Fix invalid SPIRV from continue inside switch inside for loop

[jkwak-work]

For a for-loop with a switch containing a continue, the IR has this shape:

  loop(target=%body, break=%after, continue=%incr)
  %body:
    switch(x, break=%post_switch, ...)
      case 0: unconditionalBranch(%incr)        <- 'continue'
      case 1: unconditionalBranch(%post_switch) <- 'break'
  %post_switch:
    unconditionalBranch(%incr)   <- normal post-switch flow
  %incr: i++; unconditionalBranch(%body)  <- back-edge
  %after: ...                              <- loop break

The continue inside the switch branches to %incr (the loop's continueBlock).
Because continueBlock (%incr) != targetBlock (%body) in a for-loop, this is a
branch that exits the switch region to reach an exit block of the enclosing loop
region -- a multi-level branch that must be transformed for valid SPIRV.

The bug: populateExitBlocks() stored continueBlock but did not add it to
region->exitBlocks, so mapExitBlockToRegion never mapped %incr to the loop
region. gatherInfo() therefore never flagged the branch from case 0 to %incr as
a multi-level branch, needsContinueElimination stayed false, and the raw IR
branch reached the SPIRV emitter -- producing unstructured control flow.

The fix: add continueBlock to region->exitBlocks whenever continueBlock !=
targetBlock (i.e. for for-loops). This makes %incr visible in
mapExitBlockToRegion, gatherInfo() detects the multi-level branch, and
eliminateContinueBlocksInFunc() is called to wrap the loop body in an inner
breakable region. After that transformation the continue becomes a break from
the inner region -- a valid SPIRV structured exit to its merge block -- and the
outer loop handles the actual iteration.

Also fix a stack leak: after processing a loop region, pop continueBlock from
the global exitBlocks stack unconditionally. For for-loops it is already in
info.exitBlocks and removed by the existing loop; for while-loops (where
continueBlock == targetBlock and is not in info.exitBlocks) it was pushed to
the global stack but never popped, which could affect sibling constructs.

Fixes #9585, #10198.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 434d7737-f3d5-4aa3-80ab-edce55b5d8fb

📥 Commits

Reviewing files that changed from the base of the PR and between 695944c and a90cb7c.

📒 Files selected for processing (2)

• source/slang/slang-ir-eliminate-multilevel-break.cpp
• tests/bugs/nested-switch-continue-in-loop.slang

🚧 Files skipped from review as they are similar to previous changes (2)

• tests/bugs/nested-switch-continue-in-loop.slang
• source/slang/slang-ir-eliminate-multilevel-break.cpp

---

📝 Walkthrough

Walkthrough

Handle continue targets when eliminating multi-level breaks: loop headers are captured as IRLoop, continue blocks are recognized and treated as exit points when distinct from loop targets, and continue blocks are removed from exit-block sets after region processing to preserve structured control flow.

Changes

Cohort / File(s)	Summary
IR Control Flow Fix source/slang/slang-ir-eliminate-multilevel-break.cpp	Capture loop headers as IRLoop, obtain continueBlock via getContinueBlock(), assert non-null, add it to exitBlocks when it differs from the loop target; on region pop, remove info.continueBlock from exitBlocks to keep exit set consistent.
Regression Test tests/bugs/nested-switch-continue-in-loop.slang	Add a test exercising continue inside a switch within for/while loops; writes results to a buffer and asserts expected outputs (3, 36, 39) to validate correct control-flow handling across backends.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 I hopped through IR blocks, a twist in the loop,
A switch made me skip—what a puzzling loop-de-loop!
I nudged the continue, marked its exit with care,
Now structured flow's tidy, no SPIR‑V nightmare.
The rabbit applauds: small hops, big repair. 🐇

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and accurately summarizes the main fix: addressing invalid SPIRV generation when continue appears inside a switch within a for loop.
Description check	✅ Passed	The description is well-detailed and directly related to the changeset, explaining the bug, root cause, and the specific fixes applied to handle multi-level branches in nested control structures.
Linked Issues check	✅ Passed	The changes fully address the linked issue #9585 by implementing IR transformation to detect and handle multi-level branches from continue in switch-within-loop structures, and fix a stack leak affecting while-loops.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the nested switch-continue-in-loop issue and its related stack leak; no unrelated modifications are present.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

📝 Coding Plan

• Generate coding plan for human review comments

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[coderabbitai]

Actionable comments posted: 1

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 282e06c7-f085-4f90-8559-0e44e6d10e10

📥 Commits

Reviewing files that changed from the base of the PR and between 38b5b83 and 695944c.

📒 Files selected for processing (2)

• source/slang/slang-ir-eliminate-multilevel-break.cpp
• tests/bugs/nested-switch-continue-in-loop.slang

Automated review dismissed: bot reviews must use COMMENT only, not APPROVE or REQUEST_CHANGES.

[jhelferty-nv]

LGTM, thanks!

[jkwak-work]

Having a trouble with CI.
I force-push-ed to see if it helps.