1609887 - Prevent uses of webassembly results from being reordered past another webassembly result

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, defect, P2)

Description

[Andy Wingo [:wingo]]

WebAssembly functions may have multiple results, and there may (in the future) be more than one register result. Consider the instruction sequence:

MWasmCall (() -> (i32, i32))
x = MWasmRegisterResult(%rax)
y = MWasmRegisterResult(%rcx)
z = MConstant(1)
w = MAdd(truncate, i32, x, z)

We wouldn't want it to optimize to:

z = MConstant(1)
MWasmCall (() -> (i32, i32))
x = MWasmRegisterResult(%rax)
w = MAdd(truncate, i32, x, z)
y = MWasmRegisterResult(%rcx)

Reordering the z to the top of the block is fine, because it won't be between a call and the result definition, but moving w is not fine, because it could clobber the result that y is meant to capture.

Fix will be to add something to InstructionReordering.cpp. However it can't be based on isEffectful because the instruction being reordered could be pure, without aliases.

Comment 1

[Andy Wingo [:wingo]]

Attachment: Bug 1609887 - Prevent instruction reordering between wasm call and result

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/66cf77a1b464
Prevent instruction reordering between wasm call and result r=lth

Comment 3

[Andrei Ciure[:aciure]]

https://hg.mozilla.org/mozilla-central/rev/66cf77a1b464