 41,642,609 -17.99% + memset (this) 40,338,153 -20.57% mem* routines fall from 24.41% to 4.84% of guest cycles. No existing AIR changes: CPU stays at 38 columns and the new table only adds senders to existing buses.
The DMA memcpy ecall already snapshots its entire source range before writing (all reads at T+1, all writes at T+2), so one chunk has memmove semantics for free. Chunking is what breaks it: copying [0,256) -> [4,260) clobbers source bytes a later forward chunk still needs. So the memmove stub walks chunks from the END backwards exactly when the destination starts inside the source range (src < dst < src+n); every chunk then reads bytes no earlier chunk has written. Disjoint regions, and dst below src, keep forward chunking. This costs one guest symbol and nothing else — no table, no syscall, no constraint. Measured on real mainnet block 25368371: memcpy + memset 40,338,153 + memmove (this) 39,867,443 -0.93% Cumulative vs the 50,781,394 baseline: -21.49%. The guest test covers both overlap directions at offsets either side of the 256-byte chunk boundary, plus exact aliasing.
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/bench |
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Benchmark Results for modified programs 🚀
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Benchmark — real block (
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| Metric | main | PR | Δ |
|---|---|---|---|
| Peak heap | 48519 MB | 52081 MB | +3562 MB (+7.3%) 🔴 |
| Prove time | 166.381s | 129.635s | -36.746s (-22.1%) 🟢 |
🎉 Improvement on the real block — prove time down 22.1%.
Prove-time spread 1.9% (129.635s / 128.678s / 131.142s)
Commit: 77a5467 · Baseline: cached · Runner: self-hosted bench
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Automated review pass (high-effort, adversarially verified). Scoped to this PR's own diff on top of 874's current head. Findings:
Soundness of the new |
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