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denoland-deno/ops/README.md
Divy Srivastava ca66978a5a
feat(ops): fast calls for Wasm (#16776) 
This PR introduces Wasm ops. These calls are optimized for entry from
Wasm land.

The `#[op(wasm)]` attribute is opt-in. 

Last parameter `Option<&mut [u8]>` is the memory slice of the Wasm
module *when entered from a Fast API call*. Otherwise, the user is
expected to implement logic to obtain the memory if `None`

```rust
#[op(wasm)]
pub fn op_args_get(
  offset: i32,
  buffer_offset: i32,
  memory: Option<&mut [u8]>,
) {
  // ...
}
```
2022-11-27 19:24:28 +05:30

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deno_ops

proc_macro for generating highly optimized V8 functions from Deno ops.

// Declare an op.
#[op(fast)]
pub fn op_add(_: &mut OpState, a: i32, b: i32) -> i32 {
  a + b
}

// Register with an extension.
Extension::builder()
  .ops(vec![op_add::decl()])
  .build();

Performance

The macro can optimize away code, short circuit fast paths and generate a Fast API impl.

Cases where code is optimized away:

  • -> () skips serde_v8 and rv.set calls.
  • -> Result<(), E> skips serde_v8 and rv.set calls for Ok() branch.
  • -> ResourceId or -> [int] types will use specialized method like v8::ReturnValue::set_uint32. A fast path for SMI.
  • -> Result<ResourceId, E> or -> Result<[int], E> types will be optimized like above for the Ok() branch.

Fast calls

The macro will infer and try to auto generate V8 fast API call trait impl for sync ops with:

  • arguments: integers, bool, &mut OpState, &[u8], &mut [u8], &[u32], &mut [u32]
  • return_type: integers, bool

The #[op(fast)] attribute should be used to enforce fast call generation at compile time.

Trait gen for async ops & a ZeroCopyBuf equivalent type is planned and will be added soon.

Wasm calls

The #[op(wasm)] attribute should be used for calls expected to be called from Wasm. This enables the fast call generation and allows seamless WasmMemory integration for generic and fast calls.

#[op(wasm)]
pub fn op_args_get(
  offset: i32,
  buffer_offset: i32,
  memory: Option<&[u8]>, // Must be last parameter. Some(..) when entered from Wasm.
) {
  // ...
}