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Download Compilation Systems Volume 2 (Concepts)
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Compilation Systems Volume 2 (Concepts) entirely from its actual arguments in a deterministic manner. This means that the compiler can eliminate the call if its result isn’t used or if its result is redundantly computed elsewhere in the caller routine (common subexpression elimination). If an actual argument is of pointer type, it is implied that the result is computed by manipulating the actual bits of the pointer value, not by referencing the object pointed to by the pointer. Obtaining Optimization Messages 20 Each compiler has a verbose (-v) option that produces output giving you more information about the compilation. Part of this output may include informative messages about optimization. These messages inform you when the optimizer has been unable to perform one or more optimizations because of the limits in effect for the compilation. You can usually correct the problem by using the -Q option to specify a higher limit. Refer to the appropriate language reference manual or compiler man page to learn how to specify the verbose option. Classes of Optimizations 20 CCG compilers perform the following classes of optimizations: • Branch optimizations Page 20-10 • Variable optimizations Page 20-12 • Expression optimizations Page 20-16 • Loop optimizations Page 20-18 • Register allocation Page 20-24 • Instruction scheduling Page 20-24 • Inline expansion of subprograms (Ada only) Page 20-26 • Optimization of constraints (Ada only) Page 20-27 Branch Optimizations 20 The compiler performs branch optimizations to minimize the number of branches in the program and to reduce memory requirements. These optimizations include the following: • Straightening blocks • Folding conditional tests • Eliminating unreachable code 20-10
