Optimization and tuning

Enables destructive copy operations for structures and unions.

Indicates whether a program contains certain categories of aliasing or does not conform to C standard aliasing rules. The compiler limits the scope of some optimizations when there is a possibility that different names are aliases for the same storage location.

Specifies the processor architecture for which the code (instructions) should be generated.

Specifies the cache configuration for a specific execution machine.

Avoids optimizations that increase code size.

Marks data as local or imported.

Informs the compiler that a given compilation unit may reference write-through-enabled or cache-inhibited storage.

Provides object files with information that the IBM® Feedback Directed Program Restructuring (FDPR®) performance-tuning utility needs to optimize the resulting executable file.

Performs high-order loop analysis and transformations (HOT) during optimization.

Allows the compiler to perform optimizations as if system calls would not modify errno.

Enables or customizes a class of optimizations known as interprocedural analysis (IPA).

Specifies functions in the source file that have no side effects other than those implied by their parameters.

Takes advantage of large pages provided on POWER4 and higher systems, for applications designed to execute in a large page memory environment.

Assumes that all functions with the name of an ANSI C library function are in fact the system functions.

Asserts that all functions with Message Passing Interface (MPI) names are in fact MPI functions and not a user function with different semantics.

Limits the amount of memory that the compiler allocates while performing specific, memory-intensive optimizations to the specified number of kilobytes.

Controls the generation of the table of contents (TOC), which the compiler creates for an executable file.

Specifies whether to optimize code during compilation and, if so, at which level.

Prepares the object files produced by the compiler for profiling.

Tunes optimizations through profile-directed feedback (PDF), where results from sample program execution are used to improve optimization near conditional branches and in frequently executed code sections.

Inserts prefetch instructions automatically where there are opportunities to improve code performance.

Marks functions as local, imported, or unknown.

Attempts to inline functions instead of generating calls to those functions, for improved performance.

Specifying this option is equivalent to adding the restrict keyword to the pointer parameters within the specified functions, except that you do not need to modify the source file.

When used with -qpdf1 and a minimum optimization level of -O2 at compile and link steps, creates a PDF map file that contains additional profiling information for all procedures in your application.

#pragma nosimd

Controls whether the compiler can automatically take advantage of vector instructions for processors that support them.

Minimizes stack usage where possible. Disables optimizations that increase the size of the stack frame.

Enables parallelization of program code.

Works with the -qtocmerge -bl:file for non-IPA links and with the -bl:file for IPA links to disable speculation at absolute addresses.

Ensures that optimizations that are done by default at the -O3 and higher optimization levels, and, optionally at -O2, do not alter the semantics of a program.

Prevents the compiler from performing induction (loop counter) variable optimizations. These optimizations may be unsafe (may alter the semantics of your program) when there are integer overflow operations involving the induction variables.

Enables TOC merging to reduce TOC pointer loads and improves the scheduling of external loads.

Tunes instruction selection, scheduling, and other architecture-dependent performance enhancements to run best on a specific hardware architecture. Allows specification of a target SMT mode to direct optimizations for best performance in that mode.

Controls loop unrolling, for improved performance.

Specifies whether the call stack can be unwound by code looking through the saved registers on the stack.

Specifies the visibility attribute for external linkage entities in object files. The external linkage entities have the visibility attribute that is specified by the -qvisibility option if they do not get visibility attributes from pragma directives, explicitly specified attributes, or propagation rules.