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[Custom Descriptors] Heap2Local: Handle definitely-failing descriptor casts #7919

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Merged
merged 3 commits into from
Sep 24, 2025
Merged

[Custom Descriptors] Heap2Local: Handle definitely-failing descriptor casts #7919

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kripken Sep 23, 2025
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kripken Sep 24, 2025
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kripken Sep 24, 2025
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29 changes: 18 additions & 11 deletions src/passes/Heap2Local.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -890,17 +890,24 @@ struct Struct2Local : PostWalker<Struct2Local> {
}
} else {
assert(allocIsCastRef);
// The cast succeeds iff the optimized allocation's descriptor is the
// same as the given descriptor and traps otherwise.
auto type = allocation->desc->type;
replaceCurrent(builder.blockify(
builder.makeDrop(curr->ref),
builder.makeIf(
builder.makeRefEq(
curr->desc,
builder.makeLocalGet(localIndexes[fields.size()], type)),
builder.makeRefNull(allocation->type.getHeapType()),
builder.makeUnreachable())));
if (!Type::isSubType(allocation->type, curr->type)) {
// The cast fails, so it must trap.
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Maybe extend the comment with something like this:

"We mark such failing casts as fully consuming their inputs, so we cannot just emit the explicit descriptor equality check below because it would appear to be able to propagate the optimized allocation on to the parent."

replaceCurrent(builder.blockify(builder.makeDrop(curr->ref),
builder.makeDrop(curr->desc),
builder.makeUnreachable()));
} else {
// The cast succeeds iff the optimized allocation's descriptor is the
// same as the given descriptor and traps otherwise.
auto type = allocation->desc->type;
replaceCurrent(builder.blockify(
builder.makeDrop(curr->ref),
builder.makeIf(
builder.makeRefEq(
curr->desc,
builder.makeLocalGet(localIndexes[fields.size()], type)),
builder.makeRefNull(allocation->type.getHeapType()),
builder.makeUnreachable())));
}
}
} else {
// We know this RefCast receives our allocation, so we can see whether it
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48 changes: 48 additions & 0 deletions test/lit/passes/heap2local-desc.wast
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Original file line number Diff line number Diff line change
Expand Up @@ -898,3 +898,51 @@
)
)

;; A definitely-failing descriptor cast. We have two pairs of descriptor/
;; describee, and create an $A2 that we try to cast to the unrelated $A.
(module
(rec
;; CHECK: (rec
;; CHECK-NEXT: (type $A (descriptor $B (struct)))
(type $A (descriptor $B (struct)))
;; CHECK: (type $B (describes $A (struct)))
(type $B (describes $A (struct)))

;; CHECK: (type $A2 (descriptor $B2 (struct)))
(type $A2 (descriptor $B2 (struct)))
;; CHECK: (type $B2 (describes $A2 (struct)))
(type $B2 (describes $A2 (struct)))
)

;; CHECK: (type $4 (func (result (ref $A))))

;; CHECK: (func $A (type $4) (result (ref $A))
;; CHECK-NEXT: (local $0 (ref (exact $B2)))
;; CHECK-NEXT: (local $1 (ref (exact $B2)))
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (block (result nullref)
;; CHECK-NEXT: (local.set $1
;; CHECK-NEXT: (struct.new_default $B2)
;; CHECK-NEXT: )
;; CHECK-NEXT: (local.set $0
;; CHECK-NEXT: (local.get $1)
;; CHECK-NEXT: )
;; CHECK-NEXT: (ref.null none)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (block (result nullref)
;; CHECK-NEXT: (ref.null none)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (unreachable)
;; CHECK-NEXT: )
(func $A (result (ref $A))
(ref.cast_desc (ref $A)
(struct.new_default $A2
(struct.new_default $B2)
)
(struct.new_default $B)
)
)
)
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