wasm-bindgen-multi-value-xform 0.2.55

Internal multi-value transformations for wasm-bindgen
Documentation

The wasm-bindgen multi-value transformation.

This crate provides a transformation to turn exported functions that use a return pointer into exported functions that use multi-value.

Consider the following function:

#[no_mangle]
pub extern "C" fn pair(a: u32, b: u32) -> [u32; 2] {
[a, b]
}

LLVM will by default compile this down into the following Wasm:

(func $pair (param i32 i32 i32)
local.get 0
local.get 2
i32.store offset=4
local.get 0
local.get 1
i32.store)

What's happening here is that the function is not directly returning the pair at all, but instead the first i32 parameter is a pointer to some scratch space, and the return value is written into the scratch space. LLVM does this because it doesn't yet have support for multi-value Wasm, and so it only knows how to return a single value at a time.

Ideally, with multi-value, what we would like instead is this:

(func $pair (param i32 i32) (result i32 i32)
local.get 0
local.get 1)

However, that's not what this transformation does at the moment. This transformation is a little simpler than mutating existing functions to produce a multi-value result, instead it introduces new functions that wrap the original function and translate the return pointer to multi-value results in this wrapper function.

With our running example, we end up with this:

;; The original function.
(func $pair (param i32 i32 i32)
local.get 0
local.get 2
i32.store offset=4
local.get 0
local.get 1
i32.store)

(func $pairWrapper (param i32 i32) (result i32 i32)
;; Our return pointer that points to the scratch space we are allocating
;; on the shadow stack for calling `$pair`.
(local i32)

;; Allocate space on the shadow stack for the result.
global.get $shadowStackPointer
i32.const 8
i32.sub
local.tee 2
global.set $shadowStackPointer

;; Call `$pair` with our allocated shadow stack space for its results.
local.get 2
local.get 0
local.get 1
call $pair

;; Copy the return values from the shadow stack to the wasm stack.
local.get 2
i32.load
local.get 2 offset=4
i32.load

;; Finally, restore the shadow stack pointer.
local.get 2
i32.const 8
i32.add
global.set $shadowStackPointer)

This $pairWrapper function is what we actually end up exporting instead of $pair.