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[Custom Descriptors] Handle a null descriptor in Heap2Local #7915

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Merged
merged 1 commit into from
Sep 23, 2025
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[Custom Descriptors] Handle a null descriptor in Heap2Local #7915

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6 changes: 5 additions & 1 deletion src/passes/Heap2Local.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -929,13 +929,17 @@ struct Struct2Local : PostWalker<Struct2Local> {
}

auto type = allocation->desc->type;
Expression* value = builder.makeLocalGet(localIndexes[fields.size()], type);
if (type != curr->type) {
// We know exactly the allocation that flows into this expression, so we
// know the exact type of the descriptor. This type may be more precise
// than the static type of this expression.
refinalize = true;
if (type.isNull()) {
// This traps.
value = builder.makeUnreachable();
}
}
auto* value = builder.makeLocalGet(localIndexes[fields.size()], type);
replaceCurrent(builder.blockify(builder.makeDrop(curr->ref), value));
}

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44 changes: 43 additions & 1 deletion test/lit/passes/heap2local-desc.wast
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@@ -1,7 +1,7 @@
;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.

;; (remove-unused-names allows the pass to see that blocks flow values)
;; RUN: wasm-opt %s -all --remove-unused-names --heap2local -S -o - | filecheck %s
;; RUN: foreach %s %t wasm-opt -all --remove-unused-names --heap2local -S -o - | filecheck %s

(module
(rec
Expand Down Expand Up @@ -856,3 +856,45 @@
(local.get $desc)
)
)

(module
(rec
;; CHECK: (rec
;; CHECK-NEXT: (type $struct (sub (descriptor $desc (struct))))
(type $struct (sub (descriptor $desc (struct))))
;; CHECK: (type $desc (describes $struct (struct)))
(type $desc (describes $struct (struct)))
)

;; CHECK: (type $2 (func (result (ref (exact $desc)))))

;; CHECK: (func $test (type $2) (result (ref (exact $desc)))
;; CHECK-NEXT: (local $0 nullref)
;; CHECK-NEXT: (local $1 nullref)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (block (result nullref)
;; CHECK-NEXT: (local.set $1
;; CHECK-NEXT: (ref.as_non_null
;; CHECK-NEXT: (ref.null none)
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This traps, as you say. But the ref.null below is what determines the type of the entire expression, and it is wrong without the unreachable this PR appends.

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Oh, I see, thanks. So another solution would be to add a ref.as_non_null, but adding unreachable is no more complex and gives better results.

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Could ref.as_non_null work? It just affects nullability, but the problem is we can't return an exact descriptor since we don't have one.

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But none <: (exact $desc), so the only problem here is the nullability.

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Oh right, I forgot exact types do have one subtype...

;; CHECK-NEXT: )
;; 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: (unreachable)
;; CHECK-NEXT: )
(func $test (result (ref (exact $desc)))
;; Read a null descriptor from a struct.new we can convert to locals. We do
;; not end up with a (ref (exact $desc)) here, since this will trap, so we
;; emit an unreachable.
(ref.get_desc $struct
(struct.new_default $struct
(ref.null none)
)
)
)
)

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