Analyze WAST

Analyze WAST

Debug WebAssembly test files by reproducing issues, analyzing compilation stages, and creating regression tests.

Workflow

1. Reproduce the Issue

Run the test to observe failures:

./​install.sh

./​wasmoon test <wast_file_path>

If user provides only a filename (e.g., "elem.wast"), automatically prefix with spec/.

Record:

• Failed assertion line numbers
• Error messages
• Expected vs actual behavior

2. Analyze Compilation Stages

Use the explore command to examine IR, VCode, and machine code:

./​wasmoon explore <wast_file_path> --stage ir vcode mc

Available stages:

• ir - Intermediate Representation (SSA form)
• vcode - Virtual code (platform-independent)
• mc - Machine code (ARM64 assembly)

What to check:

• IR: Verify WebAssembly instructions translate correctly
• VCode: Check instruction selection matches expected patterns
• MC: Verify generated assembly is correct

3. Isolate the Root Cause

Create a minimal test case:

1. Extract the failing WAST module to a standalone .wat file
2. Test with both JIT and interpreter:

   ./​wasmoon test <file>.wast          # JIT mode
   ./​wasmoon test --no-jit <file>.wast # Interpreter mode
   

3. If only JIT fails, the bug is in compilation; if both fail, it's in the executor

Locate relevant code:

• Use Glob to find implementation files (e.g., **/​*lowering*.mbt)
• Use Grep to search for instruction handlers
• Read source code in vcode/​lower/, ir/​translator/, executor/

4. Create Regression Test

Write a MoonBit test in testsuite/ to prevent regressions:

test "descriptive_test_name" {
  let source =
    #|(module
    #|  (func (export "test") (result i32)
    #|    i32.const 42
    #|  )
    #|)
  let result = compare_jit_interp(source, "test", [])
  inspect(result, content="matched")
}

Key points:

• Use #| for multiline WAT source
• compare_jit_interp(source, func_name, args) runs both JIT and interpreter
• inspect(result, content="matched") verifies they produce the same output
• For arguments, use [I32(value)], [I64(value)], [F32(value)], [F64(value)]

Test file naming:

• Use descriptive names: <feature>_test.mbt
• Examples: conversions_test.mbt, call_indirect_test.mbt

Run the test:

moon test -p testsuite -f <test_file>.mbt

5. Debug with LLDB (if needed)

For segfaults or unexplained crashes:

lldb -- ./​wasmoon test <file>.wast
(lldb) run
# After crash:
(lldb) bt  # View backtrace

Command Reference

# Build and install
moon build && ./​install.sh

# Run WAST test
./​wasmoon test spec/<file>.wast

# View compilation stages
./​wasmoon explore <file>.wat --stage ir vcode mc

# Test with interpreter
./​wasmoon test --no-jit <file>.wast

# Run single MoonBit test
moon test -p testsuite -f <test>.mbt

# Debug with LLDB
lldb -- ./​wasmoon test <file>.wast

Presentation

Explain findings to the user, including:

• Root cause of the bug
• Which component has the issue (translator, lowering, codegen, etc.)
• Suggested fix approach
• Impact assessment