instruction-latency-audit

/instruction-latency-audit — pipeline result-latency / NOP-padding hazard

Ground-truth precedence: the live sources — the pinned sfpi-gcc (latencies / xtt_delay / xtt_dynamic_bug) and the tt-isa-docs MCP — outrank every rule, table, and example baked into this skill (treat those as dated illustrations). If a live source contradicts a baked rule here, do NOT silently proceed: surface the conflict to the user and ask whether the baked rule should be overwritten, discarded, or kept. Default to the live source.

MANDATORY — before any verdict, read the shared grounding policy. The per-architecture source ladder (which docs to consult), the ground-or-abstain rule, and the Source preflight (list the sources you'll consult with their reachability + hierarchy, then PAUSE for the user) are defined once in race-audit-all → .claude/skills/race-audit-all/SKILL.md. Your FIRST action is to Read that file and follow its "Ground-truth source ladder", "Ground-or-abstain", and "Source preflight" sections — they are load-bearing: a verdict produced without them is ungrounded and MUST NOT be reported. (This audit's own authority is the pinned sfpi-gcc source — see the Fetch recipe in the freshness contract below.) If race-audit-all genuinely cannot be read, say so and abstain rather than proceed ungrounded. (If you were spawned by a race-audit-all sweep — your prompt already lists the confirmed sources — skip the Source preflight and do not pause; the orchestrator ran it once.)

Coverage — floor, not ceiling. The grep patterns and site lists in this skill are a seed, not an exhaustive enumeration. After running them, widen the search with full reasoning. The techniques here are illustrative examples, not the allowed set — use any approach your reasoning suggests, including ones not listed: e.g. semantic search (by behavior/effect, not just token), resolving macros / wrappers / typedefs / indirection the literal pattern can't match, following the call graph to callers and callees, and diffing the WH/BH/QSR variants to catch a site present in one arch and missing in another. If you can find a hazard, primitive, or site the encoded patterns don't cover — by any means — pursue and report it; do not clamp a stronger analysis to this list or to these techniques. State any residual coverage gaps explicitly (no silent caps).

Execution — parallel by default. When enumeration yields more than a few sites/files, fan out concurrent Agent calls by default (one per file/subsystem, a fresh context each), saturating the available concurrency (~10–16 at once); go inline only for a trivial set. The per-file fan-out described under Thoroughness is the default, not an exhaustive-only option. The cross-referencing/synthesis of results stays sequential (it must follow the per-unit findings). The heavyweight Workflow tool still requires explicit multi-agent opt-in — it is the opt-in exhaustive tier, not the default. Don't over-spawn a tiny diff.

Persisting results — single writer, incremental. Agents only return their findings; they never write a shared file (no concurrent-write clobbering). If findings are persisted to a file, the orchestrator/caller is the sole writer and appends each wave's returns as they arrive — incremental, never only-at-the-end — so an interrupt preserves every completed wave's findings.

The bug class (precise)

Within a single Tensix thread's instruction stream, some instructions take more than one cycle for their result to become available. If a later instruction in the same stream consumes that result before it has landed, the consumer latches a stale value → silent numerical corruption (no deadlock, no crash). This is NOT a race between agents — it is an intra-thread, intra-unit pipeline hazard. The fix is a NOP (SFPNOP / TTI_NOP) — or scheduling an independent instruction into the latency shadow — between producer and dependent consumer.

⚠️ Ground-truth freshness contract (read before deriving anything)

The decisive facts (per-instruction latencies, which arch's hardware auto-handles the hazard, and which instructions are exempt) live in the sfpi-gcc compiler, NOT the ISA docs — and they change across compiler versions. Therefore:

• Consult the live source every run; never trust a list reproduced in this document. Any instruction names / counts below are dated illustrations for calibration, subordinate to fresh derivation.
• Ground latency facts in the sfpi version this build is pinned to (resolve the pin from tt-metal's build/toolchain config) — that is what compiled the shipped kernels. Optionally also read the tip and flag divergence; the pinned version is the verdict authority.
• Source files to read & parse fresh: gcc/config/riscv/tt/rtl-rvtt-schedule.cc (the NOP pass), gcc/config/riscv/tt/rvtt.md (the xtt_delay and xtt_dynamic_bug attributes), gcc/config/riscv/tt/sfpu-ops-{wh,bh,qsr}.h (per-arch op tables). HW latency columns: tt-isa-docs MCP VectorUnit.md (WH and BH separately).
• Instruction existence/validity is resolved by that arch's tt_metal/tt-llk/tt_llk_<arch>/common/inc/ckernel_ops.h (a TT_OP_*/TTI_* with an opcode encoding = valid), never by ISA-doc page coverage — so never flag an instruction "invalid/absent on BH" from a missing page. The header settles existence only; latency / sampled registers / errata still come from the ISA + sfpi-gcc. (This is the general "never infer a negative from a missing doc" rule — grounded per race-audit-all's ladder + Ground-or-abstain, which this skill's MANDATORY banner already requires; not re-derived here.)
• Fetch recipe — the source is a submodule, NOT in the local install (runtime/sfpi/ ships only the compiled toolchain; do not stop there and fall back to a compile experiment). Resolve it each run:
  1. Read the pin in tt_metal/sfpi-version (e.g. sfpi_version='7.62.0', sfpi_repo='https://github.com/tenstorrent/sfpi').
  2. tenstorrent/sfpi at that tag has a gcc submodule → tenstorrent/sfpi-gcc; get its pinned commit: gh api repos/tenstorrent/sfpi/contents/gcc?ref=<tag> -q .sha (e.g. sfpi 7.62.0 → sfpi-gcc 40d9f44).
  3. Read the files at that commit without a full clone: gh api repos/tenstorrent/sfpi-gcc/contents/gcc/config/riscv/tt/<file>?ref=<sha> (or clone tenstorrent/sfpi-gcc and git checkout <sha>). The example commit here is a dated illustration — always re-resolve from the current pin.
• If the pinned sfpi-gcc source genuinely can't be fetched (no network / repo unreachable), emit no latency verdict — abstain and mark coverage bounded; do not fall back to a stale baked-in list or a compile experiment, and never overturn a prior source-grounded verdict.

How the compiler handles it (the model to re-derive, not memorize)

The pass rtl-rvtt-schedule.cc conditionally inserts sfpnop after Tensix insns, keyed on each insn's xtt_delay (none / static / dynamic):

• static delay → fixed pipeline bubble; a NOP is always inserted (unless one already follows). No arch exemption — padded on WH, BH, and QSR.
• dynamic delay → true data-hazard; a NOP is inserted only if a following insn actually reads the result register within the window.
• Blackhole / Quasar have hardware scoreboarding (schedule.cc: TARGET_XTT_TENSIX_BH/QSR + XTT_DYNAMIC_BUG_* mask) that auto-stalls on dynamic data hazards, so the compiler omits the dynamic NOP there — except for instructions flagged xtt_dynamic_bug for that arch (scoreboard errata), where it still inserts the NOP. Wormhole has no scoreboard → the compiler always pads dynamic hazards. Re-derive the exact errata-flagged and static-delay instruction sets from rvtt.md each run.

Provenance lens (decides the verdict — run this FIRST)

• Code compiled through sfpi (sfpi intrinsics → RTL → the scheduling pass: vFloat, dst_reg[...], sfpi:: ops) → the compiler schedules and pads it. Trust it (on both WH and BH, modulo the pinned version). Do not flag.
• Hand-written TTI_SFP* / TTI_* / inline-asm / direct opcode pushes → bypass the pass entirely → manual NOPs required. This is where hazards live.

Method

1. Establish the freshness contract (resolve pinned sfpi; load the live rule files; re-derive the static-delay set, the per-arch dynamic-bug errata set, and the latency table). State what you derived and from which version.
2. Enumerate hand-written instruction sequences (the at-risk surface):

   cd tt_metal/tt-llk
   grep -rInE "\bTTI_SFP[A-Z0-9_]+|\bTTI_[A-Z0-9_]+\(|sfpnop|TTI_NOP" tt_llk_* --include=*.h | grep -v /tests/
   

   Classify each block by provenance (sfpi-generated vs raw). Skip sfpi-generated blocks.
3. For each raw sequence, walk producer→consumer in program order. For every result a later instruction reads, check the producer's latency (and xtt_delay class) against the spacing present: enough NOPs / independent instructions to cover the latency?
4. Apply the arch matrix. WH: every static + dynamic hazard needs manual spacing. BH/QSR: static delays + the xtt_dynamic_bug errata instructions still need manual NOPs; other dynamic hazards are HW-scoreboarded. Do NOT exempt Blackhole wholesale.
5. Watch the explicit footguns the compiler flags: variable-LReg read/write (rvtt.md notes "for the user to get this right"), SFPLOADMACRO ("Complex" latency), and SFPCONFIG. Also the non-SFPU result-latencies the compiler never touches (these are pure LLK-C++, manual on every arch): MVMUL/FPU → SFPLOAD-from-Dst settle, config-write (SETC16/WRCFG) → consumer settle.

Verdict

• sfpi-compiled, or raw sequence with sufficient spacing for the arch → SAFE.
• Raw sequence consuming a multi-cycle result without enough spacing, on an arch that needs it (WH dynamic; any-arch static; BH/QSR errata insn) → LATENCY-HAZARD (real, silent corruption) — fix = insert the required SFPNOP/TTI_NOP (or reorder independent work in).
• Arch-divergent (e.g. needs a NOP on WH but HW-scoreboarded on BH) → report per-arch; never collapse to one verdict.
• Can't resolve the pinned compiler → UNCERTAIN + bounded-coverage note.

Thoroughness (optional, full sweep)

Deterministic derivation of the latency/errata tables from the pinned sfpi-gcc (script the parse), then one agent per file to classify provenance and check spacing, then adversarially re-check each SAFE-by-scoreboard verdict against the freshly-derived errata set. Only run a Workflow if the user opts into multi-agent orchestration. Never exempt BH without confirming the consuming insn is not in the dynamic-bug set, and never trust a latency number not re-derived this run.

Output

For each flagged site: file:line, the producer instruction + its latency/xtt_delay class, the dependent consumer, provenance (sfpi vs raw), spacing present vs required, per-arch verdict (SAFE / LATENCY-HAZARD / UNCERTAIN), and the one-line fix. Head the report with the compiler version consulted and the derived static-delay + per-arch errata sets (with a note that they were derived fresh). End with totals per verdict per arch.