HP C Programmer's Guide (92434-90009)

Chapter 4 65

Optimizing HP C Programs

Removing Compilation Time Limits When Optimizing

Aggressive optimizations are new optimizations or are optimizations that can change the

behavior of programs. These optimizations may do any of the following:

• convert certain library calls to millicode and inline instructions

• cause the inlined routines strcpy(), sqrt(), fabs(), and alloca() to not return the

routine's completion status in ERRNO

• alter exception handling and asynchronous interrupt handling as a result of instruction

scheduling optimization

• cause less precise ﬂoating-point results

• cause programs that perform comparisons between pointers to shared memory and

pointers to private memory to run incorrectly

Use aggressive optimizations with stable, well-structured, ANSI-conforming code. These

types of optimizations give you faster code, but are riskier than the default optimizations.

Removing Compilation Time Limits When Optimizing

You can remove optimization time restrictions at the second, third, or fourth optimization

levels by using the +Onolimit option as follows:

cc +O2 +Onolimit

sourcefile

or:

cc +O3 +Onolimit

sourcefile

or:

cc +O4 +Onolimit

sourcefile

By default, the optimizer limits the amount of time spent optimizing large programs at

levels 2, 3, and 4. Use this option if longer compile times are acceptable because you want

additional optimizations to be performed.

Limiting the Size of Optimized Code

You can disable optimizations that expand code size at the second, third, and fourth

optimization levels by using the +Osize suboption, as follows:

cc +O2 +Osize

sourcefile

or:

cc +O3 +Osize

sourcefile

or: