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The -O3 residency exception "gate that stands today" is only a debug print

Found by the invariant sweep ([[audit-a-a-comment-asserting-an-invariant-is-a-claim-about-a-sibling-arm-nobody-checked]]), pass 5. Filed as a Track U question, not a bug — I could not settle it read-only and the code landed today, so guessing at intent is the wrong move.

What the comments say

compiler/ir_codegen.inc:10100-10101:

"the exception landing pad refreshes residents FROM their frame slots, so it is the one slot reader that residency cannot see through. Anything that stops keeping the slot current must refuse a body that has one. Conservative default is True."

compiler/ir_codegen.inc:10035-10036:

"This is the axis the -O3 item-3 gate actually turns on, and it is finer than the RcProcHasExc gate that stands today."

What is measured

RcProcHasExc appears at exactly four sites:

line use
10036 the comment above, calling it a gate that stands
10102 RcProcHasExc := False;
10104 set True when an IR_EXC_ENTER is found
10261 ' exc=', Ord(RcProcHasExc), — inside if PxxDbgEnabled('a.resid')

SymWrittenInProtectedSpan — the finer axis the first comment says the gate "actually turns on" — appears at four sites too: its own declaration (10066), two lines of its own body, and 10262, the next argument of the same WriteLn.

Neither symbol appears in any if, any Exit, or any assignment that reaches codegen. The residency assignment's real early-exits are OptLevel < 3, CurProc < 0, RcSuppressAssign, TargetArch <> TARGET_X86_64, CurProcIsGenerator or CurProcIsStackless, and IR_ASM — no exception test among them.

The two readings, and why I am not choosing

  1. No gate is needed. The header at 10024-10032 lists the landing refresh as one of the choke points: "the IR_EXC_ENTER landing refresh covers the longjmp rollback". If that refresh is unconditionally correct, residency needs no refusal, both symbols are pure instrumentation for the next slice, and the defect is only that 10036 says a gate "stands today" when it does not.
  2. A gate is needed and is not there. 10100's "must refuse a body that has one" is written as a live requirement in the imperative. If it is, an -O3 x86-64 body with a try currently gets residency with no refusal, and correctness rests entirely on the refresh being complete.

Reading 1 is more likely — the surrounding prose reads like a designed instrumentation step ahead of "item 3" — but "the gate that stands today" is not a phrase you write about a debug print, and I would rather ask than assume.

What I am asking

Is the RcProcHasExc / SymWrittenInProtectedSpan pair instrumentation ahead of a gate, or a gate that was never wired?

e967f90387 (10036) and d2eafe1e39 (10100) are both from 2026-08-28/29 — hours old at filing — so this is in-flight work and its author will answer in seconds what I could not settle in an hour of reading.

Why the audit flags it at all

It is the sweep's shape with the arms one level apart: a comment asserting that an enforcement exists, and the enforcement being a print statement. The phrase "the gate that stands today" is a claim about the state of the code, adjacent to the code, and the code does not support it. Whether it is a documentation slip or a real hole, that sentence is the reason nobody has checked which.

Gate

None — this is a question. Whatever it resolves to takes Track A's gate.

RESOLVED 2026-08-30 — answered by the author. It is (a): the gate is real and cross-file.

Answered by frank-optimize-b4, the author of both comments, routed to it by the coordinator rather than left in the U queue (a U question whose author is awake is an unrouted question, not a human-judgement one). No bug-a-* follows.

RcProcHasExc — live, and enforced in a different file

Consumed by two if RcProcHasExc then Exit; in compiler/symtab.inc: ResidentSlotIsDead:5337 and FloatResidentSlotIsDead:5393 — exactly the two functions that decide whether a resident's frame slot may stop being kept current, i.e. precisely the passes the comment says it gates.

ir_codegen.inc only sets it (:10102) and prints it. So a grep scoped to ir_codegen.inc finds a variable that is assigned, displayed, and never tested — which is the exact signature of a forgotten gate. The inference was correct for the evidence; the evidence was file-scoped.

SymWrittenInProtectedSpan — instrumentation-only, and cannot leave a hole

It is unwired as reported. That is not a missing gate, and the argument is containment rather than intent: the coarse gate is strictly stronger than the fine one. RcProcHasExc refuses any body containing an IR_EXC_ENTER; SymWrittenInProtectedSpan would refuse only the symbols written inside a protected span. Every symbol the fine gate would refuse is inside a body the coarse gate already refused. So wiring it can only ever relax — which is the point of "-O3 item 3" — and leaving it unwired cannot open a hole.

Confirmed live rather than argued: test_o3_resident_exc agrees at -O0 and -O3 across all seven cases (OUT/IN/MIX/NEST/FIN/PAR/THR), binary 8cb0ce6ce208.

The real defect, and the author is fixing it

ir_codegen.inc:10036 says "the RcProcHasExc gate that stands today" without saying where it stands, and :10100 says a slot-reader "must refuse" such a body without naming the two functions that do the refusing.

"A cross-file gate that is described in file A and enforced in file B is readable exactly once — by whoever wrote it, on the day they wrote it."

Both call sites are being named in the comments, so the next reader gets the answer from the text instead of from the author being awake. That is the whole remainder of this ticket.