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Section 03: Exception Handling

Context: The EH infrastructure was added for ARC cleanup (strings, lists need ori_rc_dec on unwind). But it’s applied too broadly — recursive pure-integer functions get invoke + empty landing pads, and ARC functions get orphaned pads that are never reached. This adds IR bloat and confuses optimization passes.

Depends on: Coordinate with Section 02 (H2) since nounwind analysis code is shared.

03.1 Fix H1 — Empty Landing Pads for Recursive Functions

Journey: J3 (confirmed J3, J5, J9, J10) | Severity: HIGH File(s): compiler/ori_llvm/src/codegen/ (call emission, nounwind analysis)

When any function in a module is recursive, ALL functions switch from call to invoke with empty landing pads. For fib(10): ~354 useless landing pad blocks. The landing pads just resume — no cleanup.

; Pure integer function — needs NO landing pad:
define fastcc i64 @_ori_fib(i64 %0) personality ptr @rust_eh_personality {
  %call = invoke fastcc i64 @_ori_fib(i64 %sub)
          to label %bb4 unwind label %bb5
bb5:
  %lp = landingpad { ptr, i32 } cleanup
  resume { ptr, i32 } %lp    ; does NOTHING — no cleanup needed
}

Fix: Use invoke only when the caller has ARC-managed values that need cleanup on unwind. If the caller holds no ARC values at the call site, use call even for potentially-unwinding functions.

  • Identify the condition that triggers invoke vs call in the codegen
  • Change: use invoke only when caller has live ARC values at the call site
  • Functions with no ARC values should use call even for recursive/panicking callees
  • If a function has no ARC values AND all callees are nounwind → mark it nounwind
  • Verify: fib uses call (no ARC values) instead of invoke — zero landing pads
  • Verify: string programs still work correctly (abort-on-panic means no unwind cleanup currently needed; when unwinding is added, RC insertion will populate unwind blocks and invoke will be used)

03.2 Fix M10 — Inconsistent nounwind on _ori_main

Journey: J8 (confirmed J9) | Severity: MEDIUM File(s): compiler/ori_llvm/src/codegen/ (nounwind analysis)

_ori_main sometimes has nounwind (J1, J2, J4) and sometimes doesn’t (J8, J9), even when all callees are nounwind. The nounwind analysis doesn’t propagate through monomorphized call sites.

  • Find the nounwind analysis code (nounwind.rs — two-pass prepare/analyze/emit)
  • Verify it considers monomorphized function names when checking callees (mono propagation existed but was OUTSIDE the fixed-point loop)
  • Fix: moved mono propagation INSIDE fixed-point loop so callers of original generic names are re-analyzed after propagation
  • Verify: _ori_main consistently has nounwind when all callees are nounwind (including through generics)

03.3 Fix M11 — Orphaned Landing Pads

Journey: J9 (confirmed J10) | Severity: MEDIUM File(s): compiler/ori_llvm/src/codegen/ (ARC emitter)

The ARC emitter generates landing pad blocks for cleanup, but the actual runtime calls (ori_str_from_raw, ori_rc_dec) use call not invoke. The landing pads have no predecessors — pure dead code.

bb2:                                  ; No predecessors!  ← DEAD
  %lp = landingpad { ptr, i32 } cleanup
  call void @ori_rc_dec(...)          ; cleanup that's never reached
  resume { ptr, i32 } %lp

Fix: Either:

  • (a) Don’t emit landing pads for calls that use call instead of invoke
  • (b) Use invoke for runtime calls that may panic, connecting to the landing pads

Recommendation: Fix the emission to match — if a call uses call, don’t emit a landing pad for it.

  • Identify where ARC cleanup landing pads are generated (mod.rs unwind_blocks → landingpad emission)
  • Fix: callee_will_be_intercepted() helper detects callees handled by builtin/format/prelude handlers; dead_unwind detection skips creating LLVM blocks for their unwind targets
  • Verify: string programs have 0 orphaned landing pads (No predecessors!)
  • Verify: catch(expr:) landing pads still work correctly (declared user functions excluded from interception check)

03.4 Completion Checklist

  • Pure-integer functions use call (not invoke) — no empty landing pads
  • _ori_main consistently has nounwind when all callees are nounwind
  • 0 orphaned landing pad blocks (no blocks with “No predecessors!”)
  • ARC cleanup paths still work correctly (string/list programs don’t leak or crash)
  • ./test-all.sh green
  • ./scripts/valgrind-aot.sh — 0 errors

Exit Criteria: Generated IR for all 12 journeys has 0 orphaned blocks, 0 empty landing pads for non-ARC functions, and consistent nounwind attributes. grep "No predecessors" *.ll returns 0 matches.