> On Fri, Jun 9, 2017 at 3:16 AM, Jan Hubicka <hubi...@ucw.cz> wrote: > > > >> code compiled with -fsplit-stack, if the cold partition calls a > >> function that is not compiled with -fsplit-stack (such as a C library > >> function). The problem is that when the linker sees a split-stack > >> function call a non-split-stack function, it adjusts the function > >> header to request more stack space. This doesn't work if the call is > >> in the cold partition, as the linker doesn't know how to find the > >> header to adjust. You can see this by trying to build the Go library > >> using the gold linker with this patch. > > > > If code does not work, I wonder why we let user to overwrite the default > > by hand? In other cases we drop the flag with inform message. > > My thinking here is that there is no fundamental reason that the code > does not work, and the actual problem does not lie in GCC but rather > in the linker (specifically, gold). It's possible in principle to fix > gold to make this work, and someone who is using a fixed gold could > then direct GCC to take advantage of this optimization (and later > after that version of gold is wide-spread enough we can change GCC to > drop this patch).
Thanks for explanation. Perhaps we could have this documented, because otherwise people will think the option is simply broken. I guess even better we could have configure autodetection for the broken linker. > > > > Also bb-reorder knows how to prevent landing pads to go to different > > sections, > > so perhaps same machinery can be used to prevent splitting blocks having > > calls that needs linker adjustments? > > Unfortunately I don't see how that is possible in general, as the code > that needs adjustment is cases where code compiled with -fsplit-stack > calls functions compiled without -fsplit-stack. By definition those > calls are to functions defined in other compilation units, and the > compiler simply doesn't know whether they will be compiled with > -fsplit-stack or not. Only the linker knows. I see. We could stil block offlining all blocks that contains calls to functions that does not bind to current defs, but I guess that would prevent most of useful code plitting anyway. Thank you! Honza > > Ian
