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OptimizeInstructions: Do not remove an inner cast when there are descriptor effects in the way #9067

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kripken merged 4 commits into from
Sep 2, 2026
+99 −3
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OptimizeInstructions: Do not remove an inner cast when there are descriptor effects in the way #9067

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83db657
go
kripken Sep 1, 2026
d8211a3
fix
kripken Sep 1, 2026
5c0b0a4
expand comment
kripken Sep 1, 2026
4e09eb0
reorder as suggested
kripken Sep 1, 2026
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34 changes: 31 additions & 3 deletions src/passes/OptimizeInstructions.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2550,13 +2550,41 @@ struct OptimizeInstructions
// traps are allowed, then we cannot remove the potentially-trapping
// child, though.
bool notWeaker = Type::isSubType(curr->type, child->type);
bool safe = !child->desc || getPassOptions().trapsNeverHappen;
if (notWeaker && safe) {
auto& options = getPassOptions();
auto canTrap = !options.trapsNeverHappen;
bool safe = !child->desc || !canTrap;
bool canOptimize = notWeaker && safe;
if (canOptimize && curr->desc && canTrap) {
// There is another child here, which might trap, and we need to
// consider that in this situation:
//
// (outer.cast
// (inner.cast (inner.ref))
// (descriptor with effects)
// )
//
// =>
//
// (outer.cast
// (inner.ref) ;; inner cast was removed
// (descriptor with effects)
// )
//
// It is safe to remove the inner cast, as if it trapped, the outer one
// would still trap. But if there is a descriptor, then we are moving
// the trap across the descriptor, and shouldn't cross effects there.
EffectAnalyzer descEffects(options, *getModule(), curr->desc);
ShallowEffectAnalyzer movingEffects(options, *getModule(), curr->ref);
if (movingEffects.orderedBefore(descEffects)) {
canOptimize = false;
}
}
if (canOptimize) {
if (child->desc) {
// Reorder the child's reference past its dropped descriptor if
// necessary.
auto* block =
ChildLocalizer(child, getFunction(), *getModule(), getPassOptions())
ChildLocalizer(child, getFunction(), *getModule(), options)
.getChildrenReplacement();
block->list.push_back(child->ref);
block->type = child->ref->type;
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68 changes: 68 additions & 0 deletions test/lit/passes/optimize-instructions-desc.wast
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Original file line number Diff line number Diff line change
Expand Up @@ -1459,4 +1459,72 @@
)
)
)

;; CHECK: (func $ref.cast_desc_eq-ref.cast (type $22) (param $x anyref)
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (ref.cast_desc_eq (ref $struct)
;; CHECK-NEXT: (ref.cast (ref $struct)
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: )
;; CHECK-NEXT: (block (result (ref null $desc))
;; CHECK-NEXT: (return)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: (drop
;; CHECK-NEXT: (ref.cast_desc_eq (ref (exact $struct))
;; CHECK-NEXT: (local.get $x)
;; CHECK-NEXT: (struct.new_default $desc)
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; CHECK-NEXT: )
;; NTRAP: (func $ref.cast_desc_eq-ref.cast (type $22) (param $x anyref)
;; NTRAP-NEXT: (local $1 (ref $struct))
;; NTRAP-NEXT: (local $2 (ref null $desc))
;; NTRAP-NEXT: (drop
;; NTRAP-NEXT: (block (result (ref $struct))
;; NTRAP-NEXT: (local.set $1
;; NTRAP-NEXT: (ref.cast (ref $struct)
;; NTRAP-NEXT: (local.get $x)
;; NTRAP-NEXT: )
;; NTRAP-NEXT: )
;; NTRAP-NEXT: (local.set $2
;; NTRAP-NEXT: (block (result (ref null $desc))
;; NTRAP-NEXT: (return)
;; NTRAP-NEXT: )
;; NTRAP-NEXT: )
;; NTRAP-NEXT: (local.get $1)
;; NTRAP-NEXT: )
;; NTRAP-NEXT: )
;; NTRAP-NEXT: (drop
;; NTRAP-NEXT: (ref.cast_desc_eq (ref (exact $struct))
;; NTRAP-NEXT: (local.get $x)
;; NTRAP-NEXT: (struct.new_default $desc)
;; NTRAP-NEXT: )
;; NTRAP-NEXT: )
;; NTRAP-NEXT: )
(func $ref.cast_desc_eq-ref.cast (param $x anyref)
;; As above with ref.as_non_null, removing the inner ref.cast would allow
;; reaching the return before the cast check, so we do not optimize. (In
;; NTRAP mode we end up removing the outer cast, separately.)
(drop
(ref.cast_desc_eq (ref $struct)
(ref.cast (ref $struct)
(local.get $x)
)
(block (result (ref null $desc))
(return)
)
)
)
;; Without dangerous effects we can remove the inner cast.
(drop
(ref.cast_desc_eq (ref $struct)
(ref.cast (ref $struct)
(local.get $x)
)
(struct.new $desc) ;; this has no effects
)
)
)
)
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