1934692 - Improve codegen for wasm subtype checks

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P3)

Description

[Ryan Hunt [:rhunt]]

When we cast, we generate a testing node which is branched on at the MIR level. The failure case for the cast goes to a basic block with a single wasm trap node. We should find a way to ensure that block gets moved away from all other code in the function. Essentially move it 'out of line'.

Comment 1

[Ryan Hunt [:rhunt]]

Attachment: Bug 1934692 - wasm: Move traps for casting OOL. r?bvisness

Tweak the masm so that the final conditional branch in a cast goes
to the caller specified 'label' instead of the 'cast success' branch.

This lets us eliminate a jump instruction in the case that we're
trying to branch OOL on cast failure.

Comment 2

[Ben Visness [:bvisness]]

Besides the improvements Ryan mentioned above, we can also improve codegen in our masm routines to avoid unnecessary jumps.

Comment 3

[Ben Visness [:bvisness]]

Attachment: Bug 1934692: wasm: Move cast traps out of line. r=rhunt

Comment 4

[Ben Visness [:bvisness]]

Attachment: Bug 1934692: wasm: Avoid unnecessary branches in casts. r=rhunt

Cleans up codegen for wasm subtype checks to avoid unnecessary short
jumps and to branch directly to the destination on the most common
paths.

Depends on D239958

Comment 5

[Ben Visness [:bvisness]]

This bug ended up with two chunks of work that have turned out to be independent: moving the trap from bug 1949849's MWasmRefCast out of line, and tweaking the masm for the subtype checks to avoid unnecessary jumps. The former seems to be better done in bug 1949849 (and currently resides there anyway), and the latter can be done at any time. Therefore I'm shuffling the dependencies so we can land this tiny tweak more quickly.

Comment 6

[Ben Visness [:bvisness]]

Attachment: Bug 1934692: Add a fast path to STV checks. r=rhunt

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/01ba02821657
wasm: Avoid unnecessary branches in casts. r=rhunt
https://hg.mozilla.org/integration/autoland/rev/d36f5081af48
Add a fast path to STV checks. r=rhunt

Comment 8

[Norisz Fay [:noriszfay]]

https://hg.mozilla.org/mozilla-central/rev/01ba02821657
https://hg.mozilla.org/mozilla-central/rev/d36f5081af48