Why compiling Rust to WebAssembly is slow

6.9 relevance

Deep technical analysis of Rust-to-WASM compilation performance, actionable for developers using WebAssembly.

Languages 00f.net

Answer the board on AI readiness

Arrive with evidence, not opinion. See how your foundation benchmarks against your peers, get a clear read, and leave with a ranked place to start.

Why It Matters

For platform engineers and Rust developers targeting WebAssembly, this explains why debug builds are unusably slow and pinpoints the LLVM pass causing it, enabling informed trade-offs between debug info and build speed.

Key Takeaways

Set debug = 0 or use a custom profile for wasm builds to avoid the Register Stackify quadratic slowdown until the LLVM fix is ported.

Summary

Compiling Rust to WebAssembly with debug info (debug = 2) can be 40x slower than without, as demonstrated by a 40-line reproducer that takes 50 seconds with debug info vs 1.5 seconds without. The root cause is an LLVM bug in the Register Stackify pass for WebAssembly's stack machine: moving instructions to optimize operand stack usage also moves DBG_VALUE records, but the pass rescanning and duplicating records for each moved definition creates quadratic complexity. The fix for clang is incomplete for Rust's wasm target, and the default dev profile's debug = 2 alias triggers this for all wasm builds.

Author

Frank Denis (Jedi/Sector One)