AIX Version 4.3 Differences Guide

platform. You may recompile the code in 64-bit mode to check performance
implications in 64-bit mode on a 64-bit platform.
While most code will recompile and execute properly in 64-bit mode, some code
will behave differently, or may not function at all, due to nonportability deliberately
or accidentally written into the code. Common causes of behavior changes are
due to mixed use of long and int types in operators, especially comparison
operators that will change the code execution path. Although the usual operand
promotion rules do not change, the changed size of long types may yield
surprising and unexpected results. Function arguments and return types from
functions need to be checked for their actual value. Many library functions return
long types and take long types that are implicit, such as size_t and ptrdiff_t.
Structures, structure alignments, member alignments, bit fields, and enums are
guaranteed to change when compiled in 64-bit mode (especially if they contain
long and pointer types).
3.3.1.1 Compiler Mode
The generation of 64-bit instructions and 64-bit XCOFF is called the 64-bit
compilation mode. The compiler invocation for setting the 64-bit versus 32-bit
mode evaluates several sources. They are:
• Internal default
• Environment variable OBJECT_MODE
• Configuration file
• Command line
• Source file
The compiler evaluates the options in the given order of the items. The last one
takes precedence. For example, if the environment variable OBJECT_MODE
exists, it will replace the internal default of the compiler. Table 14 provides a list of
OBJECT_MODE settings and the compilation mode behavior.
Table 14. Settings for OBJECT_MODE and the Resulting Compiler Behavior
OBJECT_MODE setting
not set
Compilation Mode Behavior
32-bit mode
32 32-bit mode
64 64-bit mode
32_64 fatal error and stop (unless there is explicit user setting
in the config file or command line) with message:
1501-054 OBJECT_MODE=32_64 is for mixed-mode
and is not a valid setting for the compiler.
anything else
fatal error and stop (unless there is explicit user setting
in the config file or command line) with message:
1501-055 OBJECT_MODE setting is not recognized
and is not a valid setting for the compiler.
This option allows the code to function in a 32- or 64-bit environment without
excessive use of new option names. This will maintain compatibility with other
tools that can exhibit 32/64-bit mode behavior since they will all use the
OBJECT_MODE environment variable. It also maintains compatibility with
machines without 64-bit capability that want to compile in 64-bit mode. In all
64-Bit Enablement 57