>>> On 05.11.14 at 18:23, <l...@amacapital.net> wrote: > On Wed, Nov 5, 2014 at 9:13 AM, Jan Beulich <jbeul...@suse.com> wrote: >>>>> Andy Lutomirski <l...@amacapital.net> 11/04/14 8:40 PM >>> >>>On 11/04/2014 01:24 AM, Jan Beulich wrote: >>>> The main obstacle to having done this long ago was the need to >>>> determine whether annotations are needed in the first place: They need >>>> to be avoided when a frame pointer got set up. Since I can't see a way >>>> to determine this before the compilation phase, this is being achieved >>>> by inspecting the memory address generated by the compiler in an >>>> interposed assembler macro. Of course this isn't really nice code, and >>>> this the main reason I'm posting this as RFC only at this point (with >>>> the hope that maybe someone has an idea of how to achieve the same >>>> thing in a more elegant way). >>> >>>Ask binutils for help? >> >> Binutils know as little about the code the compiler generated as we do. > > Could binutils add a > .cfi_adjust_cfa_offset_if_the_cfa_depends_on_sp_right_now directive? > IIUC, the issue is that, when you push, you don't want the canonical > frame address to change as a result, but you just changed the stack > pointer, so if the CFA is computed as an offset from the stack pointer > in the current context, that offset needs to change.
While that's theoretically doable, I don't think this would be a reasonable approach. > Alternatively, is there any sane way to get the inline asm to act as > though it creates an entirely new frame? It would have CFA == rsp > initially (or rsp + 8 or whatever -- I can never keep track of what > the CFA is actually supposed to point to) and unwind instructions that > tell the unwinder that the caller pc is at a known address instead of > being stuck in the stack frame? No, that can't work: You'd have to - end the previous function (from the CFI engine's pov) - start a new function - do what you suggest above - end the "nested" function - start a continuation function for the subsequent compiler generated code - magically know the state of things at the point the original function got (artificially) ended >>>Is the issue that the CFI annotation you need is different depending on >>>whether there's a frame pointer or not? >> >> No - as said above, they need to be avoided altogether when there's a >> frame pointer. >> >>> If so, can you add some >>>comments so that mere asm mortals have some prayer of understanding how >>>your magic works and what the desired output annotations are in the >>>various cases? >> >> Honestly I have a hard time seeing where comments would help here. Plus >> the difficult part isn't how the annotations look like, but (see above) > simply >> whether to emit them at all. > > It could be something simple like an example of what the inputs to the > asm macros are in the two cases. Currently even figuring out where > those inputs come from involves following a big tangle of C > preprocessor stuff, and I don't know what it's supposed to output, and > what that's supposed to do the expansions in the inline asm, and how > that ends up influencing the gas macros. > > I.e. I sort of expect I'll need to want to one of these things some > day, and I'd like a couple pointers :) I'll see what I can do (but I'll invest time into doing so only if there are at least signs of this having a remote chance of going in at some point). Jan -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
