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react-compiler: fix N^4 time in InferMutationAliasingEffects and quadratic post-dominator frontiers - #43259
react-compiler: fix N^4 time in InferMutationAliasingEffects and quadratic post-dominator frontiers#43259robobun wants to merge 3 commits into
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…ratic post-dominator frontiers InferMutationAliasingEffects kept the values of an identifier in a list that it searched for each value it added, and each state owned a copy of every list. A local that N sequential `if`s assign holds up to N values in each of N phis, so the pass took N^4 steps: 36 seconds for 1000 `if`s. - A set of more than three values is stored once per pass in a store that all states share. A state holds its offset. The ids keep their insertion order, which the transitive freeze of a function's captures can observe. A union marks the ids it has in a stamp vector, so it is linear. - The two maps of a state are in chunks of 64 entries that clones share until one of them writes. A clone copies one pointer per chunk, and a merge skips the chunks that both sides still share. A write of the value that is already there does not copy the chunk. - infer_block no longer clones the operands of every phi on each visit. InferReactivePlaces and ValidateNoSetStateInEffects computed the post-dominator frontier of one block at a time, with a walk over every ancestor of the block. post_dominator_frontiers computes all of them from the post-dominator tree in one pass over the edges. The output of all 1,807 upstream fixtures is byte-identical.
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WalkthroughThe compiler now computes post-dominator frontiers for all blocks, shares inference-state storage through interned sets and chunked vectors, and adds performance and capture-freezing regression tests. ChangesCompiler analysis updates
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🟡 Moderate · up to Some loop-shaped components can receive incorrect React Compiler control analysis, so the frontier walk should be corrected before merge. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Inline comments:
In `@src/react_compiler/hir/dominator.rs`:
- Around line 310-325: Update the frontier walk in the dominator computation to
stop before processing target_id when it equals pred, preventing pred from being
added to its own frontier. Preserve the existing post-dominator termination and
frontier deduplication behavior.
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src/react_compiler/hir/dominator.rssrc/react_compiler/inference/infer_mutation_aliasing_effects.rssrc/react_compiler/inference/infer_reactive_places.rssrc/react_compiler/validation/validate_no_set_state_in_effects.rstest/bundler/transpiler/react-compiler.test.ts
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…ntier The test of a `do`/`while` loop decides whether its own block runs again. InferReactivePlaces needs that entry of the frontier to see that a local that only the loop changes follows the test. Without it the compiled component memoizes with no dependency and returns a stale element. Say in the doc comment that the frontier excludes only the blocks that the target strictly post-dominates, and add a test that renders such a component twice.
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I re-reviewed after the two follow-up commits and did not find any bugs, but this is a large rewrite of a fixpoint pass's state representation plus a replacement graph algorithm, so a human look is still worthwhile before merging.
What was reviewed:
post_dominator_frontiersvs the old per-target walk: traced the self-predecessor/no-post-dominator skip, the sorted-edgelast() == preddedup, and the exit-node break; each matches the oldunwrap_or(pred)semantics, including a self-looping test block landing in its own frontier.ValueIdSetStore::union/intern: insertion order is preserved (this-then-new, as the oldinsertloops did),unionreturnsaunchanged whenb ⊆ asomerge_from'schangedstays exact, and the hash-collision fallback compares slice contents so a stale offset can never alias a different set.ChunkedVec::merge_from/setandRefCelldiscipline: noborrow()is held across aborrow_mut(), shared default chunks fromVec::resizeare handled byRc::make_mut/get_mut, andmerge_abstract_valuesis idempotent so the equal-entry skip is sound.
Extended reasoning...
Overview
The diff replaces the per-target post_dominator_frontier/post_dominators_of in src/react_compiler/hir/dominator.rs with a single post_dominator_frontiers that walks each (pred, block) edge up the post-dominator tree, and switches the two callers (infer_reactive_places.rs, validate_no_set_state_in_effects.rs) to index the precomputed map. In infer_mutation_aliasing_effects.rs it deletes the unsafe inline/heap ValueIdSet and the Variables::{Dense,Sparse} enum, replacing them with a Copy 16-byte ValueIdSet backed by a pass-wide interning ValueIdSetStore behind a RefCell, and a copy-on-write ChunkedVec<T> (Vec of Rc<[T; 64]>) for both values and variables. Three tests are added to test/bundler/transpiler/react-compiler.test.ts: a CPU-time-per-statement scaling test, a test pinning that capture-freeze order follows value insertion order, and an itBundled case for a do/while test block being in its own frontier.
Security risks
None: this is compiler-internal dataflow code with no I/O, no user-controlled sizes beyond source-program shape, and the PR removes all unsafe from the file. The only panics reachable are the pre-existing PostDominator::get expect on unknown nodes (the old code called get(pred) on the same set of predecessors) and a u32::try_from on the store length, which would require billions of value ids.
Level of scrutiny
High. It is a ~640-line rewrite of the state representation of a fixpoint pass whose output is observable (the PR itself found that set ordering changes compiled output), plus a new graph algorithm replacing one ported from upstream. I traced the equivalence arguments by hand: the frontier walk stops at ipdom(pred), which is always on the chain from any successor of pred, so pred ∈ frontier(target) iff target post-dominates-or-equals the successor and does not strictly post-dominate pred; the special case for a block that is its own predecessor with no post-dominator reproduces the old unwrap_or(pred) behavior, and a successor of such a block cannot have a post-dominator either, so the chain never reaches it another way. In the store, union builds a followed by new ids of b and returns a itself when nothing was added, so ordering and the changed flag match the old insert loops; intern verifies slice contents after the hash lookup so a collision only costs a duplicate copy. ChunkedVec handles the Vec::resize-shared default Rc via make_mut/get_mut, set is a no-op on equal values to avoid unsharing, and the equal-entry skip in merge_from is valid because merge_abstract_values and union are idempotent. No Ref is held across a borrow_mut. I could not run cargo check or the test suite in this environment, so compile and fixture equivalence rest on the author's report.
Other factors
The timeline shows no CHANGES_REQUESTED review; the coderabbitai and github-actions inline comments were followed by a commit that shortened comments, and their contents are withheld. The scaling test is timing-based (user CPU ratio < 2.5, best of up to three rounds on optimized builds, one round of 300 ifs on debug, try skipped on debug with a stated reason); the margin against the reported 0.5-1.4 ratio is reasonable but a human may want to weigh the flake risk under CI load. The freeze-order test and the do/while itBundled test pin behavior a human reviewer should confirm against the upstream plugin, since they encode observable output differences.
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Problem
bun build --react-compilertakes 44 s for one component with 1000 sequentialifs that assign a local, and 6.6 s for 1000trystatements. A plain build takes 10 ms.InferMutationAliasingEffectskept the values of an identifier in a list, searched it for each added value, and copied every list into each state: N^4 steps for N suchifs.InferReactivePlacescomputed each post-dominator frontier with a walk over every earlier block (post_dominators_of,hir/dominator.rs).Fix
ValueIdSetStore, in insertion order. A state holds its offset.ChunkedVec: chunks of 64 entries that clones share until one writes. A merge skips shared chunks. Nounsafeis left.post_dominator_frontierscomputes all frontiers from the post-dominator tree, one walk per edge.test/bundler/transpiler/react-compiler.test.ts(two of four new tests fail on main) andreact-compiler-fixtures.test.ts. All 1,807 fixtures give byte-identical output. 1000ifs now take 0.12 s. Self-reviewed: 10 concerns raised, 9 addressed, 1 refuted (Notes).Background
InferMutationAliasingEffectsis a forward dataflow pass. A state maps each identifier to the values it can hold. The pass clones a state per successor and merges states at joins.Notes
Timings. Release builds on Linux x64, one component per file,
bun build --react-compiler --target=browser --external react. "main" is the canary build of b52d513, which has the samesrc/react_compileras the base of this branch. User CPU time and peak RSS. The machine was under load.if (a > i) { deep += i; }forforforfortry/catchThe output of each of these files is the same on both builds. Typical components do not change: the 1,807 fixtures, built twice, take 1.9 to 2.0 s of CPU time on both builds.
Debug build, CPU time: 400
ifs 21 s to 1.8 s, 800ifs 346 s to 4.3 s, 300try29.5 s to 3.3 s. For 800ifs,InferMutationAliasingEffectswent from 330 s to 0.4 s andInferReactivePlacesfrom 11 s to 0.25 s.Why the output does not change.
Setof the TypeScript original does. A union is the ids of the first set, then the new ids of the second, which is what the oldinsertloops built. An earlier version of this branch sorted the ids. The self-review found that the order is observable:freeze_function_captures_transitivefreezes one value of a set, and that decides whether the walk from the next value skips a capture. The testreact-compiler freezes the captures of a function in the order of its valuespins an input where a sorted set compiles a component that main andbabel-plugin-react-compiler1.0.0 leave as written.ChunkedVecis a data structure change only.merge_fromcomputes the same per-entry merge and the samechangedresult as the flat loops did.post_dominators_of(target)returns the blocks thattargetpost-dominates, for any visit order: a path from such a block totargetpasses its immediate post-dominator first, so the post-dominator is in the result before the block is examined. The blocks whose frontier getspredthrough an edgepred -> blockare thenblockand its post-dominators, up to the immediate post-dominator ofpred. A block that cannot reach the exit has no post-dominator, and upstream counts it as its own post-dominator only when it is its own predecessor. The new function keeps that case.minify.syntax. The release build of this commit and main's canary also agree on about 3,600 generated components (closures, loops,try,switch, labels).The tests. Two of the four new tests pin behaviour that main already has, so they pass on main: the order of the values of a set (above), and
react-compiler/DoWhileTestControlsItsOwnBlock. The test of ado/whileloop is in the post-dominator frontier of its own block, in the old code and in the new. A review suggested to leave it out. With that change the component of the test compiles to_c(1)with no dependency, and its second render returns the element of the first ([1,1], not[1,3]).Each compile-time test builds the same statements twice in-process: in one large component, and in components of 10 or 20 statements. It compares the user CPU time per statement. The ratio is 0.5 to 1.4 with the fix. On main's release build it is 9 for 500
trystatements, and 1000ifs run into the 5 s timeout. A debug build runs 300ifs (ratio 6 without the fix, and the timeout) and skips thetryshape: at 5 ms per statement it cannot build enough of them in the time of one test. The ASAN lane of CI is an optimized build, so it runs both shapes, with 400ifs and 300trystatements, below the stack limit of a debug build.Self-review. Ten concerns. One was refuted (print more than the ratio when the assertion fails: the neighbouring tests assert the same way). Addressed:
trytest did not separate a debug build with the fix from one without: skipped there, and sized for the ASAN lanetrysize was 2.3 times below the stack limit: 500 now, and the rounds stop once the ratio is under the boundpost_dominator_frontiersNot in this PR.
codegen_block), which is quadratic in time and memory. It is the largest cost that is left: 4000ifs take 1.6 GB. react-compiler: codegen uses memory quadratic in the number of blocks of a component #43260 tracks it.EnterSSArecurses once per block between a use and its definition. It overflows the stack at about 8000 sequentialifs or 2300trystatements on a release build. react-compiler: check the stack in every pass that recurses as deep as the source nests #43230 covers that.InferTypesis exponential for a chain of phis without a known type, for example 18 timestry { v = f(); } catch {}. react-compiler: keep a phi operand as a type variable in InferTypes #42395 covers that.post_dominator_frontier, and react-compiler: check the stack in every pass that recurses as deep as the source nests #43230 touches the same files. Whichever lands later needs a small rebase.[human-review] gate passed · iteration 0 · 5 files touched
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