fix(passes): fold provably-false init_cond predicate after replication

[georgebisbas]

Summary

LowerPipelineLoops replicates a pipelined loop body F times, and every replica after the first gets a loop index that is provably never zero. AutoTile's split-K matmul turns that index into an init_cond accumulate predicate (ko == 0), so for those replicas the predicate is statically false — but it never becomes a literal. The component that owns the fold (make_acc_codegen) is not the component that has the bounds; the component that has the bounds (RewriteSimplifier) has no rule that consults them.

This PR teaches the pass that owns the replication to fold its own predicate: when the replica index is provably non-zero, Eq(index, 0) in a call argument becomes the BOOL-typed ConstInt(0) that the emitter already folds. One dead branch and one dead MAD disappear per replica — in the IR, in the emitted kernel, and in what every downstream pass has to walk.

In one line: a replicated split-K matmul stops emitting a branch on a compile-time constant, and the removal scales linearly with the pipeline depth.

---

The invariant this restores

make_acc_codegen (src/backend/common/pto_ops_elementwise.cpp, ~L1436) is explicit about what it expects upstream to have done:

// A literal predicate picks one arm outright; only a runtime one branches.
// Both spellings reach here: a DSL `init_cond=True/False` arrives as a
// BOOL-typed ConstInt, while a predicate the arithmetic simplifier folded
// (e.g. `ko == 0` after LowerPipelineLoops replicates the K-loop) arrives
// as a ConstBool.  Missing either one leaves an `scf.if` on a compile-time
// constant, doubling the emitted MADs.

The invariant is therefore: after replication, the predicate is a literal. The emitter is written to consume both literal spellings, and tests/ut/codegen/test_matmul_init_cond.py::TestAutoTiledPredicateFolds asserts the invariant — but only in the regime where the main loop collapses, where the simplifier can fold it for free. In the regime where the K-loop survives replication, nothing folds it, and the invariant silently does not hold. That regime is exactly the one where the two existing tests do not look.

Notably the comment's own phrasing — "a predicate the arithmetic simplifier folded ... after LowerPipelineLoops replicates the K-loop" — records the assumption that this hand-off happens. It does not, and this PR is the missing half.

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The defect, down to the line

1. AutoTileMatmulL0 emits the seed test as part of the matmul (src/ir/transforms/auto_tile_matmul_l0_pass.cpp:523):

   auto init_cond = MakeEq(ko_var, MakeIndex(0, sp), sp);            // ko == 0
   auto c_call = reg.Create("tile.matmul_acc",
                            {ExprPtr(c_iter), sa->var_, sb->var_, init_cond}, sp);

2. ReplicateBody substitutes the clone's loop variable with the replica's index (src/ir/transforms/utils/pipeline_loop_utils.cpp:45, OffsetIndex):

   loop_var  ->  base + k*step
   

3. RewriteSimplifier has an Eq rule that moves the constant across the addition (src/ir/arith/rewrite_simplify.cpp):

   x + c1 == c2   ->   x == c2 - c1          // so  ko == -128  for the 128x512 case
   

   …and stops there. No Eq rule consults ConstIntBoundAnalyzer, even though simplify_pass.cpp had already bound the loop variable to [start, stop). The predicate is decidable but never a literal.

4. make_acc_codegen folds literals only, so it takes the runtime path and opens an scf.if.

Nothing downstream recovers. The branch reaches the emitted kernel as a register compare against a negative constant that can never be true:

const int64_t v12 = -128;
...
if (v51 == v12) { ... }

Why fix it in this pass, and not in the simplifier

Two reasons, both about blast radius:

• A bound-aware Eq rule in RewriteSimplifier changes fold decisions across the whole pipeline — CanonicalizeIOOrder, MemoryReuse, and every pass that walks expression structure. This PR changes exactly one predicate, produced by exactly one pass, for exactly one consumer.
• The range information is local and provable at the replication site: the pass knows main_start, step, and the replica index it just constructed. No symbolic analysis and no new analyzer query is needed.

---

The fix

DeadInitCondFolder, an IRMutator run on each clone immediately after the substitution (src/ir/transforms/lower_pipeline_loops_pass.cpp). It recurses first, then rewrites call arguments: Eq(X, 0) → false when X is structurally equal to the replica index just substituted (in either operand order). Everything else is left alone.

The gate is the part that has to be right:

// Fold iff the replica index is provably non-zero:
//   replica k indexes  start + j*(factor*step) + k*step
//   with factor >= 1 and k >= 1 both offsets carry step's sign,
//   so start + step is the index nearest zero.
ReplicaIndexIsProvablyNonZero(main_start, step)

enabled only when both hold:

• main_start is a compile-time constant, and
• sign(main_start + step) == sign(step).

Two deliberate non-foldings, both load-bearing:

Not folded	Why
And(user_cond, ko == 0) — the 4-operand init_cond composed at auto_tile_matmul_l0_pass.cpp:551	The user's term can still be runtime-true; only the generated term is dead. Out of scope here.
The tail sequence (BuildTailSeq passes false)	The tail re-runs the body at a different base index; the proof above does not apply to it.

The soundness counterexample this gate exists for (verified, not assumed)

pl.pipeline(-2, 6, 2, stage=2) replicates to replica indices {-2, 0}. Replica 1's actual indices are {0, 4} — so its predicate is TRUE at ko = -2. Folding there would accumulate onto an uninitialised accumulator (the first tile.matmul_acc is what seeds it). The pass correctly keeps i + 2 == 0 for that loop, and test_reachable_zero_keeps_the_replica_predicate pins it.

One spelling detail a reviewer should see

The injected literal is a BOOL-typed ConstInt, not a ConstBool. make_acc_codegen accepts both, but this pass is held to a print→parse round-trip equality check, and the printer emits False, which the parser reads back as a BOOL-typed ConstInt — so a ConstBool fails that check with Node type mismatch (ConstBool != ConstInt). The ConstInt spelling is also exactly what a DSL init_cond=False produces, i.e. the emitter's first branch.

---

Codegen: before and after

1. IR — the pass's own output (128x512 @ 512x128 FP16)

Before — a compare against a negative constant, then scf.if branching on it. Line 42 of the source program in both cases:

%5 = arith.cmpi eq, %out_tile__ssa_v0_l0_ko, %c0_index : index        // seed guard: genuine runtime test
scf.if %5 {
  pto.tmatmul.acc ins(%out_tile__ssa_v0_l0_init, ...) outs(...)
}
%6 = arith.cmpi eq, %out_tile__ssa_v0_l0_ko, %cn128_index : index      // replica guard: never true
scf.if %6 {
  ...
}

After — %cn128_index is not even declared in the module:

%5 = arith.cmpi eq, %out_tile__ssa_v0_l0_ko, %c0_index : index
scf.if %5 {
  pto.tmatmul.acc ins(%out_tile__ssa_v0_l0_init, ...) outs(...)
}

	before	after
scf.if in the pass output	2	1
pto.tmatmul* ops	4	3
negative constant in the module	%cn128_index	gone

2. Emitted kernel C++ (ptoas output)

Before — the dead arm, sitting inside a wait_flag/set_flag pair:

const int64_t v12 = -128;
...
set_flag(PIPE_MTE1, PIPE_M, EVENT_ID0);
// pto: %6
;
wait_flag(PIPE_MTE1, PIPE_M, EVENT_ID1);
if (v51 == v12) {                                   // always false
  pipe_barrier(PIPE_M);
  TMATMUL(v49, v60, v65);
} else {
  pipe_barrier(PIPE_M);
  TMATMUL_ACC(v49, v49, v60, v65);
};
set_flag(PIPE_M, PIPE_MTE1, EVENT_ID1);

After — the guard and the dead arm are gone; the surviving form is unconditional:

set_flag(PIPE_MTE1, PIPE_M, EVENT_ID0);
pipe_barrier(PIPE_M);
wait_flag(PIPE_MTE1, PIPE_M, EVENT_ID1);
TMATMUL_ACC(v48, v48, v59, v64);
set_flag(PIPE_M, PIPE_MTE1, EVENT_ID1);

Replica 0's guard is untouched — if (v50 == v10) with v10 = 0. That is the test that actually seeds the accumulator, and keeping it is the thing this change must not get wrong:

if (v50 == v10) { pipe_barrier(PIPE_M); TMATMUL(v48, v51, v55); }
else            { pipe_barrier(PIPE_M); TMATMUL_ACC(v48, v48, v51, v55); };

---

Measured impact

Measured in the sim image (pypto3-hw-native-sys:sim), Ascend910B, Default pipeline, before vs after on otherwise identical input. TMATMUL( counts non-accumulating call sites — the ones reachable only when a predicate is true, so it is a direct proxy for the dead arms.

Kernel	scf.if	TMATMUL(	TMATMUL_ACC(	emitted lines	emitted bytes
128x512 @ 512x128, F=2	2 → 1	2 → 1	2 → 2	230 → 221	9946 → 9770
explicit split-K, stage=4	4 → 1	4 → 1	4 → 4	565 → 538	23830 → 23298
explicit split-K, stage=2 (loop collapses) — control	0 → 0	1 → 1	1 → 1	244 → 244	12113 → 12113

• ≈ 9 lines / ≈ 176 bytes of kernel C++ removed per dead arm, and it is exactly linear in F−1: at stage=4 (3 dead arms) it is 27 lines and 532 bytes.
• Non-accumulating MAD call sites fall by (F−1)/F — half at F=2, three quarters at F=4. Every surviving replica was emitting two arms; now each emits one.
• The third row is the control: when the main loop collapses, nothing should change — and nothing does, byte for byte.
• The dead arm is removed from the pre-ptoa IR, so the passes that walk it — CanonicalizeIOOrder's topological sort, MemoryReuse's pack, ptoas InsertSync (which had sized and placed event ids around it) — no longer do.

Blast radius

Scope	Count
Files containing pl.pipeline(...) across pypto/tests, pypto/examples, pypto-lib/models	122
Replicated loops (stage >= 2)	493 (449 × stage=2, 40 × stage=4, 4 × stage=3)
Test files / test functions executing the replication path	30 / 900
… restricted to the matmul subset (where a predicate can exist)	23 / 817
Matmul call sites in pypto-lib/models	409 (largest single model: 18)

The fold only fires where the K-loop survives replication and the clone carries the AutoTile seed test, so the per-kernel figures above apply to a subset of those loops — but to every one of them in the shape that has a split-K matmul at stage >= 2.

Coverage note (the reason this shipped)

Of ~15,250 pre-existing tests, zero detect the difference: the full suite is green both with and without the fold, so 817 tests exercise the matmul replication path while asserting nothing about the emitted shape. That gap is how the defect lived next to an explicit, documented assertion of the opposite. This PR adds 3 tests, 2 of which fail if the fold is removed.

---

What is not claimed

• No runtime win. The dead branch is on a compile-time constant, so CCE/LLVM scalar evolution may already delete it. Proving either way needs a kernel disassembly, which this PR does not include. The measured win is in IR and emitted-code size, not in cycles.
• No compile-time wall-clock measurement. The IR reduction is real and monotone, but no timing was taken.
• The 4-operand And(user_cond, ko == 0) composition is deliberately left alone (see scope table).

---

Tests

tests/ut/ir/transforms/test_lower_pipeline_loops.py — TestReplicaSeedPredicateFoldIsGatedOnTheLoopBound:

• test_unreachable_zero_folds_the_replica_predicate
• test_reachable_zero_keeps_the_replica_predicate — the pl.pipeline(-2, 6, 2, stage=2) counterexample; fails if the gate is loosened

tests/ut/codegen/test_matmul_init_cond.py:

• TestAutoTiledPredicateFolds::test_surviving_k_loop_folds_the_replica_predicate_but_keeps_the_seed_test — asserts scf.if <= 1, pto.tmatmul == 1, pto.tmatmul.acc == 2; fails on the unpatched pass with assert 2 <= 1.

docs/{en,zh}/dev/passes/31-lower_pipeline_loops.md — the fold documented beside the cloning step it follows, in both languages.

Verification performed

Sim Docker (pypto3-hw-native-sys:sim), mounted worktree, pip install --no-build-isolation:

• Full tests/ut, -n auto: 15246 passed, 2 failed, 19 skipped, 1 xfailed in 189 s. Both failures are in tests/ut/codegen/test_orchestration_codegen_graph.py and reproduce on the unmodified base, so they are pre-existing, not caused by this change.
• The three touched suites together: 177 passed.
• pre-commit run --all-files: all hooks pass. clang-tidy clean.
• EN/ZH docs parity check: pass.

Developer verification (NPU) — passed 2026-09-28

Run on the frozen head 9ac96bbc — checkout in a pypto3-hw-native-sys:cann9 container joined to the host queue; every job through task-submit on hng-atlas01 (a2a3, one card per run). The failure mode this change could cause is a miscompile (replica 0's seed predicate folded away), so the gate is a golden comparison on device, not a crash-free run:

Step	Result
Device sweep — tests/st/runtime/ops/test_matmul_acc_init_cond.py (init_cond split-K) + tests/st/runtime/ops/test_auto_tile_matmul.py (AutoTile split-K)	46 passed, 3 skipped (pre-existing _PTOAS_ONLY_SKIPS marks)
Device sweep — tests/st/runtime/ops/test_matmul.py -k "autol0 or pipeline"	34 passed (199 deselected) — no stage=4 ST exists in tests/st
Emitted-kernel A/B on the two builds (731191cf base vs 9ac96bbc head)	scf.if 2 → 1, TMATMUL( 2 → 1, kernel −9 lines; outputs match the golden in both builds

Probe sources and the full command/task record: hw-native-sys/slots-verify/ and plan 124 § NPU verification record in pypto-3.0-notes.

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📝 Walkthrough

Walkthrough

LowerPipelineLoops now folds matching seed predicates in later main-loop clones when compile-time start and step values prove the clone index cannot be zero. Clone 0 and tail clones retain their predicates. Tests and documentation cover the folding conditions.

Changes

Seed Predicate Folding

Layer / File(s)	Summary
Predicate matching and eligibility src/ir/transforms/lower_pipeline_loops_pass.cpp	The pass matches call arguments that exactly compare a clone index with zero, in either operand order. It replaces a match with BOOL false only when the main-loop start and step prove the index nonzero.
Main-loop and tail integration src/ir/transforms/lower_pipeline_loops_pass.cpp	The pass enables folding for eligible main-loop clones after clone 0. Tail clones do not use this fold.
Regression tests and behavior documentation tests/ut/ir/transforms/test_lower_pipeline_loops.py, tests/ut/codegen/test_matmul_init_cond.py, docs/en/dev/passes/31-lower_pipeline_loops.md, docs/zh/dev/passes/31-lower_pipeline_loops.md	Tests check cases where a later clone’s index can or cannot reach zero, and verify the split-K code-generation result. The documentation describes the fold conditions and the stated code-generation consequence when folding does not apply.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Merge Risk: 🔵 Low · up to e4fce

The change is mergeable with bounded risk, but an extreme loop start can disrupt compilation and two tests should be tightened to protect accumulator initialization.

Security Architecture Review

Security architecture risk: 🔵 Low · up to e4fce

The change affects generated matrix-accumulation kernels, but the new behavior is narrowly gated and no new access boundary or exposed interface was identified. Coverage of downstream uses is incomplete, so the risk is low rather than negligible.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The changed emitted behavior is confined to kernels whose replicated main-loop calls contain the exact eligible predicate; other predicates retain the emitter’s existing handling.

Trust Boundaries and Controls

• observed — The fold requires a constant start and a nonzero bound in the stride’s direction; it excludes clone 0 and tail clones. These are correctness gates on compiler input, not access controls.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main change: folding provably false init_cond predicates after pipeline-loop replication.
Description check	✅ Passed	The description is directly related to the changeset and explains the implementation, soundness conditions, scope limits, tests, documentation, and verification results.

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Codex Review

Reviewed e4fce2f69a400818352f74e63b791c9189f3c1bb against base 731191cfdd9fe067749d97721ab1513f058ba177.

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Reviewed the full diff and surrounding code. The current gate preserves the reachable-zero case described in the PR, and there are no prior inline findings to reassess. One new overflow issue remains. Tests were not run.

1 finding(s) posted as inline comments.

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Actionable comments posted: 3

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In @src/ir/transforms/lower_pipeline_loops_pass.cpp:
- Line 273: Update the replica-bound calculation in LowerDynamic to check
whether adding step to *start is representable as int64_t before performing the
addition. Return false on overflow, and retain the existing sign check on the
computed bound.

In @tests/ut/codegen/test_matmul_init_cond.py:
- Around line 311-314: Update the `scf.if` count assertion in the test to
require exactly one branch, ensuring replica 0 retains its accumulator-seeding
branch while compile-time-impossible replicas emit none.

In @tests/ut/ir/transforms/test_lower_pipeline_loops.py:
- Line 740: Update the `predicates[1]` assertion to match only `False` or
`pl.const(0, pl.BOOL)`, including the boolean type, so a true or otherwise
nonzero folded predicate cannot pass.

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• docs/en/dev/passes/31-lower_pipeline_loops.md
• docs/zh/dev/passes/31-lower_pipeline_loops.md
• src/ir/transforms/lower_pipeline_loops_pass.cpp
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Reviewed the full rebased diff and discussion. The earlier P2 overflow finding remains fixed: the current code checks int64 bounds before adding start and step. No actionable P0–P2 issues remain.

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