Bug: Function inlining produces invalid WASM

[avrabe]

Problem

The inline optimization pass produces invalid WASM that fails validation.

Reproduction

# Test with simple file
python3 /tmp/test_idempotence.py /tmp/test_inline.wasm --passes inline

Result:

• Input: 83 bytes, ✅ valid
• Output: 108 bytes, ❌ INVALID
• Error: type mismatch: values remaining on stack at end of block (at offset 0x59)

Test Case

(module
  (func $add_two (param $x i32) (result i32)
    local.get $x
    i32.const 2
    i32.add
  )
  
  (func $main (param $a i32) (result i32)
    local.get $a
    call $add_two  ;; This gets inlined incorrectly
    call $add_two
  )
  
  (export "main" (func $main))
)

Root Cause

The inline pass (from PR #29) has a bug in stack management:

1. ✅ Correctly identifies inlining opportunities
2. ✅ Substitutes function body at call site
3. ❌ Leaves extra values on stack or corrupts stack discipline

Impact

Severity: HIGH

• Affects loom.wasm self-optimization
• Produces invalid WASM that cannot be executed
• All other 9 optimization passes work correctly

Systematic Test Results

Pass	Valid Output	Idempotent
inline	❌	✅
precompute	✅	✅
constant-folding	✅	✅
cse	✅	✅
advanced	✅	✅
branches	✅	✅
dce	✅	✅
merge-blocks	✅	✅
vacuum	✅	✅
simplify-locals	✅	✅

See Issue #30 for full test methodology.

Recommendation

Immediate action: Disable inline pass by default until fixed.

// In optimize_module, comment out inline pass:
// loom_core::optimize::inline_functions(&mut module)?;

Related

• Issue LOOM cannot optimize itself (self-optimization fails) #30: Component model optimization (where this was discovered)
• PR feat: enable function inlining in optimization pipeline #29: Function inlining implementation
• Commit 99d19b0: feat: enable function inlining in optimization pipeline

[avrabe]

Fixed in PR #32

The bug was in the function inlining implementation. It wasn't consuming function arguments from the stack before inlining the callee body.

Solution:

• Store arguments from stack to temporary locals using LocalSet
• Remap parameter accesses in inlined body to use temporary locals
• Remap callee's local variables to avoid index conflicts

Verification:
✅ All tests pass including 6 new comprehensive inlining tests
✅ Idempotence verified
✅ Tested on real fixtures

The test case from this issue now produces valid WASM.