On Tue, 2009-05-05 at 15:51 +0200, Segher Boessenkool wrote: > >>> When we add "-pg" to gcc, it automatically causes frame pointers > >>> to be > >>> used. > >> > >> Nope, it does no such thing. > > > > Well, mcount is expected to be able to get to not just who called > > mcount, but also the parent of that function. The way mcount is > > implemented does not let you do that. If mcount was the first thing to > > be called in a function, then it would have been perfect. We could get > > to the caller, its parent, and even the parameters. But unfortunately, > > mcount is called after the stack is set up. Thus, without frame > > pointers > > (the way to find a previous frame) there's no way (on some archs) to > > find the parent. Nor can we figure out the parameters, which really > > sucks. > > Yes, and this is (supposedly) why GCC does not like seeing -pg and > -fomit-frame-pointer at the same time -- because that cannot work > *on some architectures*. These are the same architectures that > do not enable -fomit-frame-pointer automatically at -O. > > >> NO_NO_OMIT_FRAME_POINTER ? Or better, just never use -fno-o-f-p, > >> I don't see why you would ever need it. > > > > Because on x86_64 it gives better back traces. x86_64 has no way to > > get > > to the previous frames without it. There's code to use other debug > > metadata to get back tracing, but for uses of things like the stack > > tracer, we need to be able to use the actual stack frames. > > > > As you said above, -fomit-frame-pointer is default when we > > optimize, and > > that is how the kernel is built. If we optimize on x86_64 and do > > not use > > -fno-omit-frame-pointer, the stack tracer is useless. > > No. -fomit-frame-pointer is only the default when optimising on > archs/ABIs where it doesn't hinder debugging and -pg and all that > goodness; specifically, you do not get it by default on x86, not > at any optimisation level.
OK, so what's the status with this patch series? I don't want to pull it in unless I have an ack from Sam, and now there's issues with having -fno-omit-frame-pointer. Should we add a patch instead that simply removes that? If we eliminate the -fno-omit-frame-pointer, would that solve the PPC problem? And would it cause any other issues with other archs? -- Steve _______________________________________________ Linuxppc-dev mailing list Linuxppc-dev@ozlabs.org https://ozlabs.org/mailman/listinfo/linuxppc-dev
