Re: Patch RFC: disable block partitioning with split stack

[Jan Hubicka <hubicka@ucw.cz>]

> On Fri, Jun 9, 2017 at 3:16 AM, Jan Hubicka <hubicka@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