Running wasm-opt

By default cargo wasi will run wasm-opt over optimized WebAssembly binaries. The wasm-opt program is a tool in the binaryen toolkit which is a wasm-to-wasm transformation that optimizes the input wasm module. Often wasm-opt can get 10-20% size reductions over LLVM's raw output.

There are a number of heuristics that are used to configure how wasm-opt is run though and it's important to keep those in mind!

Which wasm-opt executed?

Every release of cargo wasi is hardcoded to download a precompiled version of wasm-opt. This binary will be lazily downloaded and then executed. You can also request that a specific wasm-opt binary is used via the WASM_OPT environment variable.

Note that we're interested in feedback on this strategy, so please don't hesitate to file an issue if this doesn't work for you!

Disabled with DWARF debuginfo

If DWARF debug information is requested for a build (default on for debug builds, default off for release builds) then wasm-opt will be disabled. At the time of this writing wasm-opt does not support preserving DWARF debug information through its transformations, so wasm-opt is skipped.

In effect this means that wasm-opt will not run in debug mode, but it will run in release mode. If you enable debug info in release mode, though, then it will not run.

You can configure debuginfo through your Cargo.toml:

debug = 1

Selected Optimization Level

The wasm-opt tool, like most compilers, supports multiple levels of optimization. The optimization level is by default selected to match rustc's own optimization level. If rustc's optimization level is "0", then wasm-opt will not be run.

This effectively means that in debug mode this is another reason that wasm-opt is disabled (because debug mode uses optimization level 0). In release mode we will by default execute wasm-opt -O3 because rustc is executed with -C opt-level=3.

You can configure rustc's and wasm-opt's optimization level through your Cargo.toml. For example to optimize for size instead of speed:

opt-level = 's'

Disabled via configuration

You can also outright disable wasm-opt via configuration by updating your Cargo.toml:

wasm-opt = false

Disabled when wasm-bindgen is used

Finally, as one last caveat, wasm-opt is automatically disabled if wasm-bindgen is used as part of the build. If wasm-bindgen is used it's assumed that WebAssembly Interface Types are also used, and currently wasm-opt (at the time of this writing) does not have support for WebAssembly Interface Types. If we were to run wasm-opt it would produce a broken binary!