-qipa

The -qipa option enables interprocedural analysis (IPA) by the compiler. This enables the compiler to identify optimization opportunities across procedural boundaries. It does this by extending the area that is examined during optimization and inlining from a single procedure to multiple procedures (possibly in different source files) and the linkage between them. This option should be included in both the compile and link phases.

There are a rich collection of suboptions of the form -qipa=suboption. See the compiler man pages for more details about these suboptions. Some useful options are the -qipa=level=n options where n can be 0, 1, or 2. These determine the amount of interprocedural analysis and optimization that is performed. As with the -On options, the higher the number, the greater the amount of optimization performed.

-qipa=level=0

This does only minimal interprocedural analysis and optimization.

-qipa=level=1 (-qipa)

This is the default level for ipa. The two options -qipa=level=1 and -qipa are identical.

This level turns on inlining, limited alias analysis, and limited call-site tailoring.

Inlining a procedure causes a procedure call to be replaced by the procedure itself to eliminate the overhead of the call.

An alias occurs when different variables in a program refer to the same area of storage. If the compiler is unsure whether a given global variable is aliased, it will assume that every procedure call might cause the variable to be read or changed. For this reason it will generate extra loads and stores to preserved the value of the variable when the procedure is called instead of storing it in a register.

Call-site tailoring is IBM's generic term for optimizations that are performed on a function-call basis like cloning and inlining.

-qipa=level=2

This ipa level performs full interprocedural data flow and alias analysis.