Use multi-value with interface types #1764
Merged
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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:
```rust
pub extern "C" fn pair(a: u32, b: u32) -> [u32; 2] {
[a, b]
}
```
LLVM will by default compile this down into the following Wasm:
```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:
```wasm
(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:
```wasm
;; 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`.
Contributor
alexcrichton
left a comment
alexcrichton
left a comment
There was a problem hiding this comment.
Looks great to me! The transform itself is quite pristine and looks fantastic, and I've mostly just got some nits about the integration here and there.
While you're working on this, mind prototyping the support necessary to integrate this into wasmtime as well?
Contributor
Author
My hope was to get this landed default off with an env var to enable it, and then get wasmtime working with it, and then enable it by default in interface types mode. How does that sound? |
Contributor
Author
|
@alexcrichton any idea what's up with these CI failures related to sccache? |
Contributor
👍
Hm no idea, would assume spurious,but if it keeps coming back feel free to disable sccache and I can deal with it. |
This tiny crate provides utilities for working with Wasm codegen conventions (typically established by LLVM or lld) such as getting the shadow stack pointer. It also de-duplicates all the places in the codebase where we were implementing these conventions in one-off ways.
fitzgen
force-pushed
the
multi-value-xform
branch
from
a2738bb to
f88dd81
Compare
It is failing to install / setup on CI.
fitzgen
force-pushed
the
multi-value-xform
branch
from
f88dd81 to
b2d1165
Compare
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
2 participants
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
No description provided.