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A NilPy generator fails on wasm32 while three other targets agree

def g(n):
    i = 0
    while i < n:
        yield i
        i = i + 1
s = 0
for x in g(4):
    s = s + x
print(s)

native / i386 / arm32 / aarch64: 6, rc=0. wasm32: Unhandled exception, rc=1, exit with invalid exit status outside of [0..126), backtrace at wasm function 1751. The BUILD exits 0 and prints ok:.

Not the double-write bug ([[bug-a-wasm32-emits-a-separate-function-per-compileast-call-so-a-proc-built-in-two-calls-loses-a-body]]): the rewrite counter prints nothing for this source, and it is proven working on the same binary — it fires on procedure Fill(out s: string) and on test/test_managed_var_param.pas.

Correction — this ticket was filed with three rows that were already fixed

As first filed it claimed a default argument, two defaults and a user-written __init__ also trapped on wasm32 (rc=134), and was ranked 55 on the strength of "ordinary NilPy is unusable there". All three pass at 6f86e8f48, printing the same values as every other target. Only the generator survives, so the ticket is retitled and re-ranked to 40.

How the error was made, because it is the reusable part. The traps were measured at b0275ecc1 and were real then. Between that and filing I pulled to 6f86e8f48 and rebuilt — and re-ran the six shapes only to read the REWRITE COUNTER, never to re-read whether they still failed. So a stale run outcome travelled beside a fresh instrument reading, in the same table, and the stale half looked as current as the fresh one. The fix that landed in between is somewhere in 34179225a..6f86e8f48, which contains two wasm32 commits (8fb2668c0, 9b67b266d); NOT bisected, so this names a range and not a cause.

Caught by franka-29, who could not reproduce a single failing row from the descriptions and said so instead of assuming its own minimisation was at fault.

The causal story the first filing gave — that these shapes call one of the two bodies emitted as unreachable (PyBindHostKwArgs, PyBoundFnCallvnMaskBody) — was INFERENCE from the census line and was never traced. Those two bodies are present in PASSING builds too, so their presence was never the discriminator. Whether the generator reaches one of them is open and is the first thing to check here.

DIAGNOSIS 2026-09-06 (frankwasm) — it is the PARAMETER, not the loop

Parked with the diagnosis banked; no fix. Compiler f2f11cd439e7, and every row below reproduces on stable_pinned too, so none of it is recent.

The repro in this ticket points at the wrong feature, and that cost me an hour. Its generator has a while loop, so the obvious reduction is "loops in generators", and a six-shape battery appeared to confirm it — including m_yield_after_loop, whose loop sits before the only yield and which still fails. I wrote down "any loop inside a generator body breaks it on wasm32".

That was wrong, and the row that broke it has no control flow at all:

def g(n):
    yield n
for x in g(7):
    print(x)

native 7, wasm32 None. Every failing shape I had took a bound parameter and every passing one did not; I had varied two things and read the correlation off the one I was looking at.

The boundary, stated as the shape that fails

A NilPy generator that READS a parameter yields None on wasm32.

shape native wasm32
def g(n): yield n 7 None
def g(a, b): yield a; yield b 4, 9 None
def g(s): yield s (string param) hi None
def g(n): yield 7 — param declared, NEVER READ 7 7
def g(): yield 7 7 7
def g(): yield 1; yield 2; yield 3 1,2,3 1,2,3
def g(): i=0; while i<3: i=i+1; yield i (loop, no param) 3 3

So: resumption across yields is fine, multiple yields are fine, and a loop in a generator is fine when nothing is parameterised. def g(n): i = n + 1; yield i yields 1 on wasm32 — i.e. 0 + 1 — which is the same fact with the value visible: the parameter reads as zero.

The discriminator that says where NOT to look

A Pascal generator; stackless; with an Integer parameter WORKS on wasm32:

function Gen(n: Integer): Integer; generator; stackless;
var i: Integer;
begin i := 0; while i < n do begin yield i; i := i + 1; end; end;

sum=6 on both native and wasm32 — the exact program shape this ticket's NilPy repro has. So the shared slgen state-machine transform, the one-word slot path, the $pc dispatch and wasm32's unstructured-CFG lowering are all fine. What differs is how a NilPy parameter is REPRESENTED.

Ruled out, by experiment rather than by reading

bug-a-the-wasm32-scope-exit-release-loop-consults-neither-skip-predicate looks like the same bug and is not. WasmEmitManagedLocals genuinely does not consult SymSkipScopeExitRelease (zero occurrences), and its own comment in ir_codegen.inc (the UNCHECKED note, at 13969 as of 2026-09-06; this ticket said 13757 and that line is AstDumpTree today) says that path is UNCHECKED — so it reads like a confirmed cause. I added the guard, exactly as the other six arms have it:

    for i := Procs[CurProc].ScopeBase to SymCount - 1 do
      if (not SymSkipScopeExitRelease(i)) and
         (Syms[i].Kind = skLocal) and not Syms[i].IsRef and (i <> retSym) then

It made things worse. No failing row was fixed, and two PASSING rows regressed: def g(): yield 1; yield 2 went from 1,2 to 1. Reverted. That is data about the model, not a partial fix — whatever the release loop is doing for a generator on wasm32, the no-parameter cases currently DEPEND on it, so that ticket cannot be closed by adding the guard alone and the two tickets should not be merged on resemblance.

Where to look next

AssignStacklessSlots (compiler/pasparser_stmt.inc:~2410) decides a symbol's slot region per type, and its two multi-word arms are the candidates, since the Pascal Integer parameter that works takes the one-word arm:

The target-dependence is suggestive and is NOT on its own an explanation: i386 and arm32 are also 32-bit and this ticket records them passing. So a plain "32-bit assumption" story is already contradicted by the ticket's own data, and whatever is wrong is wasm32-specific on top of the representation.

Also worth reading first: if SymCellPtr[i] >= 0 then Continue; at the top of that loop skips cell-promoted names entirely. If a NilPy parameter is cell-promoted, its slot is the cell POINTER rather than the value, and that is a different object to get wrong. Unmeasured — I did not check whether NilPy parameters are cell-promoted, and it should be the first thing checked rather than assumed, because it would explain the Pascal/NilPy split directly.

bug-a-a-stackless-generator-with-a-variant-parameter-yields-nothing-on-native-and-a-wrong-value-on-wasm32 — found while reducing this. A Pascal Gen(n: Variant) generator produces zero iterations on native, so it is an all-targets bug with a six-line Pascal repro. It resembles this one and has a different signature (this one is correct on native), which is exactly why it is its own ticket.

2026-09-06 (frankwasm, later) — narrowed to ONE step, with the probe

Re-measured at 0f6b627d7 (frankS's Variant-parameter fix), compiler c8dc944237a5. Still broken, unchanged, and that is informative rather than disappointing: it separates this row from the Variant one for good.

repro native wasm32
def g(n): yield n 7 None
def g(n): i=0; while i<n: yield i; i=i+1 6 Unhandled exception
def g(n): for i in range(n): yield i 6 0
def g(s): yield s hi None

frankS predicted this: NilPy Variant params are already IsRef, so they take the by-ref arm and never reach the new predicate. Confirmed by measurement, not taken on trust.

The parameter has NO instance slot at all

PXXDBG=a.slslot on def g(n): yield n:

caller: proc=g nargs=1 instsize=72 paramcount=2
  arg1: storeoff=48 paramsym=… symgenslot=-1 realoff=40 tk=22

symgenslot=-1, and there are no assign: lines whatsoever — AssignStacklessSlots gives this generator no persistent slots, because the parameter is CELL-PROMOTED and the loop's if SymCellPtr[i] >= 0 then Continue; skips it by design ("its storage is the heap cell, and every read and write already spells itself cell^").

So the for-in desugar stores the cell pointer at SL_OFF_SLOTS + 8*(k-1) = 48 by ARGUMENT INDEX, into a slot nothing reads. realoff=40 is the probe computing 48 + 8*(-1), i.e. an address BELOW SL_OFF_SLOTS — the instrument reporting a nonsense offset honestly rather than a plausible wrong one.

That table is byte-identical on native and wasm32, and native works. Third time in this neighbourhood that every compile-time quantity agrees while the behaviour diverges, so the slot machinery is exonerated here the same way it was for the Variant bug.

The step that is actually missing

def g(n):
    n = 5
    yield n

5 on BOTH targets. So the cell storage, the resume, the yield and the whole state machine are fine on wasm32 once the cell holds a value. What is missing is only the initialisation of the cell from the ARGUMENT — GenMakeVariantArgCell's pycell_new followed by cell^ := arg, or the delivery of that pointer to where the body's cell^ reads it.

That is the one step to instrument next, and it is a runtime question on wasm32, not a layout one. Everything either side of it is measured working.

What this retires

The earlier section's "look at AssignStacklessSlots's multi-word arms" is wrong and should not be followed: this parameter never reaches any of those arms. The SymCellPtr line it flagged as "unmeasured, and it should be the first thing checked" was the right instinct, and the answer is yes — it is cell-promoted, and that is why it has no slot.

RESOLVED 2026-09-06 — the cause was a pointer store, not a generator

The narrowing that got there, each row measured on both targets:

shape native wasm32 (before)
def g(n): yield n 9 None
def g(n): yield 1; yield n 1 9 1 None
def g(n): yield n; n = 5; yield n; n = n+1; yield n 9 5 6 None 5 6
def g(n): n = 5; yield n; yield n 5 5 5 5
def g(a,b): a = 7; yield 0; b = 8; yield a; yield b 0 7 8 0 7 8
def g(a,b): a = 7; yield 0; yield b 0 2 0 None

Row 3 is the one that carries the weight: the initial read is empty, then a write lands and SURVIVES two further yields including a read-modify-write. So the cell, the resume, the yield and the state machine were all correct on wasm32 and only the INITIALISATION of the cell from the argument was missing.

Row 5 kills the null-pointer reading: two parameters do not alias, so each cell is distinct and valid, and every value there crosses a yield so no fold can explain it. Row 6 is the sharpest — within ONE generator, one cell is correct and the other empty, differing only by whether the body ever wrote to it. A never-written cell reading None rather than aliasing means a FRESH cell, and pycell_new zeroes VType, which is exactly what None is.

The IR was structurally identical on both targets (caller and callee both diffed; every difference was symbol/proc renumbering), which is what moved the search into the wasm backend.

Root cause and fix: WasmVariantAddr in compiler/ir_codegen_wasm32.inc dispatched on IRKind = IR_LOAD_SYM alone. That opcode covers a load of a VARIANT symbol, whose address is its slot, and a load of a POINTER symbol, whose address is the slot's VALUE; it answered the first for both. Measured bytes for pv^ := 42 into a variant holding 0:

native  1 0 0 0 0 0 0 0 | 42 0 0 0 0 0 0 0    tag=1, payload=42
wasm32 42 0 0 0 0 0 0 0 |  0 0 0 0 0 0 0 0    tag=42, payload=0

Minimal repro, no NilPy and no generator: p: ^Variant; New(p); p^ := 42; answers 42 native and 0 on wasm32.

Two earlier hypotheses in this ticket are now positively retired, not merely unconfirmed. The while loop was never involved. And bug-a-the-wasm32-scope-exit-release-loop-consults-neither-skip-predicate is confirmed a separate defect — adding that guard fixed nothing here and regressed two passing rows.

What the diagnosis got wrong, recorded because the shape recurs: this ticket's earlier storeoff=48 / realoff=40 reading was a true measurement of a.slslot and a false lead. realoff=40 is the probe computing 48 + 8*(-1) for a symbol with symgenslot=-1, i.e. a nonsense address reported honestly. It looked like a caller/callee offset disagreement and there was none: the generator prologue reads offset 48, the caller writes offset 48, and the IR agrees on both targets.

Log

2026-09-06 — BOTH EXPLANATIONS OF THE REGRESSION ARE DEAD, and the residual is open

Recorded because a relay was about to hand bug-a-the-wasm32-scope-exit-release-loop-consults-neither-skip-predicate to another session with this written off as understood. It is not.

Two candidate explanations for why adding the guard regressed def g(): yield 1; yield 2 from 1,2 to 1:

  1. WRONG PREDICATE — that the patch used SymSkipScopeExitRelease where the stackless skip is StacklessPersistentSlotSym. DEAD. symtab.inc:13950 is SymSkipScopeExitRelease := StacklessPersistentSlotSym(i) or ... — the first CONSULTS the second, as reason (1) of two. Reason (2) is the NilPy except C as e binder during unwind-pad emission (4edf60ff9). The two are merged into one predicate deliberately. This was my error, caught by frank-coordinator.
  2. BLANKET vs PER SYMBOL — that the patch skipped the whole loop where the register arms test per symbol, which is the failure f891bbe8e fixed on those arms (a blanket exit also discards the step function's hidden temps, which are not live state and DO need releasing). DEAD. The diff recorded above is per symbol: for i := ... do if (not SymSkipScopeExitRelease(i)) and ..., the same shape the other six arms use.

So: the CORRECT predicate, applied at the CORRECT granularity, in the same shape as the six arms that are right — and two passing rows regressed. That is the finding. Whatever WasmEmitManagedLocals is doing for a generator on wasm32, the no-parameter cases currently DEPEND on the release it performs, and neither of the two obvious models explains why.

Positive control for whoever takes it, verified at cc18bc028 on both targets rather than measured once: def g(): yield 1; yield 2 must print 1 2.

2026-09-06 (later) — MY NEGATIVE RESULT IS MISATTRIBUTED. The patch cannot have caused it.

Retracting the causal claim above, not the observation. frank-coord-core built the same predicate in the shape recorded here, took it to fixedpoint, and found the emitted wasm module for def g(): yield 1; yield 2 BYTE-IDENTICAL with and without the patch. Confirmed here by inspection rather than by a second rebuild, which would have failed the same way: WasmEmitManagedLocals releases only managed locals (PXXStrRelease, PXXDynArrayRelease, and since d58828d8c PXXObjRelease). That program has nothing but integers. The loop has no symbol to iterate, so a skip predicate added to it is a no-op for this program and cannot change its behaviour in either direction.

So 1,2 -> 1 was real and its cause was NOT the patch. I do not know what it was, the tree state is gone, and I am not going to invent one. The likeliest candidate given the same session's other two incidents is a stale binary: I was mid revert/rebuild cycle, and that session separately hit both the git checkout <sha> -- <file> index trap and a genuinely walked seed. A measurement taken against a binary that did not contain the change under test looks exactly like this.

WHAT THIS COSTS, stated because it travelled. The row was relayed around the fleet as a positive control that any change to this loop must keep passing, and it cannot fail for the patch it was guarding — a control drawn from the wrong population, which passes and certifies the instrument. That is this repo's own rule and my row is now an instance of it. The population it must come from is a generator that HOLDS A MANAGED LOCAL ACROSS A YIELD; frank-coord-core measured three such shapes moving the module 35-40KB.

WHAT SURVIVES: nothing about the wasm32 release loop is established by this ticket. Not "the arm has a dependency the other six do not" — that was my inference from a misattributed measurement. bug-a-the-wasm32-scope-exit-release-loop-consults-neither-skip-predicate should be read as untouched by this ticket entirely.

AND A SECOND GAP I MISSED, from frank-coord-core (d58828d8c): WasmEmitManagedLocals was also missing the tyClass arm that all six other copies carry, so every NilPy object bound to a local leaked once per call on wasm32 and no other target. The reason this ticket said "one gap and not two" is worth more than the correction: a MISSING arm answers "does it consult the skip predicate" exactly the same way a present-but-unguarded one does. I compared wasm32 against the register arms on the question I was already asking, and absence and presence-without-a-guard are the same answer to it.