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Fortran 90, Fortran 77, C, aC++: Exemplar Programming Guide

Chapter 3 Compiler optimizations

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Optimization levels

Using the optimizer

General guidelines
Fortran guidelines
C and C++ guidelines

+O0 level optimizations

Constant folding
Partial evaluation of test conditions
Simple register assignment
Data alignment on natural boundaries

+O1 level optimizations

Branch optimization
Dead code elimination
Faster register allocation
Instruction scheduler
Peephole optimizations

+O2 level optimizations

Global register allocation
Strength reduction of induction variables and constants
Common subexpression elimination
Advanced constant folding and propagation
Loop-invariant code motion
Store/copy optimization
Unused definition elimination
Software pipelining
Register reassociation
Loop unrolling

+O3 level optimizations

Inlining within a single source file
Cloning within a single source file
Test promotion
Data localization
Preventing loop reordering
Strip mining
Loop distribution
Loop interchange
Loop blocking
Loop fusion
Loop unroll and jam
Parallelization
Node-parallelism vs. thread-parallelism
Parallel optimizations
Inhibitors of parallelization
Reductions
Preventing parallelization
Other parallelization directives and pragmas
Parallelism in HP aC++

+O4 level optimization

Inlining across multiple source files
Cloning across multiple source files
Global and static variable optimizations

This chapter discusses the various optimization levels available with the Exemplar compilers and explains the optimizations performed at each level.

The Fortran 90 compiler is located at /opt/fortran90/bin/f90.

The two Fortran 77 compilers are:

f77 is the Fortran 77 compiler and is located at /opt/fortran/bin/f77
fort77 is the POSIX-conforming Fortran 77 compiler and is located at /opt/fortran/bin/fort77

The remainder of this manual refers to the f77 compiler. Any f77 example applies to the fort77 compiler.

The two C compilers are:

cc is the C compiler and is located at /opt/ansic/bin/cc
c89 is the POSIX-conforming C compiler and is located at /opt/ansic/bin/c89

The remainder of this manual refers to the cc compiler. Any cc example applies to the c89 compiler.

The aC++ compiler is located at /opt/aCC/bin/aCC.

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