Special subexpressions

We will identify three classes of expressions that need special treatment: composite expressions, multiply referenced values, and multiply referenced variables.

• A composite expression is an expression that requires some preliminary statements to be executed before the expression can be evaluated. Typically composite expressions are special expressions of the extended language. In general, the pretranslation scheme will treat composite expressions differently according to whether they appear as the right-hand side of a top-level assignment (or as standalone statement expressions), or anywhere else. In the former cases it not usually necessary for the pre-translation phase to introduce temporaries.
• A multiply referenced value is any expression--possibly an ordinary Java expression--that appears in a context requiring the translated code to use the value of the expression more than once. Often multiply referenced values are subexpressions of composite expressions.
• A multiply referenced variable is an expression that has the syntax of a program variable, and appears in a context that means the translation scheme will use the expression more than once, with at least one of these uses being a definition (for example, one of the uses may be as the left-hand side of an assignment). In the scheme described here, the only multiply referenced variables that need special treatment are those on the left hand side of compound assignments (+=, etc).

The singled-out expressions in the current HPJava translation scheme are listed in Figure A.4. To a large extent this table encapsulates the HPJava-specific features of the pre-translation scheme described in this section. With minor changes here and there, the rest of the specification of the pretranslator could probably be adapted easily to any source-to-source translator that was adding new aggregate types to the base Java language.

Figure A.4: Special expressions in the current HPJava translation scheme.

Composite expressions:

1. Multiarray creation expressions.
2. Multiarray-valued method invocation expressions.
3. Multiarray section expressions.
4. Multiarray restriction expressions.

Multiply referenced values:

1. The target object in accesses to multiarray-valued fields.
2. The boolean expression in a multiarray-valued conditional expressions.
3. The multiarray operand in array element reference and array section expression¹.
4. The multiarray operand in array-restriction expression.
5. The array operand of multiarray inquiry operations (rng(), grp(), etc).
6. The range and group parameters of multiarray creation expressions.
7. The range expression in the header of overall and at constructs.

¹ Assuming the language enforces the restriction discussed in section A.2.1--that the target of multiarray subscripting be the name of a final variable--the pretranslator will not need to do any simplification to ensure this operand is simple.