On Tue, Aug 4, 2015 at 10:43 AM, Segher Boessenkool <seg...@kernel.crashing.org> wrote: > On Tue, Aug 04, 2015 at 10:28:00AM -0700, H.J. Lu wrote: >> >> Any comments on my middle-end patch? >> > >> > So, if the answer is the same as frame_address (0), why not have the >> > fallback just expand to that? Then, one can use this builtin everywhere >> > that frame address is used today. People that want a faster, tighter port >> > can then implement the hook and achieve higher performance. >> >> The motivation of __builtin_stack_top is that frame_address requires a >> frame pointer register, which isn't desirable for x86. __builtin_stack_top >> doesn't require a frame pointer register. > > If the target just returns frame_pointer_rtx from INITIAL_FRAME_ADDRESS_RTX, > you don't get crtl->accesses_prior_frames set either, and as far as I can > see everything works fine? For __builtin_frame_address(0). > > You might have a reason why you want the entry stack address instead of the > frame address, but you didn't really explain I think? Or I missed it. >
expand_builtin_return_addr sets crtl->accesses_prior_frames = 1; for __builtin_frame_address, which requires a frame pointer register. __builtin_stack_top doesn't set crtl->accesses_prior_frames and frame pointer register isn't required. -- H.J.
