Miscellaneous Changes to Existing Optimizers

We have modified following existing optimizers to use profile feedback:

register referencing

Register referencing pass decides what register class the given pseudo register should be allocated in. This is done by combining preferences of all instructions mentioning the given register. We now weight the preferences by expected frequencies of basic blocks.

register allocation

GCC register allocator simply assigns each pseudo register a priority and then allocates registers to pseudo registers in the priority order giving each pseudo register the first register available in its preferred class and then the first available register in the alternate class if it did not succeed in the preferred class.

We have modified the code to compute pseudo register priority to use frequencies instead of simple loop depth heuristics and have important success with this change speeding up the resulting code by about 0.6%.

final pass

Final pass outputs code alignment recommended for function body by target machine description. We omit the alignment, where allowed, for functions not executed in the training run when profile feedback is available.

register-stack conversion

Register stack conversion pass has property that it always produces code optimal only for one entry path to each basic block. We have modified its traversal to take into account frequencies in order to optimize the most frequent paths.