[WasmGC] Heap2Local: Optimize RefCast failures #6727
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This reverts commit d36281b.
kripken
commented
Jul 11, 2024
| ;; CHECK-NEXT: (local.get $3) | ||
| ;; CHECK-NEXT: ) | ||
| ;; CHECK-NEXT: (local.set $2 | ||
| ;; CHECK-NEXT: (local.get $4) |
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These changes are what I meant in the second paragraph - $B has an extra field now, which is also handled here (with value 1). This does not affect what the test is testing.
tlively
approved these changes
Jul 11, 2024
| // succeeds or fails. | ||
| if (Type::isSubType(allocation->type, curr->type)) { | ||
| // The cast succeeds, so it is a no-op, and we can skip it. | ||
| replaceCurrent(curr->ref); |
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Even if the cast is known to succeed, we might still need it to make the static types work out, right? This replacement is only valid if curr->ref->type is a subtype of curr->type, not necessarily if allocation->type is a subtype.
This isn't a new issue, so I must be missing something that makes this safe.
Aha, it's because we're going to remove the allocation entirely. This is worth a comment!
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Sounds good, I added a comment.
kripken
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Jul 18, 2024
Followup to #6727 which added support for failing casts in Struct2Local, but it turns out that it required Array2Struct changes as well. Specifically, when we turn an array into a struct then casts can look like they behave differently (what used to be an array input, becomes a struct), so like with RefTest that we already handled, check if the cast succeeds in the original form and handle that.
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Previously we just did not optimize cases where our escape analysis showed an
allocation flowed into a cast that failed. However, after inlining there can be
real-world cases where that happens, even in traps-never-happen mode (if the
cast is behind a conditional branch), so it seems worth optimizing.
Note that a field was added to the struct
$Bin the test, which is needed for some ofthe new checks. That extra field causes minor changes to existing tests (the extra
value is dropped).