HP 3000 Manuals HP 3000 Manuals
Optimizer Levels
The levels of code optimization that the MPE/iX Optimizer provides are
Level 0, Level 1, and Level 2.
Level 0 optimization is the default optimization level and provides
simple optimizations that minimize compile time. Use this level when
debugging a program or running a program that will not be run many times
(for example, a student job program that is run a few times and
discarded). Level 0 functions include:
* Simple branch delay slot scheduling.
* Dead code elimination.
* Faster register allocation (including copy elimination).
Level 1 optimization is local to basic blocks, but does not optimize
globally. Use Level 1 to achieve some optimization without spending
excessive time compiling. Level 1 functions include:
* Branch optimization.
* Dead code elimination.
* Faster register allocation (including copy elimination).
* Instruction scheduling.
* Peephole optimization.
Level 2 optimization is global optimization. It provides the greatest
saving of space and time achievable with the Optimizer and produces the
most compact and fastest running program of all the levels. Use Level 2
when running a debugged program a large number of times. This is the
correct level to choose for most applications. Level 2 functions are:
* All Level 1 optimizations.
* Coloring register allocation.
* Induction variable elaboration and strength reduction. (An
induction variable is a variable dependent on the value of a loop
counter.)
* Common subexpression elimination.
* Constant folding.
* Loop invariant code motion.
* Unused definition elimination.
* Promotion of variables to registers.