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npu: the draft loop commits a verify prefix instead of rolling back, and gates the chain on tau #798

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bong-water-water-bong merged 2 commits into from
Nov 2, 2026
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npu: the draft loop commits a verify prefix instead of rolling back, and gates the chain on tau #798

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npu: the draft loop commits a verify prefix instead of rolling back, …
Nov 2, 2026
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Merge branch 'main' into npu/mtp-deferred-commit
bong-water-water-bong Nov 2, 2026
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75 changes: 48 additions & 27 deletions npu/mtp/draft.h
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Original file line number Diff line number Diff line change
Expand Up @@ -20,11 +20,20 @@
// M=16 twins verify up to 16 rows for the price of one row's weight stream. So the draft head is worth
// its cost exactly when it turns "one pass per emitted token" into "one pass per (accepted + 1) tokens".
//
// Rollback is a truncation, not a snapshot. A snapshot of a 400-token context is ~100 MB and one per
// iteration would cost more than the drafts save, while every cache here is position-indexed: the caller
// supplies `pos`, so "forget everything past N" is a length bump for attention. A GDN layer's recurrence is
// the exception (it is not position-indexed), which is why BaseModel::truncate() may refuse -- the loop then
// falls back to plain decode rather than emitting wrong tokens.
// Rollback is a commit, not a snapshot, and not a rollback at all where it can be avoided.
//
// A snapshot of a 400-token context is ~100 MB and one per iteration would cost more than the drafts save, and
// attention state needs no rollback whatsoever: it is position-indexed, the caller supplies `pos`, so
// "forget everything past N" is a length bump. The one state that cannot be re-indexed is a GDN layer's
// recurrence. So instead of rolling it back, the verify pass DEFERS it: the rows of a verification batch
// store their recurrence inputs rather than applying them, and BaseModel::commit_verify(accepted) then
// applies exactly the accepted rows and drops the rest. Nothing is recomputed and nothing is snapshotted;
// the cost is the storage of k rows of update inputs (per layer: the conv-ring rows plus the rank-1 update
// inputs), which is kilobytes, not the 100 MB a state copy would take.
//
// A base that has not implemented deferral reports it from commit_verify() and the loop falls back to a
// synchronous commit + position truncation -- correct, just not free. Either way the emitted text is the
// same; the difference is only what a rejected draft costs.
//
// The loop is policy only -- it owns no weights and no device state. The base model arrives through
// BaseModel (Q4nxNpuForward satisfies it through the adapter in npu/mtp/forward_base.h) and the draft head
Expand Down Expand Up @@ -77,11 +86,15 @@ class BaseModel {
virtual bool step_batch_all(const std::vector<int>& tokens, int start_pos,
std::vector<float>& logits_all) = 0;

// Forget every position at or past `tokens`, so the next step can run at `tokens`. Cheap by design:
// attention state is position-indexed (the caller passes `pos`), so for a pure-attention model this is
// a length bump; a GDN/hybrid model must also roll its recurrent state back and returns false until it
// can (see the note at the top of this file -- that is the one engine gap speculation has).
virtual bool truncate(int tokens) = 0;
// After a verification batch of `rows` rows, keep the first `accepted` of them and discard the rest.
// * attention: pure bookkeeping -- drop the positions of the rejected rows (they are position-indexed,
// so nothing has to be undone);
// * GDN/hybrid: apply the deferred recurrence updates of the accepted rows, drop the rejected ones.
// That is the design this interface exists for: the updates were stored during the verify pass, not
// applied, so committing a prefix is cheap and no state copy is involved.
// Returns false if this base cannot defer (no GDN deferral implemented); the loop then stops rather than
// pretending the state is consistent.
virtual bool commit_verify(int accepted) = 0;

// The last row's hidden state (H floats) after the last step/step_batch_all: the MTP head's input.
virtual const std::vector<float>& last_hidden() const = 0;
Expand All @@ -92,14 +105,22 @@ class DraftHead {
virtual ~DraftHead() = default;
// The base model has just consumed `last_token` and `hidden` is the state after it. Propose up to k
// tokens; return how many were written to out (0 = no proposal, the iteration then costs nothing).
// `conf`, when given, receives each proposal's own probability (softmax of the head's logits at that
// position) so the loop can apply the tau gate; a head that cannot report it leaves `conf` empty and the
// gate is skipped for that iteration rather than guessed.
virtual int draft(const std::vector<float>& hidden, int last_token, int k,
std::vector<int>& out) = 0;
std::vector<int>& out, std::vector<float>* conf = nullptr) = 0;
virtual void reset() {}
};

struct Options {
int k = 4; // drafts per iteration; clamped to max_batch_rows()
int max_tokens = 64; // tokens to emit, prompt excluded
// The chain is gated on the drafter's own confidence: keep drafting while the token's probability is at
// least `tau` (0.6 is the published setting for this head), stop the chain otherwise -- a low-confidence
// draft is verified anyway (it is already in the batch), it just does not get followed by more of them.
// tau <= 0 disables the gate.
float tau = 0.6f;
};

struct Stats {
Expand Down Expand Up @@ -159,10 +180,18 @@ inline Result run(BaseModel& base, DraftHead* head, const std::vector<int>& prom
const int have = ++pos;

std::vector<int> d;
int nd = head->draft(base.last_hidden(), t1, k, d);
std::vector<float> conf;
int nd = head->draft(base.last_hidden(), t1, k, d, &conf);
if (nd <= 0) continue;
nd = std::min(nd, k);
d.resize((size_t)nd);
// tau gate: the chain only continues while the drafter is confident; the first draft below tau is
// still verified (it is in the batch anyway) but nothing is drafted after it.
if (opt.tau > 0.0f && conf.size() == d.size()) {
for (int i = 0; i < nd; i++)
if (conf[(size_t)i] < opt.tau) { nd = i + 1; break; }
d.resize((size_t)nd);
}

if (!base.step_batch_all(d, have, all)) { r.err = "verify batch failed"; return r; }
r.stats.iterations++;
Expand All @@ -189,21 +218,13 @@ inline Result run(BaseModel& base, DraftHead* head, const std::vector<int>& prom
r.stats.accepted += a; // acceptance is a property of the model, not of the budget
if (a == 0) r.stats.first_reject++;

if (take < nd) {
// Forget the rows we are not using (refused drafts, or the budget cut us short), and take the
// logits that predict the next position from the batch row we stopped at.
if (!base.truncate(have + take)) { r.err = "truncate refused"; return r; }
if (take > 0)
logits.assign(all.begin() + (size_t)(take - 1) * (size_t)V,
all.begin() + (size_t)take * (size_t)V);
pos = have + take;
} else {
// Everything landed: the batch state is already right, and the last row's logits predict the
// next position (one free token per fully accepted iteration).
logits.assign(all.begin() + (size_t)(nd - 1) * (size_t)V,
all.begin() + (size_t)nd * (size_t)V);
pos = have + nd;
}
// Commit the accepted rows of the verify batch and discard the rest. With a deferring base this
// applies only the accepted rows' recurrence updates; without one it truncates by position.
if (!base.commit_verify(take)) { r.err = "commit_verify refused"; return r; }
if (take > 0)
logits.assign(all.begin() + (size_t)(take - 1) * (size_t)V,
all.begin() + (size_t)take * (size_t)V);
pos = have + take;
if ((int)r.tokens.size() >= opt.max_tokens) break;
}

Expand Down
61 changes: 49 additions & 12 deletions tests/mtp_draft_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -67,24 +67,32 @@ class ToyBase : public BaseModel {
for (size_t m = 0; m < tokens.size(); m++)
fill_logits(start_pos + (int)m, logits_all, m * (size_t)V);
consumed_ += (int)tokens.size();
pending_ = (int)tokens.size(); // these rows are a verify batch until commit_verify() decides
batches_++;
return true;
}

// Attention-only toy: truncation is a length bump, exactly like the real position-indexed caches.
bool truncate(int tokens) override {
if (tokens < 0 || tokens > consumed_) return false;
consumed_ = tokens;
restores_++;
// Toy base: no GDN deferral, so committing a verify prefix is the position bookkeeping a pure-attention
// model needs (drop the rejected rows). It records the call so the test can check the loop commits
// exactly the accepted count and never more.
bool commit_verify(int accepted) override {
if (accepted < 0 || accepted > pending_) return false;
consumed_ = consumed_ - pending_ + accepted;
pending_ = 0;
commits_++;
last_commit_ = accepted;
return true;
}

int commits() const { return commits_; }
int last_commit() const { return last_commit_; }
int pending() const { return pending_; }

const std::vector<float>& last_hidden() const override { return hidden_; }

int consumed() const { return consumed_; }
int steps() const { return steps_; }
int batches() const { return batches_; }
int restores() const { return restores_; }
int bad_pos() const { return bad_pos_; }
int over_batch() const { return over_batch_; }

Expand All @@ -97,14 +105,17 @@ class ToyBase : public BaseModel {
}

int batch_rows_;
int consumed_ = 0, steps_ = 0, batches_ = 0, restores_ = 0, bad_pos_ = 0, over_batch_ = 0;
int consumed_ = 0, steps_ = 0, batches_ = 0, commits_ = 0, last_commit_ = -1, pending_ = 0;
int bad_pos_ = 0, over_batch_ = 0;
std::vector<float> hidden_{0.0f};
};

// Knows the truth (reads the position out of the hidden state): every draft should be accepted.
class PerfectHead : public DraftHead {
public:
int draft(const std::vector<float>& hidden, int, int k, std::vector<int>& out) override {
int draft(const std::vector<float>& hidden, int, int k, std::vector<int>& out,
std::vector<float>* conf = nullptr) override {
if (conf) conf->assign((size_t)k, 1.0f);
const int have = (int)hidden[0]; // positions consumed so far
out.clear();
for (int i = 0; i < k; i++) out.push_back(truth(have + i));
Expand All @@ -115,7 +126,9 @@ class PerfectHead : public DraftHead {
// Proposes the token AFTER the true one at every position: the base must refuse all of it.
class GarbageHead : public DraftHead {
public:
int draft(const std::vector<float>& hidden, int, int k, std::vector<int>& out) override {
int draft(const std::vector<float>& hidden, int, int k, std::vector<int>& out,
std::vector<float>* conf = nullptr) override {
if (conf) conf->assign((size_t)k, 0.1f);
const int have = (int)hidden[0];
out.clear();
for (int i = 0; i < k; i++) out.push_back((truth(have + i) + 1) % V);
Expand All @@ -126,7 +139,9 @@ class GarbageHead : public DraftHead {
// Right for the first draft, wrong afterwards: exercises the longest-prefix accept AND the rollback.
class HalfHead : public DraftHead {
public:
int draft(const std::vector<float>& hidden, int, int k, std::vector<int>& out) override {
int draft(const std::vector<float>& hidden, int, int k, std::vector<int>& out,
std::vector<float>* conf = nullptr) override {
if (conf) { conf->assign((size_t)k, 0.1f); if (!conf->empty()) (*conf)[0] = 1.0f; }
const int have = (int)hidden[0];
out.clear();
if (k > 0) out.push_back(truth(have)); // accepted
Expand Down Expand Up @@ -192,8 +207,8 @@ int main() {
// [1] k=0 must be a bit-for-bit plain decode
{ ToyBase m(8); PerfectHead ph; Options o = base_opt; o.k = 0;
const Result r = onebit::npu::mtp::run(m, &ph, P, o);
check(r.ok && same(r.tokens, ref.tokens) && m.batches() == 0 && m.restores() == 0,
"k=0: no batched pass, no rollback, same tokens"); }
check(r.ok && same(r.tokens, ref.tokens) && m.batches() == 0 && m.commits() == 0,
"k=0: no batched pass, no commit, same tokens"); }
}

// [2] the win condition, stated as a number: a perfect head at k=8 amortises the pass
Expand All @@ -217,6 +232,28 @@ int main() {
r.stats.passes_per_token(), r.stats.rows_verified);
}

// [6] tau gate: a head whose first draft is below tau gets verified but not followed, so exactly one
// row goes through the batch every iteration -- and the text still matches plain decode.
{
ToyBase m(8); GarbageHead gh; Options o = base_opt; o.k = 8; o.tau = 0.6f;
const Result r = onebit::npu::mtp::run(m, &gh, P, o);
check(r.ok && same(r.tokens, ref.tokens), "tau gate: text still identical to plain decode");
check(r.stats.rows_verified == r.stats.iterations,
"tau gate: a below-tau first draft is verified alone (1 row per iteration)");
std::printf("INFO tau=0.6 garbage head: rows verified=%d over %d iterations, passes/token=%.3f\n",
r.stats.rows_verified, r.stats.iterations, r.stats.passes_per_token());
}
// [7] the loop commits exactly the accepted prefix, never the whole batch when the tail is refused
{
ToyBase m(8); HalfHead hh; Options o = base_opt; o.k = 4;
const Result r = onebit::npu::mtp::run(m, &hh, P, o);
check(r.ok && m.pending() == 0, "commit: no verify rows left uncommitted at the end");
check(m.last_commit() <= r.stats.drafted && m.last_commit() >= 0,
"commit: the last commit was a prefix of the verified rows");
std::printf("INFO half head k=4: commits=%d last_commit=%d accepted=%d drafted=%d\n", m.commits(),
m.last_commit(), r.stats.accepted, r.stats.drafted);
}

std::printf(fails ? "\n%d CHECK(S) FAILED\n" : "\nall checks passed\n", fails);
return fails ? 1 : 0;
}
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