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Enabling JIT compilation (optional, experimental)

This directory implements a JIT compiled runtime for proc macros as an optional performance improvement over the interpreted runtime that Watt comes with by default. Installing the JIT is entirely optional; all Watt macros will work exactly the same whether or not the user has the JIT installed, but will usually be quite a bit faster on the JIT runtime.

JIT compilation relies on the host to provide a dynamic library which implements the proposed Wasm C API. Any conforming implementation of this API would be fine; a good one is wasmtime_api which runs on the Wasmtime runtime based on Cranelift.

$  git clone https://github.com/bytecodealliance/wasmtime
$  cd wasmtime/crates/api
$  cargo build --release

Then ensure the following environment variable is defined, and Watt will begin using the JIT runtime when expanding macros.

$  export WATT_JIT=/path/to/wasmtime/target/release/libwasmtime.so

Note that WATT_JIT would have to be set by users that depend on a Watt-based macro (as opposed to the author of the macro) since they're the ones that need a runtime to run the macro on. But again, JIT is optional and Watt-based macros will work the same even on systems without a JIT installed.