Daniel Towner wrote: > register renaming is able to change the registers used by the function > from callee-save to caller-save, removing any need for the > save/restore/stack adjust code in the prologue/epilogue. However, the > instructions which perform these operations are still emitted.
I doubt that any port does this optimization. You would have to redo the bulk of reload: register elimination, assigning stack slot addresses, recomputing frame size and register save offsets, etc. Gcc isn't set up to do this. In the simplest case, if you have no locals, no frame pointer, no alloca, no outgoing args on the stack, etc, then a simple RTL scan of the function may prove that SP/FP/AP are never used, allowing us to remove code in the prologue and epilogue. However, I'm skeptical that this would occur often enough to be useful. And it would require some target dependent code to figure out how to do the rewrites, which may make this complicated. It is probably easier to try to avoid this situation, by getting better register allocation earlier. Make sure that the caller-save registers occur before the callee-save registers in REG_ALLOC_ORDER. Verify that the caller-save.c code which is used in reload is working for your target. We could try moving some of the regrename optimizations to before reload. -- Jim Wilson, GNU Tools Support, http://www.specifix.com
