HP Pascal/HP-UX Release Notes Version A.10.08

22 Chapter 1

New and Changed Features

Proﬁle-Based Optimization (PBO)

Proﬁle-based optimization (PBO) is a set of code-improving

transformations that are based on feedback concerning the run-time

characteristics of an application. Run-time proﬁle data is collected

during program execution. This information is fed back to the optimizer,

which performs a variety of optimizations based upon how frequently

certain code is executed and how frequently calls are made between

different code segments. In general, each higher level of optimization

takes increasing advantage of the PBO-generated information.

One of the goals of PBO is to improve the efﬁciency of memory access by

increasing the hit rates for the instruction cache, memory pages, and the

Translation Lookaside Buffer (TLB). PBO can also improve the

compiler's inlining decisions. One basis for the decision is call frequency,

which, without PBO, the compiler can only estimate. With PBO, however,

the compiler uses actual frequencies as the basis for its decision.

For complete details on PBO, see HP-UX Linker and Libraries Online

User Guide.

NOTE HP Pascal/HP-UX is limited to procedure repositioning, not basic block

repositioning.

Invoking PBO

You must perform the following steps to invoke PBO:

1. Compile and link with the +I option to produce the instrumented

program.

2. Collect execution proﬁle statistics by running the instrumented

program.

3. Optimize the program with the +P option.

Instrumenting the Program

To instrument the program, use the +I compile-line option when

compiling and linking the object ﬁles of an application. Instrumenting

the program inserts code into the program to collect execution proﬁle

statistics. Execution proﬁle statistics include a count of the calls between