The GNU Modula-2 front end to GCC
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This section discuss the issues surrounding assignment, expression and parameter compatibility, their effect of the additional fixed sized datatypes and also their effect of runtime checking. The data types supported by the compiler are:
GNU Modula-2 scope switches ============================================= INTEGER pervasive LONGINT pervasive SHORTINT pervasive CARDINAL pervasive LONGCARD pervasive SHORTCARD pervasive BOOLEAN pervasive BITSET pervasive REAL pervasive LONGREAL pervasive SHORTREAL pervasive CHAR pervasive SHORTCOMPLEX pervasive COMPLEX pervasive LONGCOMPLEX pervasive LOC SYSTEM -fiso BYTE SYSTEM WORD SYSTEM ADDRESS SYSTEM The following extensions are supported for most architectures (please check SYSTEM.def). ============================================= INTEGER8 SYSTEM INTEGER16 SYSTEM INTEGER32 SYSTEM INTEGER64 SYSTEM CARDINAL8 SYSTEM CARDINAL16 SYSTEM CARDINAL32 SYSTEM CARDINAL64 SYSTEM BITSET8 SYSTEM BITSET16 SYSTEM BITSET32 SYSTEM WORD16 SYSTEM WORD32 SYSTEM WORD64 SYSTEM REAL32 SYSTEM REAL64 SYSTEM REAL96 SYSTEM REAL128 SYSTEM COMPLEX32 SYSTEM COMPLEX64 SYSTEM COMPLEX96 SYSTEM COMPLEX128 SYSTEM
The Modula-2 language categorises compatibility between entities of possibly differing types into three subcomponents: expressions, assignments, and parameters. Parameter compatibility is further divided into two sections for pass by reference and pass by value compatibility.
For more detail on the Modula-2 type compatibility see the Modula-2 ISO standard BS ISO/IEC 10514-1:1996 page 121-125. For detail on the PIM type compatibility see Programming in Modula-2 Edition 4 page 29, (Elementary Data Types).
Modula-2 restricts the types of expressions to the same type. Expression compatibility is a symmetric relation.
For example two sub expressions of INTEGER and CARDINAL are not expression compatible (http://freepages.modula2.org/report4/modula-2.html and ISO Modula-2).
In GNU Modula-2 this rule is also extended across all fixed sized data types (imported from SYSTEM).
This section discusses the assignment issues surrounding assignment compatibility of elementary types (INTEGER, CARDINAL, REAL and CHAR for example). The information here is found in more detail in the Modula-2 ISO standard BS ISO/IEC 10514-1:1996 page 122.
Assignment compatibility exists between the same sized elementary types.
Same type family of different sizes are also compatible as long as the MAX(type) and MIN(type) is known. So for example this includes the INTEGER family, CARDINAL family and the REAL family.
The reason for this is that when the assignment is performed the compiler will check to see that the expression (on the right of the :=) lies within the range of the designator type (on the left hand side of the :=). Thus these ordinal types can be assignment compatible. However it does mean that WORD32 is not compatible with WORD16 as WORD32 does not have a minimum or maximum value and therefore cannot be checked. The compiler does not know which of the two bytes from WORD32 should be copied into WORD16 and which two should be ignored. Currently the types BITSET8, BITSET16 and BITSET32 are assignment incompatible. However this restriction maybe lifted when further runtime checking is achieved.
Modula-2 does allow INTEGER to be assignment compatible with WORD as they are the same size. Likewise GNU Modula-2 allows INTEGER16 to be compatible with WORD16 and the same for the other fixed sized types and their sized equivalent in either WORDn, BYTE or LOC types. However it prohibits assignment between WORD and WORD32 even though on many systems these sizes will be the same. The reasoning behind this rule is that the extended fixed sized types are meant to be used by applications requiring fixed sized data types and it is more portable to forbid the bluring of the boundaries between fixed sized and machine dependant sized types.
Intemediate code runtime checking is always generated by the front end. However this intemediate code is only translated into actual code if the appropriate command line switches are specified. This allows the compiler to perform limited range checking at compile time. In the future it will allow the extensive GCC optimisations to propagate constant values through to the range checks which if they are found to exceed the type range will result in a compile time error message.
Parameter compatibility is divided into two areas, pass by value and pass by reference (VAR). In the case of pass by value the rules are exactly the same as assignment. However in the second case, pass by reference, the actual parameter and formal parameter must be the same size and family. Furthermore INTEGER and CARDINALs are not treated as compatible in the pass by reference case.
The types BYTE, LOC, WORD and WORDn derivitives are assignment and parameter compatible with any data type of the same size.