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… catch Dead code elimination can empty a try body. PruneMaybeThrows then removes the catch handler and turns the try terminal into a goto. remove_unnecessary_try_catch removed only the first block of the handler from the predecessors of the fallthrough. A handler of more than one block reaches the fallthrough from its last block, so that stale predecessor stayed and the fallthrough was not merged into the end of the try block. BuildReactiveFunction visits that block only from a try terminal, so every statement after the try statement was dropped. Remove every removed block from the predecessors of the fallthrough. With one predecessor left, a phi of that block can still have the operand of the removed handler, when the catch body assigns a local that is read later. merge_consecutive_blocks asserts on that. Report it before the merge as the invariant error that upstream has there, so the function is skipped. Before, a catch body of one block that assigns such a local aborted the build on that assert.
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Updated 7:40 PM PT - Oct 6th, 2026
✅ @robobun, your commit 4ac1313f0a20464f321c0ae4504b0e75a75631c9 passed in 🧪 To try this PR locally: bunx bun-pr 43603That installs a local version of the PR into your bun-43603 --bun |
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Status: fix pushed in this PR (#43603). The diff is ready for a maintainer. How I reproduced it (main 26e7a4b and 1.4.3-canary.1+367d939d9, linux x64): // t7.jsx
export function Comp(props) {
try {
window.localStorage;
} catch (e) {
if (props.debug) console.warn(e);
}
return <div>{props.a}</div>;
}
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Included review availability: Your plan provides up to 10 included reviews per hour; 1 remains after this review. WalkthroughThe React Compiler now removes stale CFG predecessors, reports invalid phi operand counts after pruning, and adds regression coverage for dead Try paths, branching catches, stale phi operands, and valid folded arguments. ChangesReact Compiler CFG handling
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Inline comments:
In `@src/react_compiler/optimization/prune_maybe_throws.rs`:
- Around line 42-49: Update the invariant message in the phi validation branch
to use count-neutral wording indicating that the phi does not have exactly one
operand, matching the `phi.operands.len() != 1` condition. Keep the condition
and diagnostic details unchanged.
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The reason now carries the operand count, as the invariant in upstream's mergeConsecutiveBlocks and the assert in merge_consecutive_blocks do.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
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🟣
src/react_compiler/optimization/prune_maybe_throws.rs— pre-existing:bun build --react-compilerstill aborts with a panic instead of skipping the function when a phi operand gets rewritten to a block that no longer exists. The rewrite at prune_maybe_throws.rs:66-90 takesmapped_terminalfromterminal_mapping(built before the merge) and inserts it as a new phi key without checking it is still inblock.preds; the new pre-scan at :40 only covers the single-pred case. Fix: after the rewrite, reject any phi operand whose key is not inblock.predswith the same Invariant diagnostic, so every stale-operand shape becomes a per-function skip. Shape:try { try { p.n; } catch {} p.m; x = foo(); } catch {}panics at infer_reactive_places.rs:410. [also at: src/react_compiler/optimization/prune_maybe_throws.rs:78 - pre-existing: a function whose catch-handler phi keeps an operand of a removed block still abortsbun buildwith a panic instead of being skipped.; src/react_compiler/optimization/prune_maybe_throws.rs:92 - Pre-existing:bun build --react-compilerstill aborts with a panic instead of skipping the function for some nested try shapes, e.g.try { try { p.n; } catch {} p.m; x = foo(); } catch {}.; +1 more]Extended reasoning...
The PR adds a pre-scan so a stale phi operand in a one-predecessor block becomes an Invariant error rather than the assert in merge_consecutive_blocks.rs:106. The sibling path is a block with two or more stale preds: the pre-scan skips it, the merge skips it, then mark_predecessors inside merge_consecutive_blocks (merge_consecutive_blocks.rs:179) recomputes preds. The rewrite loop at prune_maybe_throws.rs:57-93 then finds the operand key missing from preds and maps it through terminal_mapping at :66. terminal_mapping was filled in prune_maybe_throws_impl before the RPO and before the merge, so the mapped source block can itself have been merged into its predecessor at merge_consecutive_blocks.rs:154 or dropped…
Verification: pre-existing; acknowledged in diff: the PR description's "Found outside this change, not fixed here" note names exactly this route (
try { try { p.n; } catch {} p.m; x = foo(); } catch {}"aborts on main and with this change (infer_reactive_places.rs:410) ... the rewrite loop maps it to a block that the merge removed"), and the note's bound (same abort as base, not made worse) matches the…
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On the finding outside the diff ( A check that rejects every operand key that is not in A narrower check, for a key whose block is gone after the merge, turns this abort into a skip. It also skips the variants that compile today because the phi is already reactive ( |
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Additional findings (outside the current diff — GitHub can't attach inline comments there):
-
🟣
src/react_compiler/hir/cfg_utils.rs— pre-existing: a component whose catch body starts with a dead try statement is silently left unmemoized, on the base branch and after this change alike. The Try terminal at cfg_utils.rs:209-214 becomes aGoto { variant: GotoVariant::Break }to the try block; when the block holding it is the catch handler (BlockKind::Catch), merge_consecutive_blocks.rs:91 refuses to absorb the try block into it, so the goto survives. Fix: a dead try's goto must be printable from any block kind that can hold a Try terminal (Block and Catch), e.g. let the merge absorb a goto-terminated Catch predecessor, or have build_reactive_function.rs:894 continue into a goto target that is not a scheduled break target instead of erroring.Extended reasoning...
Trigger:
function C(p) { try { p.load(); } catch { try { window.localStorage; } catch (e) { report(e); } p.fallback(); } return <div>{p.a}</div>; }, or simplytry {} catch {}as the first statement of a catch body. The outer catch handler block is entered withBlockKind::Catch(lowering/build_hir/stmt.rs:1369) and the inner try statement terminates that same block with a Try terminal. DCE drops the unusedwindow.localStorageload, PruneMaybeThrows2 prunes the inner MaybeThrow edges, the inner handler is dropped by get_reverse_postordered_blocks, and remove_unnecessary_try_catch replaces the Catch block's terminal withGoto { block: try_block, variant: GotoVariant::Break }(cfg_utils.rs:209-214). With this change the inner fallthrough now merges into the inner try end, but the try block's only predecessor is the Catch block, and merge_consecutive_blocks.rs:87-96 requiresp.kind == BlockKind::Block, so the try block is not merged into it and the goto stays. In BuildReactiveFunction, traversing the handler (build_reactive_function.rs:935) reaches that Goto,…Verification: pre-existing — the base branch fails by the exact same route and this change neither widens nor fixes it; the defect lives in the untouched half of the function the diff edits (
remove_unnecessary_try_catch) and is the same "dead try + removed handler" class the PR sets out to fix. Trigger: a component whose catch body begins with a try statement whose body DCE empties (e.g. `try { p.load();…
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On the second finding outside the diff (a dead try as the first statement of a catch body): confirmed, and main does the same. The goto that replaces the |
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Related: #43658 changes the same pass for two sibling defects (a function skipped with
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…t-compiler-dead-try-branching-catch
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Problem
bun build --react-compilerdrops every statement after atrystatement when dead code elimination empties the try body and the catch body has more than one basic block (if,?:,??,?.). The component compiles tofunction Comp(props) { ; }, with no error.remove_unnecessary_try_catch(src/react_compiler/hir/cfg_utils.rs:223). After PruneMaybeThrows removes the catch handler, it drops only the first handler block from the predecessors of the block after the try statement. A longer handler reaches that block from its last block. That stale predecessor stopsmerge_consecutive_blocks, and only the removedTryterminal visits the block (build_reactive_function.rs:915).Fix
remove_unnecessary_try_catchdrops every removed block from those predecessors. The block then merges into the end of the try block, as it does for a catch body of one block.prune_maybe_throwsfirst reports the invariant error that upstream has there. The function is skipped, as in Babel. On main two such shapes abort the build.test/bundler/transpiler/react-compiler.test.ts(two new tests, both fail on main). Alsoreact-compiler-fixtures.test.ts.Background
trybody each instruction ends its basic block with aMaybeThrowterminal, which has an edge to the catch handler. PruneMaybeThrows removes that edge for a block that cannot throw.Tryterminal becomes a goto.merge_consecutive_blocksappends a block to its only predecessor. A phi joins the values of a local from each predecessor.Notes
Provenance. Found by differential fuzzing against babel-plugin-react-compiler. There is no issue to close. Upstream has the same bug: babel-plugin-react-compiler 1.0.0 prints
export function Comp(props) { ; }for the repro with aCompileSuccessevent, and facebook/react main has the same code inHIRBuilder.ts(removeUnnecessaryTryCatch) andPruneMaybeThrows.ts.Repro (main 26e7a4b and 1.4.3-canary.1+367d939d9):
With
catch (e) { console.warn(e); }the output is correct (_c(2), the try statement is removed, thereturnstays). With this change the branching catch prints the same.Trace. Blocks after the second PruneMaybeThrows on main:
bb0: Goto bb5,bb5: MaybeThrow (no handler),bb6: MaybeThrow (no handler),bb7: Goto(Try) bb1,bb1: preds=[bb7, bb3] ... Return.bb3is the last block of the removed handler.bb1has two predecessors on paper, so it is not merged intobb7. BuildReactiveFunction prints nothing for aTrygoto, because theTryterminal normally continues at the fallthrough.Why only the second run. The first PruneMaybeThrows runs before SSA. The standalone MergeConsecutiveBlocks pass that follows sees fresh predecessors and merges the block. Nothing merges after the second run (
pipeline.rs:445).Other shapes with the same cause. The rest of a loop body (
for (...) { try { it.n; } catch { if (c) log(); } out.push(it); }losesout.push(it)), the rest of an outer try body, the rest of a labeled block, the rest of auseMemocallback (the memoized value becomesundefined), and a catch body withreturnorthrowin a branch.The two aborts.
let ok = true; try { window.localStorage; } catch { ok = false; }failsassertion left == right failed: Found a block with a single predecessor but where a phi has multiple (2) operands(merge_consecutive_blocks.rs:106). With a catch body of one block the stale predecessor ishandler_id, so main already removes it, and the phi keeps two operands.try { try { p.n; } catch { x = 2; ({ ...p }); } p.f(x); } catch {}ends inpanic: called Option::unwrap() on a None value(infer_reactive_places.rs:410): the rewrite loop inprune_maybe_throwsmaps the stale operand to another removed block, because the inner catch ends in an object literal, which cannot throw. Both now stop at the new check. Babel 1.0.0 reports an invariant for both and leaves the function as written.Relation to #42456. That PR turns the assert at
merge_consecutive_blocks.rs:106into an error. The two changes agree: the functions of its test stay as written with this change too. After it lands, the check inprune_maybe_throwscan go, because the merge then reports the same invariant.Alternative not taken.
mark_predecessorsbefore the merge, asconstant_propagationdoes. It also corrects the predecessors of the blocks of a do-while whose test becomes unreachable in this pass. The merge then flattens that loop, and that flattening is wrong for some shapes (see below). On main such a loop always ends inExpected a break target, and this change keeps that.DeadTryInDoWhilein the second test covers it. A guard in BuildReactiveFunction for aTrygoto that is not the innermost break target was tried too. It skipped a function that main compiles (a reactive scope that starts in the try block and ends in the catch body), so it is not part of this change.Generated corpus. 2484 try/catch shapes (18 try bodies, 15 to 18 catch bodies, 9 contexts). Against main: 2216 print the same bytes, 160 are truncated on main and now compile, 75 abort on main and are now skipped, 33 abort on main and still abort (see below). None goes from compiled to skipped, to truncated, or to an abort. All 59 upstream fixtures that contain
tryprint the same bytes.One known difference the corpus does not have.
let x = p.q; try { p.f(); try { p.n; } catch { x = 1; ({ ...p }); } } catch { x = 2; }compiles on main and is now skipped. On main the phi after the inner try keeps an operand of a removed block and nothing reads it, becausexis already reactive. Withlet x = 0the same function aborts on main (infer_reactive_places.rs:410). The catch body has to end in a literal statement for main to get that far. Babel 1.0.0 skips this function too (Expected non-existing phi operand's predecessor to have been mapped to a new terminal).Found outside this change, not fixed here.
try { try { p.n; } catch {} p.m; x = foo(); } catch {}aborts on main and with this change (infer_reactive_places.rs:410). The operand of the outer catch phi is keyed by a block that stops throwing in the second run, and the rewrite loop maps it to a block that the merge removed.try { do { if (p.a) break; return null; } while (p.c); p.f(); } catch {}printstry { if (p.a) p.f(); return null; } catch {}, anddo { try { p.g(); break; } catch { return null; } } while (p.c); p.f();movesp.f()into the try block. Babel 1.0.0 prints the first one the same way.try { (() => { if (props.c) props.onRender(); })(); } catch { ... }loses its try statement: the blocks of an inlined function of more than one block have noMaybeThrowterminal. Babel 1.0.0 prints the same.try { p.f(); } catch { try { p.n; } catch { if (p.c) log(); } p.g(); }) is skipped withExpected a break target, on main and with this change. The goto that replaces theTryterminal sits in aCatchblock, and the merge merges only into aBlockpredecessor. The output is correct, but the function is not memoized.Not changed. The private copy of
remove_unnecessary_try_catchinlowering/hir_builder.rs: the predecessors are empty there, andmark_predecessorsruns right after it.constant_propagationcallsmark_predecessorsafter the helper, so the change has no effect there. When the try body never completes and the handler has more than one block, the unreachable placeholder of the fallthrough stays in the block map, as on main.What the tests check. Each test bundles one file with
--target=browserand runs the output against a fakereact/compiler-runtimewhosecrecords the cache sizes.[]means the function was left as written. On main the first test prints"undefined"for four functions,[]for the loop body andfCalls: 0. The second test aborts the build on main. Without the check inprune_maybe_throwsboth tests abort, the first one throughAssignedInBranchingCatch.Suites run with the debug build:
test/bundler/transpiler/react-compiler.test.ts(75 pass),test/bundler/transpiler/react-compiler-fixtures.test.ts(3293 pass, 320 skip).cargo clippy -p bun_react_compilerandcargo fmt --checkare clean.Self-reviewed in three rounds: 18 concerns raised, 17 addressed. The review rejected three earlier shapes of this fix: a stricter rewrite of stale phi operands (it skipped
try { x = foo(60 * 1000); } catch {}, now a control in the second test), a filter on existing blocks (an unreachable placeholder passed it and the merge aborted), andmark_predecessorswith a guard in BuildReactiveFunction (see "Alternative not taken"). Not addressed: the known difference above. To keep that function compiled, the stale phi operand has to be dropped, and then thelet ok = true; try { ... } catch { ok = false; }shapes compile to a function without the try statement, which Babel does not do and #42456 decided against.[human-review] gate passed · iteration 1 · 3 files touched
fails on main (without fix)
passes on PR (with fix)
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gate history · 2 passed · 0 rejected · iteration 1
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