fixing -fsplit-stack

[Martin Uecker]

Am Dienstag, dem 20.10.2026 um 10:58 +0200 schrieb Thomas Neumann via Gcc:
> Hi,
> 
> I would like to use -fsplit-stack for some recursion heavy C++ code, but 
> I noticed that it currently has severe issues, which prevents practical 
> usage, but which are fixable: 1) currently a stack switch costs about 
> 167ns due to signal blocking, which is prohibitive in hot loops 2) the 
> stack computation for signal handlers is wrong, which leads to crashes 
> on machines with AVX-512 support, and 3) a signaler handler with split 
> stack causes a crash if the signal happens during exception unwinding.
> 
> I have prototype fixes for all three (x86-64 only for now). By making 
> __morestack re-entrant safe we can avoid signal blocking, bringing stack 
> switch cost to about 5ns, which fixes the hot loop problem. The other 
> two are largely classical bug fixes.
> 
> Before working on this more, are you actually interested in reviving 
> this functionality? I got the impression that the code is hardly used at 
> all (except for go, which uses a separate signal stack and thus avoids 
> some of these problems). But I think it would be useful to make it work, 
> as it makes recursive functions much more practical.

Only speaking for myself and I will not be able to help, but I would
definitely be interested in using this in C and I think others would too.

> 
> Thomas
> 
> PS: I noticed that alloca/VLA with split stack is broken, too, because 
> the memory allocated by __morestack_allocate_stack_space is not freed 
> properly. I don't care about this too much, and fixing it requires 
> compiler changes, thus I did not try to fix it. But perhaps it should be 
> addressed, too.

VLAs would be important for me.

What about longjmp / setjmp?

Martin