Hi! On Tue, Jul 08, 2025 at 08:51:30AM +0000, Cui, Lili wrote: > > rs6000 does not *have* a hard frame pointer! > > Oh, I see. The handling of HARD_FRAME_POINTER_REGNUM seems redundant for > rs6000.
The Power Architecture, Power ISA, nor any of our ABIs has a frame pointer. GCC generic code requires one to exist though (for no reason at all), so when given -fno-omit-framepointer we dedicate GPR for it (we use 31, I had to look it up to confirm it even, we never actually want to use it!) > Yes, -fomit-frame-pointer does help performance. Here is a simple small case > https://godbolt.org/z/5Tc3jM7qc . Do you mean to optimize the %rbp here? You tell GCC you want a frame pointer. It follows your instructions. There is a function call, so the frame pointer is saved before it; the compiler does not realise it does not actually use the frame pointer ever, it could optimise this whole stuff out (that's what in rs6000 the frame_pointer_needed_indeed thing is for: frame_pointer_needed_indeed = frame_pointer_needed && df_regs_ever_live_p (HARD_FRAME_POINTER_REGNUM); ) Segher
