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fix: prove stack-carried locals by predecessor coverage - #4544
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Included review availability: Your plan provides up to 8 included reviews per hour; 6 remain after this review. 📝 WalkthroughWalkthroughThe compiler replaces phi-like local detection with stack-coverage analysis. The analysis validates control-flow predecessors and terminators, including short-circuit paths, calls, loops, entry blocks, catch handlers, awaits, and unwind paths. Compiler and BAML tests cover these cases. ChangesStack coverage for local bindings
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In `@baml_language/crates/baml_compiler2_emit/src/analysis.rs`:
- Around line 1195-1210: Update predecessors_cover_block to immediately reject
body.entry and catch-region handler blocks before consulting predecessors or
marking coverage, so implicit function-entry and VM-unwind entry paths cannot be
proven covered solely by CFG edges. Reuse the existing MIR body and
handler-region symbols to identify these blocks, while preserving normal
predecessor-based coverage analysis for other blocks.
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baml_language/crates/baml_compiler2_emit/src/analysis.rsbaml_language/crates/baml_tests/baml_src/ns_fixtures/ns_short_circuit_locals/short_circuit_locals.bamlbaml_language/crates/baml_tests/tests/short_circuit_locals.rs
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is_phi_like accepted a join whose predecessors all assign the local, but never checked that those were the local's only definitions: an earlier `let x = 0` was still emitted as a bare push that nothing popped, so the shape grew the operand stack once per loop iteration. is_short_circuit_phi had the mirror hole on the other side. Requiring the ShortCircuit's join to be the use block does not make the other edges into that block safe, because merge_passthrough_blocks can retarget a join onto a block that also carries an unrelated Branch edge. The name ban that covered that case in practice also gave up the optimization for every named single-use short-circuit binding. Both are now one predicate. A local is stack-carried when its single use sits in a join whose every incoming edge provably leaves the value on the stack -- a trailing assignment plus Goto, a ShortCircuit joining there, or an empty passthrough proven the same way one level up -- and when every definition of the local is one of the pushes that walk accounted for.
ns_fixtures/ns_short_circuit_locals holds one function per shape of the predecessor-coverage rule, so its bytecode snapshot is where a classification change shows up: a fully covered chain and a mixed short-circuit/assignment join stay on the stack, while a merged join with a Branch edge and an initialized local assigned in both arms of a branch get slots. The two loop shapes also get result-asserting engine tests. Regenerated snapshots: - ns_parser_expressions: `let d = true || false && false; return d` is back to its ten-instruction form, with no slot for `d`. - ns_while_let: `body_local`'s `cur` now gets a slot. It has an assignment in a block that is not a predecessor of the loop header, which was emitted as a push with nothing to pop it -- one leaked operand per iteration through that branch, latent because the loop's own values never reach the branch's condition. - ns_operators and the two stdlib namespaces: `a || call(...)` shapes lose their carry, because the call terminator that defines the local on the rhs edge is not one of the covered predecessor forms. Correct, one store/load short of what it was.
A predecessor whose Call or VirtualCall returns into the join leaves the
result on the stack there: both emit the call opcode, then
emit_store_place on a destination that is stack-carried (a no-op), then
the jump to target. Treat that as covering the join.
Without it `a || b.starts_with(c)` kept its destination in a slot, which
cost baml.ops.Compare.le and every `x catch (e) { ... }` or `a?.m()` join
a store/load pair.
Await, AwaitAny and SysOp stay out: all three suspend the engine, and the
note on is_call_result_immediate records that the await opcodes rewind and
re-execute across that suspend. Spawn defines a future at `resume`, not a
call result. None of them covers a join anywhere in the corpus, so
admitting them would be speculation rather than a proof. A call that
unwinds into the join is rejected too -- Terminator::successors reports
the throwing edge as well, and nothing is pushed along it.
The regenerated snapshots only ever drop a store_var/load_var pair and
renumber the slots freed by doing so; no namespace gains an instruction.
baml.ops.Compare.le, operators.ord_in_short_circuit and
testing.pattern_may_match_descendant are byte-identical to their form
before the short-circuit locals work started.
The coverage walk reads `build_predecessors`, which is built purely from `Terminator::successors`, so it only sees entries that traverse a CFG edge. Two blocks are also entered without one, with nothing on the stack: the entry block on every call of the function, and a catch region's handler when the VM unwinds into it. A call's `unwind` target is a real `successors` edge, so a handler reached that way already has the predecessor recorded. A same-frame panic does not: an overflowing `add_int` or a division by zero enters the handler from the middle of a block with no edge at all. `compute_rpo` already has to seed those handlers separately because nothing reaches them from the entry block, and `collect_def_use` records the VM's write of the error local at the handler as an implicit use. Refuse both as the block being covered. A handler that is merely a predecessor is untouched -- it reaches its successor over a real edge, having pushed the value like any other predecessor -- so no corpus snapshot moves.
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Stacked on #4508.
The parent PR's guards fix #4421 but overshoot in one direction and undershoot in another:
let x = a && bloses the stack-carry optimization, even where it is sound.is_phi_likepredicate still accepts the same broken shape. At the parent head, this program leaves one value on the VM operand stack per loop iteration, because nothing checks for definitions outside the covering predecessors:for (let i in items) { let x = 0; if (i > 0) { x = 1 } else { x = 2 } total = x + total }This replaces
is_phi_likeandis_short_circuit_phiwith one predicate that proves the property the emitter actually relies on: a local may ride the operand stack only when every predecessor of its single use block leaves its value on top — a trailing assignment falling through, aShortCircuitjoining there, an empty passthrough whose own predecessors are covered, or aCall/VirtualCallreturning into it — and no definition of the local exists outside that covered set. The stack-carry simulation starts at the use block and never inspects definitions or predecessors, so this predicate carries the entire soundness burden. In particularjoin == use_blockalone is not enough:merge_passthrough_blockscan retarget a short circuit's join onto a block with unrelated incoming edges, as inlet x = false; if (c) { x = a && b } x.The local's name no longer matters. Before (parent) and after (this PR) for
let d = true || false && false; return d;:The coverage proof also extends to joins reached through call results —
a || b.starts_with(c),expr catch (e) { ... },a?.m()— sobaml.ops.Compare.leand the other short-circuits-over-calls return to their pre-#4508 bytecode, and catch/optional-chain joins across the corpus drop a store/load pair each. Every changed snapshot hunk removes instructions; none adds any. The stray-definition rule also fixes a second latent leak, visible inwhile_let.body_local's snapshot, wherecur = nullin a non-predecessor block used to be an unbalanced push.Summary by CodeRabbit
Bug Fixes
Tests