HP aC++/HP C A.06.25 Programmer's Guide

Specifically, level 2 provides the following:
• Coloring register allocation.
• Induction variable elimination and strength reduction.
• Local and global common subexpression elimination.
• Advanced constant folding and propagation. (Simple constant folding is done by
default.)
• Loop invariant code motion.
• Store/copy optimization.
• Unused definition elimination.
• Software pipelining.
• Register reassociation.
Level 2 can produce faster runtime code than level 1 if programs use loops extensively.
Loop-oriented floating-point intensive applications may see run times reduced by 50%.
Operating system and interactive applications that use the already optimized system
libraries can achieve 30% to 50% additional improvement. Level 2 optimization produces
faster programs than level 1 and compiles faster than level 3 optimization.
Level 3 Optimization
Level 3 optimization includes level 2 optimizations, along with full optimization across
all subprograms within a single file. Level 3 also inlines certain subprograms within
the input file. Use +O3 to get level 3 optimization.
Level 3 optimization produces faster runtime code than level 2 on code that does many
procedure calls to small functions. Level 3 links faster than level 4. But level 3 does not
work with the debugger options -g0 and -g1.
Level 4 Optimization
Level 4 optimization includes level 3 optimizations, along with full optimizations across
the entire application program. Level 4 includes global and static variable optimization
and inlining across the entire program. Optimizations are performed at link time rather
than at compile time. Use +O4 to get level 4 optimization.
Level 4 optimization produces faster runtime code than level 3 if programs use many
global variables or if there are many opportunities for inlining procedure calls. But
level 4 does not work with the debugger options -g0 and -g1.
Additional Options for Finer Control
In addition to basic optimization levels, optimization options are provided should you
require a more precise level of control.
Some introductory examples follow:
206 Optimizing HP aC++ Programs