Check for side effects on descriptor in OptimizeInstructions::skipCast - #9190
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In traps-never-happen mode, skipCast removes casts whose type does not matter to the parent, which it does for the operands of ref.is_null (with no required type) and ref.eq (requiring eqref). When the cast is a descriptor cast (ref.cast_desc_eq), removing it also removes its descriptor operand, along with any side effects the operand has. For example: (ref.is_null (ref.cast_desc_eq (ref null $A) (local.get $x) (call $get))) was optimized to: (ref.is_null (local.get $x)) dropping the call. Skip descriptor casts in skipCast while still removing casts without a descriptor.
kripken
reviewed
Oct 1, 2026
| Type::isSubType(cast->ref->type, requiredType)) { | ||
| // Removing a descriptor cast would also remove the descriptor operand, | ||
| // along with its side effects. | ||
| if (!cast->desc && (requiredType == Type::none || |
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How about checking if the descriptor has side effects? We can optimize if it doesn't, which is the common case.
gkdn
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kripken
approved these changes
Oct 2, 2026
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In traps-never-happen mode,
skipCastremoves casts whose type does not matter to the parent, which it does for the operands ofref.is_null(with no required type) andref.eq(requiringeqref). When the cast is a descriptor cast (ref.cast_desc_eq), removing it also removes its descriptor operand, along with any side effects the operand has. For example:was optimized to:
dropping the call.
Check that the descriptor operand has no side effects before removing a descriptor cast in skipCast.
Note: we didn't hit this; it is a pass-by fix.