HP Fortran Programmer's Guide (March 2010)

Performance and optimization
Using options to control optimization
Chapter 6 151
where opt-level is an integer in the range 0-2. If you use the -g option at a
higher level of optimization, the compiler lowers the level to 2 and compiles for
debugging.
Table 6-1 Optimization levels
Option
Optimizations
performed Advantages Disadvantages
Recommended
use
+O0
default
Constant folding and
partial evaluation of
test conditions.
Compiles
fastest;
compatible
with the
debugger
option -g.
Does very little
optimization.
During program
development.
+O1 Level 0 optimizations,
plus branch
optimization, dead
code elimination, more
efficient use of
registers, instruction
scheduling, and
peephole optimization.
Produces faster
programs than
level 0;
compiles faster
than level 2;
compatible
with the
debugger
option -g.
Compiles slower
than level 0.
During program
development.
+O2, -O Default level
optimizations,
including level 1, plus
coloring register
allocation, induction
variable elimination
and strength
reduction, common
subexpression
elimination, loop
invariant code motion,
store/copy
optimization, unused
definition elimination,
software pipelining,
and register
reassociation.
Can
significantly
increase
performance
over level 1;
works with
debugger
option -g.
Compiles slower
than level 0 and
1.
During program
development and
when building
the production
version;
especially
effective in
optimizing loops
that perform
arithmetic
operations on
large float and
double arrays.