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IFACMPXX.md

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Implementation of IF_ACMPEQ/IF_ACMPNE opcodes:

Simplified pseudocode

  • If none of the 2 types are recognized as Scalar Value Type, standard reference comparison is used. This also covers comparing VT arrays or vt array and an object.
  • If both sides are a Value Type and their Value Type class is different, comparison will be hardcoded as "not equal"
  • If any of the sides is recognized as null (normal null or VT null constant), then the comparison is converted into explicit null check of the non-null argument. Method ValueTypeUtils.isNull(long) is called for a scalar, non-boxed Value Type argument, otherwise normal null comparison is used (IFNULL/IFNONNULL opcode)
  • If none of the cases above is chosen, issue a warning about reference comparison changed to value comparison
  • If none of the 2 types is a non-boxed Value Type, replace the comparison with call to bool ValueType.isIdentical(ValueType, Object).
  • If one of the 2 types is a non-boxed Value Type, try to unbox the other side object as the same ValueType by calling the unboxing method.
  • Finally, replace the comparison with ValueTypeUtils.isIdentical(long, long) or ValueTypeUtils.isIdentical(long, Object) depending on the type of the other argument

Following hardcoded method names are expected to be defined by the user

  • static bool ValueTypeUtils.isNull(long) - Compares ValueType Value with null, used if one of the arguments is identified as null constant and the opcode is processed as ISNULL/IFNONNULL
  • static bool ValueTypeUtils.isIdentical(long, long) - Compares a nullable ValueType Value to another ValueType Value
  • static bool ValueTypeUtils.isIdentical(long, Object) - Compares a nullable ValueType Value to an Object (possibly a boxed Value Type)
  • static bool ValueType.isIdentical(ValueType, Object) - Compares a boxed nullable ValueType Object with arbitrary Object (possibly another boxed ValueType)

ValueType stands for user-defined Value Type class, ValueTypeUtils stands for user-defined utility class for ValueType. User is responsible for the methods described above having the same semantics, and being exception-safe, regardless of if the ValueType appearing as value or as reference in any given context. Comparison is assumed to be commutative, argument order may be swapped as necessary.