Finding and Understanding Missed Optimizations in WebAssembly Optimizer (Experience Paper)

As WebAssembly (Wasm) expands from web applications to high-performance domains, the standard optimizer, wasm-opt, is critical but frequently suffers from missed optimizations (MOs). This paper presents an experience report on detecting MOs in wasm-opt and understanding their root cause through the lens of Wasm’s tree-structured intermediate representation (tree IR). To this end, we adapt an established marker-based technique from C compilers, overcoming the constraints of Wasm’s structured control flow via a novel structure-aware instrumentation strategy. Complementing this, we design a cross-optimization differential testing strategy leveraging the monotonicity of optimization levels as an oracle. Together, these strategies enable the systematic identification of fine-grained MOs that are overlooked by existing cross-architecture methods. Our evaluation uncovered 24 distinct MOs (20 fixed, 100% confirmation rate, 0% false positive rate), demonstrating the high actionability and effectiveness of our approach. The performance impact, especially in code size, yields an average 1.30% improvement and no regressions on the Emscripten benchmark suite, further proving their practical value. Beyond detection, our analysis distills three practical lessons for designing MO testing techniques and understanding the optimization trade-offs imposed by wasm-opt’s tree IR.