11.16.1 Overall control of optimizations

-O N

--optimization-level N

--optimisation-level N

--opt-level N

Set optimization level to N. Optimization level -1 means no optimization while optimization level 6 means full optimization. The option ‘--output-optimization-options’ lists the optimizations enabled at each level.

Note that some options are not enabled automatically at any optimization level. These include options that are too new and experimental for large scale use, and options that generate speedups in some use cases, but slowdowns in others, require situation-specific consideration of their use.

Likewise, if you want the compiler to perform intermodule optimizations, where the compiler exploits information about the non-public parts of the modules it imports (which it gets from their .opt files) for optimization purposes then you must enable that separately, partially because they affect the compilation process in ways that require special treatment by mmake. This goes double for transitive intermodule optimizations, where the compiler exploits information about the non-public parts of not just the modules it imports, but also from the modules that they) import, directly or indirectly. (It gets this info from the .trans_opt files of the directly or indirectly imported modules.)

--optimize-space

--optimise-space

--opt-space

Turn on optimizations that reduce code size, and turn off optimizations that significantly increase code size.