metainterp: review follow-ups, guard-failure and back-edge costs, and the loop header's missing virtualizable normalization - #1657
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Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review. WalkthroughThe change updates bridge skip numbering, entry-token resolution, guard ownership propagation, resume virtual-information caching, guard failure checks, loop-header normalization, and recorder panic handling. It also updates related comments and triage documentation. ChangesRuntime control flow
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🔵 Low · up to The PR changes an exported JIT resolver name and callback contract, so downstream callers outside this repository may require coordinated migration; in-repository usage is clean, making the change mergeable with explicit owner awareness of external API compatibility. Sequence Diagram(s)sequenceDiagram
participant CompiledRun
participant CompileResult
participant JitDriver
participant ResumeStorage
CompiledRun->>CompileResult: provide rd_loop_token and exit_layout
CompileResult->>JitDriver: provide owning green key
JitDriver->>ResumeStorage: request virtual_infos()
ResumeStorage-->>JitDriver: return cached VirtualInfo slice
JitDriver->>JitDriver: call must_compile_with_owning_key
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Full details: Title checkExplanation The title clearly summarizes the main changes: metainterpreter review follow-ups, guard-failure and back-edge cost reductions, and loop-header virtualizable normalization. It is specific and related to the changeset, although somewhat long. Full details: Docstring CoverageExplanation Docstring coverage is 81.25% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 6 files. (1 skipped: 1 too large.) ✨ Finishing Touches📝 Generate docstrings
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…sured against, and start the bridge sequence for MAJIT_SKIP_BRIDGES alone `guard_may_bridge` compared the register banks against the spans taken before the blackhole ran, and only then asked whether the walk had crossed a frame boundary. Each pop rebinds `bh` to its caller, so the spans describe a bank that is no longer the one being indexed; a caller whose bank is shorter panics before the frame-count test can reject the bridge. Put that test first, where the comment beside it already said the answer was decided. `bridge_fuel_take` returned before allocating a sequence number whenever `MAJIT_MAX_BRIDGES` was unset, so a run configuring only `MAJIT_SKIP_BRIDGES` numbered nothing and skipped nothing — the gate-triage entry and the commit that added it both describe declining the listed numbers. The two gates name positions in one sequence, so either one alone now starts the count. `MAJIT_BRIDGE_FUEL_LOG=1 MAJIT_SKIP_BRIDGES=0` on `logo.aheui` reports `bridge #0 SKIPPED` and then `bridge #1`, against no fuel line at all before; with neither gate set nothing is numbered. The gate-triage entries for `MAJIT_SKIP_BRIDGES` and `MAJIT_GUARD_RESUME_REFUSE_PENDING_FIELDS` are reworded for the same reason: the first named `MAJIT_MAX_BRIDGES`'s sequence for numbers that are not its alone, and the second used `siting` where `placing` is the ordinary word. Assisted-by: Claude
The ban was justified by "what the recorder panicked on is a property of the jitcode it is walking, not of the values this attempt happened to see". The comment on the `catch_unwind` it reads from says otherwise, and says it about the only class it was built for: "Catch panics from BigInt overflow in runtime stack operations." An overflow is a property of the values, so a key whose loop overflows on one iteration was disabled for every later one. What the ban bought was small. It did not stop a retry loop — `abort_trace` already converges through `MAX_TRACE_ABORT_COUNT`, which is what banned the aheui key that motivated it — it only moved the ban from the fifth attempt to the first, and the panic that key raised was fixed at its source by the argument-register coloring commit. `TraceCtx::recorder_panicked` had no other reader and goes with it; `abort_after_panic`, which records that the frames name no resumable position, is unaffected. Assisted-by: Claude
`effectinfo.py:14` and `:147` are `EffectInfo._cache` and the `key in cls._cache` probe in `EffectInfo.__new__`. `unroll.py:454-457` is the force-then-flush-then-virtual-state that opens `UnrollOptimizer.export_state`, and in the checkout at hand those line numbers land on `_expand_info` instead, which is the drift the line-citation check exists to catch. Assisted-by: Claude
`guard_resume_pc` was resolved through `get_merge_point_pc` right after the `must_compile` tick, on every guard-failure event. Its single reader is the fallback block reached when the resume chain raised and `deliver_blackhole_exception` returned `None`; every other exit from the guard arm returns the pc the bridge or the blackhole reported. The resolution now happens in that block. Assisted-by: Claude
`execute_assembler_at_dispatch_key` built a `CompiledExitLayout` for every non-finish exit, including a plain loop back-edge JUMP (`fail_index == u32::MAX`). Every reader of the field opens it past its own JUMP arm: the two driver run loops restore the exit values and return, and the drain loop breaks. The layout build and the `descr_owning_jct` resolution that feeds its fallback now run on the guard arm only, and `CompileResult::exit_layout` is `None` for a JUMP exit. Assisted-by: Claude
The exit gathering walked `descr_owning_jct` to seed the layout fallback and `must_compile_with_values` walked it again on the same descr in the same guard-failure event; each walk is a registry lock, a `Weak::upgrade` and an `Arc` clone. `CompileResult` now carries the resolved `rd_loop_token`, the two driver run loops hand it to the new `must_compile_with_owning_key`, and `must_compile_with_values` becomes the wrapper that resolves the key for callers that hold only the descr. The walk itself moves to `MetaInterp::owning_key_for_descr`, with the comments that described it. Assisted-by: Claude
The blackhole resume arm converted every `RdVirtualInfo` on the failing guard's storage into a `VirtualInfo` on each guard-failure event. `resume.py ResumeDataReader._prepare_virtuals` is a bare assignment because `ResumeDataVirtualAdder.finish` builds the reader's virtuals at compile time. The conversion reads nothing but the `RdVirtualInfo` — the tagged fieldnums stay tagged — and `rd_virtuals` is immutable once the guard owns it, so `ResumeStorage::virtual_infos` now builds the list on the first resume off a guard and shares it. `rd_virtual_to_virtual_info` keeps its reader-shaped signature and forwards to the pure `virtual_info_from_rd`. Assisted-by: Claude
The comment on the guard-only exit-layout build listed the run loops and the drain loop that read the field. State the property of a back-edge exit instead. Assisted-by: Claude
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| let guard_resume_pc = self | ||
| .get_merge_point_pc(owning_key, trace_id, fail_index) | ||
| .map(|pc| pc as usize) | ||
| .unwrap_or(target_pc); |
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When guard recovery reaches this fallback and owning_key == green_key (the normal same-loop case), remove_compiled_loop(green_key) has already removed the entry that get_merge_point_pc searches. The lookup therefore returns None and silently substitutes target_pc instead of the guard's recovery-header PC, potentially resuming at the loop entry and re-executing bytecode against already-recovered state. Capture the recovery PC before removing the compiled metadata.
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Accepted and fixed in 30d3cac3563 — the mechanism is exactly as you describe: get_merge_point_pc opens with compiled_loops.get(&green_key)?, so after remove_compiled_loop the ? short-circuits and .unwrap_or(target_pc) silently substitutes the loop entry against state recover_after_compiled_run has already rewound.
The resolution now sits at the head of the fallback block, ahead of the retirement calls, and reads the recovery pc straight off the exit_layout the caller still holds (recovery_layout.frames.first().header_pc — the same value get_merge_point_pc's success path returns, with zero index lookups), keeping the keyed lookup only as the fallback for a synthesized layout without recovery metadata. The cost win that motivated sinking the resolution is preserved: it is still paid only on this path.
One honest caveat: the corpus gates cannot exercise this block (it is unreachable for an exception-less interpreter), so the verification is the code reading plus unchanged counters/outputs on all 8 workloads, not a behavioural repro.
— commented by Claude
`back_edge_internal` resolved the cell key, then read the resolved key's procedure token, then read that same cell's `JC_TEMPORARY` flag — three walks of one bucket chain for one cell, plus a `compiled_loops` probe that the metadata fetch below repeated. `WarmEnterState::resolved_cell_procedure_token` now reports the token and applies the `JC_TEMPORARY` filter off the borrow its own walk produced, and takes `make_key` as an `Option` so the hash-only door shares it. Its `gate` parameter is gone: the remaining conjunct is `compiled_loops`, whose VALUE the entry needs anyway, so the door reads it once instead of probing for presence and then fetching. `JitDriver::resolved_runnable_procedure_token` is renamed `resolved_entry_procedure_token` to match what it now answers — the cell-owned half of the predicate, not the whole of it — and is the back edge's resolver. Counter-identical on logo, aheui.aheui+quine and aheui.aheui+99bottles (whole MAJIT_STATS block, timings excluded); byte gates unchanged. Assisted-by: Claude
`resolved_cell`'s miss arm re-entered `cell_by_key(hash)`, which re-derives the slot and traverses the same chain the resolve had just finished. It has to when `hash` was minted for another bucket, since the cell then sits in a different slot; when `bucket_of(hash) == hash` — every key that took its own bucket's raw hash — the lookup would search the slot already walked, and a cell key names at most one live cell. The walk now notes the cell filed under `hash` itself as it passes, so the miss arm answers from that note and reaches `cell_by_key` only for the minted-elsewhere case. One extra compare per chain node in place of a second traversal, on the arm a never-compiled green key takes every back edge. Assisted-by: Claude
`maybe_start_tracing` asked `is_ceiling_latched_for_key` and then called
`on_back_edge_typed_decision`, which resolves the same cell through the same
typed chain walk and re-tests the same condition — two walks and two
thread-local decision-key borrows per cold back edge, where
`maybe_compile_and_run` binds its cell once and answers every question about
it off that binding.
The decision's own ceiling arm sits above its counter tick and above the
`decay_all_counters` inside `bound_reached`, which is the timing the early
refusal existed to preserve, so dropping the mirror changes neither the
answer nor when it is taken. Slot 81 keeps its population: only one of the
two refusals could fire for an edge, since every state in which the mirror
answered `false` either returns above the decision's ceiling arm or fails
its condition.
`WarmEnterState::is_ceiling_latched{,_for_key}` stay as the readers
`is_ceiling_latched_agrees_with_the_decision_it_mirrors` pins the equality
with.
Counters byte-identical to an unmodified build on logo, aheui.aheui+quine
and aheui.aheui+99bottles; byte gates unchanged.
Assisted-by: Claude
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Inline comments:
In `@majit/majit-metainterp/src/warmstate.rs`:
- Around line 3325-3380: Add a regression test for the ambiguous no-key path in
resolved_cell: create a fixture with two cells sharing the same bucket, call
resolved_cell(bucket, None), and assert its returned key and cell match the
first result from cell_keys_at followed by cell_by_key. Do not rely on a test
that only exercises sole_cell_key or cell_by_key.
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majit/majit-metainterp/src/jitdriver.rsmajit/majit-metainterp/src/lib.rsmajit/majit-metainterp/src/pyjitpl.rsmajit/majit-metainterp/src/warmstate.rs
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Merging this PR will degrade performance by 29.57%
|
| Benchmark | BASE |
HEAD |
Efficiency | |
|---|---|---|---|---|
| ❌ | fib_recursive |
2.6 s | 4.6 s | -44.6% |
| ❌ | fib_loop |
1.2 s | 1.8 s | -34.32% |
| ❌ | fannkuch |
3.3 s | 4.9 s | -32.78% |
| ❌ | nbody |
1.9 s | 2.7 s | -31.51% |
| ❌ | inline_helper |
841.8 ms | 1,092.2 ms | -22.92% |
| ❌ | spectral_norm |
1 s | 1.3 s | -19.41% |
| ❌ | float_loop |
1.2 s | 1.4 s | -17.39% |
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Comparing aheui (86c2957) with main (55e6fb8)
Footnotes
-
6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩
…eader `reached_loop_header` (pyjitpl.py) runs `remove_consts_and_duplicates` over `virtualizable_boxes[:-1]` in place before the list is appended to `live_arg_boxes`. The state-field dispatch model ran it only inside the two merge-point registration branches, and there only over a copy, so the closing JUMP — `collect_jump_args_with_boxes`, which splices a fresh copy of `virtualizable_boxes` — carried whatever the list held. For a guard-origin bridge that list is what `seed_bridge_virtualizable_boxes` decoded, where a TAGCONST element becomes a constant `OpRef`, so an element position of the JUMP into the parent loop was a bare literal. Run the normalization at the top of the loop-header equivalent and hand the result back with `adopt_normalized_virtualizable_elements`, so the splice sees the same list. Assisted-by: Claude
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…n is dropped The unrecoverable-resume fallback resolved `guard_resume_pc` after `invalidate_loop` / `remove_compiled_loop`. `get_merge_point_pc` opens with `compiled_loops.get(&green_key)`, and `owning_key == green_key` on a same-loop guard, so the lookup missed the entry that had just been removed, returned `None`, and substituted `target_pc` — the loop entry, entered against state `recover_after_compiled_run` had already rewound. The resolution moves to the head of the block, ahead of the retirement. It reads the failing guard's own `recovery_layout` header pc first, which is the same value with no index lookup, and keeps `get_merge_point_pc` as the fallback for a layout that carries no recovery metadata. Assisted-by: Claude
`resolved_cell`'s `(count > 1, make_key: None)` arm reports the raw hash and reaches the cell through the `by_key` candidate noted during the walk, where the readers it folds together reach it by walking back from the key. The test builds a two-candidate bucket, calls `resolved_cell(bucket, None)`, and pins the returned key against `cell_keys_at` and the returned cell against `cell_by_key` by pointer identity. Assisted-by: Claude
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Inline comments:
In `@majit/majit-metainterp/src/jitdriver.rs`:
- Around line 7004-7005: Update the comment near the recovery_layout header_pc
parity logic to replace the line-based upstream reference “compile.py:710” with
the relevant upstream symbol; only use an allow-line-citation marker if the
exact line number is essential.
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The comment named `compile.py:710`. That line is the `else` arm of `AbstractResumeGuardDescr.handle_fail`, where a guard failure that does not compile goes to `resume_in_blackhole` and the rebuilt frame chain is `setposition`ed at the pc the guard's resume data encodes. Name those symbols and what they say instead of the line. Assisted-by: Claude
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Two batches. The first carries the three changes #1649's review produced after its merge snapshot; the second removes four per-event costs from the guard-failure path, each a divergence from the upstream shape that the fix closes.
Review follow-ups to #1649
metainterp: test the frame count before indexing the banks it was measured against, and start the bridge sequence for MAJIT_SKIP_BRIDGES aloneA panic reachable from the blackhole resume loop.
guard_may_bridgecomparedbh.registers_i[sf_i]and its two siblings against spans captured before the resume walk. The resume loop rebindsbhto the caller on every pop, so once a frame boundary was crossed the spans described a bank that is no longer the one being indexed — and if the caller's bank is shorter, the slice access panics beforebh_frames_popped == 0gets to reject the bridge. The frame-count test now comes first inside the arm, so&&short-circuits the three indexings unlessbhis still the frame the spans were measured against.MAJIT_SKIP_BRIDGESdid nothing on its own.bridge_fuel_takereturned early wheneverMAJIT_MAX_BRIDGESwas unset, so no sequence number was ever allocated and the skip list named positions nothing allocated. Either gate alone now starts the count. Verified —MAJIT_SKIP_BRIDGES=0alone prints@@@FUEL bridge #0 SKIPPEDfollowed by@@@FUEL bridge #1.metainterp: stop banning the green key on the first recorder panicReverts the
permanent || ctx.recorder_panickedpromotion added in #1649. Thecatch_unwindthat produces the flag documents its own class as value-dependent ("Catch panics from BigInt overflow in runtime stack operations"), so the same key can panic on one set of values and record cleanly on the next.ir, metainterp: cite two upstream references by symboleffectinfo.rsnow citesEffectInfo._cacheand thekey in cls._cacheprobe inEffectInfo.__new__;optimizer.rsnames the force-then-flush-then-virtual-state sequence that opensUnrollOptimizer.export_state.Guard-failure path: four per-event costs removed
An audit of the path from a compiled guard's failure through blackhole resume found four costs paid on every event (~250 of 251 events never bridge) that upstream does not pay. Each fix moves the code toward the upstream shape; all JIT counters are byte-for-byte unchanged (verified against a control build on 8 workloads — loops/bridges/aborts/guard_failures identical, outputs byte-identical).
metainterp: compute the guard resume pc inside its only readerguard_resume_pcwas resolved throughget_merge_point_pc— three IndexMap lookups — on every guard failure, and read only in the fallback reached when the resume chain raised anddeliver_blackhole_exceptionansweredNone.handle_failhas no such lookup; the resolution now happens in that block alone.metainterp: build the exit layout for guard exits onlyexecute_assembler_at_dispatch_keybuilt aCompiledExitLayout— anExitTypescopy, threeArcclones, and aBox— for every non-finish exit, including a plain loop back-edge JUMP whose callers never read it. The layout build and thedescr_owning_jctresolution feeding its fallback now run on the guard arm only. (Plus a comment-only follow-up removing a caller name from the new comment.)metainterp: carry the failing guard's owning loop key out of the runThe exit gathering walked
descr_owning_jct(registry lock +Weak::upgrade+Arcclone) andmust_compile_with_valueswalked it again on the same descr in the same event.compile.py handle_failreadsresumedescr.rd_loop_tokenonce andmust_compilenever re-derives identity.CompileResultnow carries the resolved key;must_compile_with_valuesremains as the resolving wrapper for callers that hold only the descr.metainterp: build a guard's reader-shaped virtuals onceThe blackhole resume arm converted every
RdVirtualInfoon the failing guard's storage into a reader-shapedVirtualInfoon each event —2N+1allocations plus descrArcbumps — although the conversion is a pure function of the storage (rd_virtual_to_virtual_infoignores three of its four parameters).ResumeDataVirtualAdder.finishbuilds the reader's virtuals at compile time and_prepare_virtualsis a bare assignment;ResumeStorage::virtual_infosnow builds the list once behind aOnceLockand shares it.Verification
cargo test -p majit-metainterp -p majit-ir --features majit-metainterp/dynasm— 0 failed.7fcdbff…); self-interpreted quine/40col/99bottles--jit≡--no-jitbyte-for-byte.Back-edge path: cell questions answered from one chain walk
A guest interpreter whose merge point sits on every guest branch pays the back-edge door on every taken branch.
maybe_compile_and_runinlines its cell walk precisely "to avoid computing the hash several times"; the port had grown three chain walks and two map probes per warm edge.metainterp: answer a back edge's cell questions from one chain walkback_edge_internalresolved the cell key, walked back for the procedure token, walked back again for theJC_TEMPORARYfilter, and probedcompiled_loopsfor presence right before fetching its value.WarmEnterState::resolved_cell_procedure_tokennow reads the token and applies the temporary filter off the borrow its own walk produced, and the presence probe becomes the fetch. Warm entry: 3 walks + 2 probes → 1 walk + 1 probe.metainterp: decide a resolve's miss arm inside the walk that found itThe resolve's miss arm re-entered
cell_by_key(hash), re-traversing the chain just walked — on the arm a never-compiled green key takes on every back edge. The walk now notes the cell filed under the hash itself as it passes.metainterp: let the counter decision own the abort-ceiling refusalmaybe_start_tracingpre-tested the abort ceiling and then delegated to the typed decision, which resolves the same cell and re-tests the same condition. The decision's ceiling arm already sits above its counter tick, which is the ordering the early refusal existed to preserve; the mirror was pure duplication. Slot counters are unchanged — only one of the two refusals could ever fire per edge.Verification
Same gates as above (2380 tests / 0 failed; logo + self-interpreted quine and 99bottles byte-exact,
--jit≡--no-jit; the full stats dict — every counter — byte-identical to a control build). Child-CPU on the arm that isolates the door (MAJIT_THRESHOLD=1e9, compilation off): 0.95–0.96x across four independent interleaved runs; the fully-jitted arms sit inside host-load noise.A silent wrong answer: the loop header never normalized the virtualizable elements
A guest interpreter driving its virtualizable through the state-field model produced deterministic wrong answers (and one hang) across a wide band of bridge-threshold settings — silently, with stable counters. The root cause is one upstream statement the port never ran:
reached_loop_headerrewritesvirtualizable_boxesin place throughremove_consts_and_duplicates, so no element position holds a Const (or a repeated box) by the timecompile_tracecloses a bridge. The port had faithful implementations of both halves —TraceCtx::remove_consts_and_duplicates, andadopt_normalized_virtualizable_elements, whose own doc comment states the write-back requirement — but the adopter had zero callers, so the closing JUMP spliced the raw decoded list and aTAGCONSTelement landed as an unguarded literal in the jump into the parent loop.cut_trace_from_with_consts'sremap_refshort-circuits on constants, so the literal was never rewritten to the LABEL's inputarg.metainterp: normalize the virtualizable element boxes at every loop headerRuns the normalization over the element block (identity excluded, upstream's
endindex) at the loop header, beforeGUARD_FUTURE_CONDITION, and hands the result back through the adopter. Alternatives measured and rejected: emitting a live array read would read stale memory (nothing writes a token-less state vable back on the guard-failure path — 236 box slots also disagreed with the heap across 93 seeds), and aGUARD_VALUEwould never fire (the decoded const values are correct; only the bridge's use of them was wrong). A mode switch isolating the two halves showed the constant half is causal; both are kept, as upstream.Verification: the self-interpreted quine is byte-correct at every bridge threshold in 12..120 (109 values, previously wrong or hanging across 15..110) and at the default; logo and the self-interpreted 99bottles byte-exact,
--jit≡--no-jit; 77-program corpus differential clean (the five divergent rows are byte-identical to the pre-fix control — non-terminating orundefined/family); loops/bridges/guard-failure counters unchanged, ops-after-opt unchanged (the addedSAME_ASops fold away).Summary by CodeRabbit
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