User's Guide

Linkage Convention for Function Calls

A routine has two symbols associated with it: a function descriptor (name) and an entry point (.name). When a call is made to a routine, the program branches to the entry point directly. Excluding the loading of parameters (if any) in the proper registers, compilers expand calls to functions to the following two-instruction sequence:

       BL    .foo                # Branch to foo
       CROR  15,15,15            # Special NOP

The linker does one of two things when it encounters a BL instruction:

1. If foo is imported (not in the same object module), the linker changes the BL to .foo to a BL to .glink (global linkage routine) of foo, and inserts the .glink into the object module. Also, if a NOP instruction (CROR 15,15,15) immediately follows the BL instruction, the linker replaces the NOP instruction with the LOAD instruction L R2, 20(R1).

2. If foo is bound in the same object module as its caller, and a LOAD instruction (L R2,20(R1) for 32-bit, and L R2,40(R1) for 64-bit) or (ORI R0,R0,0) immediately follows the BL instruction, the linker replaces the LOAD instruction with a NOP (ORI R0,R0,0).

Pointers to Functions

A function pointer is a data type whose values range over procedure names. Variables of this type appear in several programming languages, such as C and Fortran. In Fortran, a dummy argument that appears in an EXTERNAL statement is a function pointer. Fortran provides support for the use of function pointers in contexts such as the target of a call statement or an actual argument of such a statement.

A function pointer is a fullword quantity that is the address of a function descriptor. The function descriptor is a 3-word object. The first word contains the address of the entry point of the procedure. The second has the address of the TOC of the object module in which the procedure is bound. The third is the environment pointer for some non-Fortran languages. There is only one function descriptor per entry point. It is bound into the same object module as the function it identifies if the function is external. The descriptor has an external name, which is the same as the function name but with a different storage class that uniquely identifies it. This descriptor name is used in all import or export operations.

Function Values

Functions return their values according to type:

• INTEGER and LOGICAL of kind 1, 2, and 4 are returned (right justified) in R3.

• In 32-bit mode, INTEGER and LOGICAL of kind 8 are returned in R3 and R4.

• In 64-bit mode, INTEGER and LOGICAL of kind 8 are returned in R3.

• REAL of kind 4 or 8 are returned in FP1. REAL of kind 16 are returned in FP1 and FP2.

• COMPLEX of kind 4 or 8 are returned in FP1 and FP2. COMPLEX of kind 16 are returned in FP1-FP4.

• Character strings are returned in a buffer allocated by the caller. The address and the length of this buffer are passed in R3 and R4 as hidden parameters. The first explicit parameter word is in R5, and all subsequent parameters are moved to the next word.

• Structures are returned in a buffer that is allocated by the caller. The address is passed in R3; there is no length. The first explicit parameter is in R4.

The Stack Floor

The stack floor is a system-defined address below which the stack cannot grow. All programs in the system must avoid accessing locations in the stack segment that are below the stack floor.

All programs must maintain other system invariants that are related to the stack:

• No data is saved or accessed from an address lower than the stack floor.

• The stack pointer is always valid. When the stack frame size is more than 32 767 bytes, you must take care to ensure that its value is changed in a single instruction. This step ensures that there is no timing window where a signal handler would either overlay the stack data or erroneously appear to overflow the stack segment.

Stack Overflow

The linkage convention requires no explicit inline check for overflow. The operating system uses a storage protection mechanism to detect stores past the end of the stack segment.