1951158 - Get wasm ref cast/test source types from MIR

Status: RESOLVED WONTFIX

(Core :: JavaScript: WebAssembly, enhancement, P1)

Description

[Ben Visness [:bvisness]]

Once we have wasm ref types in MIR, we can get our source types from MIR instead of from wasm validation. This would also allow us to put AliasSet::None on subtype checks, since they would always have correct source type information regardless of where they were ordered in the MIR graph.

Comment 1

[Ben Visness [:bvisness]]

Attachment: Bug 1951158: Use ref type from input in cast/test operations. r=rhunt

Comment 2

[Ben Visness [:bvisness]]

From my experimentation, it seems that applying this on its own is perhaps not as effective as we would have hoped. Particularly, it seems that a significant number of casts actually get worse when pulling the source type from MIR; in particular, they lose information in cases that use br_on_cast or br_on_null.

It may make sense to leave this alone and focus everything on bug 1949849, which would add dedicated MIR nodes for casting and optimization passes that refine the type information on those nodes. On the other hand, if we can demonstrate an overall win from this patch on its own, we could still go ahead with it in the meantime.

Regardless, the cast codegen change in bug 1934692 has proven more important and largely independent from this work, so I am shuffling the dependencies around to do that first.