XL Fortran Enterprise Edition for AIX : User's Guide - IBM

Function Values
Functions return their values according to type:
v INTEGER and LOGICAL of kind 1, 2, and 4 are returned (right justified) in R3.
v In 32-bit mode, INTEGER and LOGICAL of kind 8 are returned in R3 and R4.
v In 64-bit mode, INTEGER and LOGICAL of kind 8 are returned in R3.
v
REAL of kind 4 or 8 are returned in FP1. REAL of kind 16 are returned in FP1
and FP2.
v COMPLEX of kind 4 or 8 are returned in FP1 and FP2. COMPLEX of kind 16
are returned in FP1-FP4.
v
v
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:
v
No data is saved or accessed from an address lower than the stack floor.
v 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.
Prolog and Epilog
On entry to a procedure, you might have to do some or all of the following steps:
1. Save the link register at offset 8 for 32-bit environments (or offset 16 for 64-bit
environments) from the stack pointer if necessary.
2. If you use any of the CR bits 8-23 (CR2, CR3, CR4, CR5), save the CR at
displacement 4 for 32-bit environments (or displacement 8 for 64-bit
environments) from the current stack pointer.
3. Save any nonvolatile FPRs that are used by this procedure in the caller’s FPR
save area. You can use a set of routines: _savef14, _savef15, ... _savef31.
4. Save all nonvolatile GPRs that are used by this procedure in the caller’s GPR
save area.
5. Store back chain and decrement stack pointer by the size of the stack frame.
Note that if a stack overflow occurs, it will be known immediately when the
store of the back chain is done.
366 XL Fortran Enterprise Edition for AIX : User’s Guide