Fix #137: for-loop continue skips update on --target web#178
Merged
proggeramlug merged 1 commit intomainfrom Apr 24, 2026
Merged
Fix #137: for-loop continue skips update on --target web#178proggeramlug merged 1 commit intomainfrom
proggeramlug merged 1 commit intomainfrom
Conversation
In the WASM emitter's Stmt::For lowering, the update expression was
placed at the end of the loop body just before the explicit Br(0)
restart. When a continue statement fired (via Br(rel) targeting the
loop label), it jumped back to the loop start and skipped both the rest
of the body AND the update — causing the iterator to pin forever on the
last active index.
Fix: wrap the loop body in an inner block so that continue targets the
inner block's exit rather than the loop top. The new structure is:
block $break
loop $loop_top
<condition>; br_if 1 ($break)
block $body_end ← continue targets this block's exit
<body>
end ← continue falls through here
<update> ← now always runs before loop restart
br 0 ($loop_top)
end
end
loop_depth now tracks $body_end instead of $loop_top, so the existing
Stmt::Continue formula (block_depth - loop_depth.last()) correctly emits
a relative branch that exits $body_end and falls through to the update.
break_depth still tracks $break, so Stmt::Break is unaffected.
Verified with wasm-tools: both continue statements in the repro emit
br 1 (;@3;) — targeting the inner body block — and br 0 (;@2;) at
the bottom correctly restarts the loop after the update runs.
https://claude.ai/code/session_01BBbsJYwY1ZWTjESXffRC6C
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Root cause
In
crates/perry-codegen-wasm/src/emit.rs,Stmt::Forlowered the update expression at the end of the loop body, just before the explicitBr(0)restart:When
continuefired (asBr(rel)targeting the loop label), it jumped back to the loop start and skipped the update — causing the iterator to pin forever on the last active index.The bug was latent before v0.5.161 because
continuewas hardcoded toBr(0)(always targeting the innermost block, i.e. the loop). After #135's fix madecontinueuse the formulablock_depth - loop_depth.last(), continue correctly escaped nestedifblocks — but now exposed that theforupdate was always in the wrong place relative to wherecontinuelanded.Fix
Wrap the loop body in an inner
blockso thatcontinueexits that inner block and falls through to the update before the loop restarts:loop_depthnow tracks$body_endinstead of$loop_top, so the existingStmt::Continueformula (block_depth - loop_depth.last()) produces the correct relative branch without any changes to theStmt::Continuehandler.break_depthstill tracks$break, soStmt::Breakis unaffected.Verification
WAT disassembly of the repro from #137 (via
wasm-tools print) confirms:Both
continuestatements emitbr 1 (;@3;)targeting the inner body block, the update always runs beforebr 0restarts the loop. Native target output (walker 2/done) matches — no non-WASM code paths touched.Scope
Single-file change in
crates/perry-codegen-wasm/src/emit.rs. No version bump, no CLAUDE.md changes (external contributor PR per project rules).Generated by Claude Code