Techniques used in optimization levels 4 and 5
At optimization levels 4 and 5, all of the techniques in optimization levels
2 and 3 are used, plus the following:
- High-order transformations, which provide optimized handling
of loop nests (equivalent to compiling with the -qhot=level=1 option).
- Interprocedural analysis, which invokes the optimizer at link time to
perform optimizations across multiple source files (equivalent to compiling
with the -qipa option).
- Hardware-specific optimization (equivalent to compiling with the -qarch=auto, -qtune=auto, and -qcache=auto options).
- At optimization level 5, more detailed interprocedural analysis (the equivalent
to compiling with the -qipa=level=2 option). With
level 2 IPA, high-order transformations are delayed until link time, after
whole-program information has been collected.