[LLVMdev] Segmented Stacks: Breaking libgcc compatibility

[Sanjoy Das]

Hi llvmdev!

I've been working on implementing support for segmented stacks in LLVM
(towards GSoC '11). Currently I'm working on adding intrinsics for
coroutines. The problem is this:

Till now I had been depending on libgcc for runtime support (and was
being 100% libgcc compatible in the process). However, since all the
stack allocation routines in libgcc depend on TLS variables to manage
dynamically allocated stack blocks (stacklets), it may not be possible
to have more than one stack in a single thread. This makes coroutines
less interesting, since the whole point was to use multiple, cheap
stacks from a single thread (and switch between them, at will).

My current idea is to port segmented stack related routines to
compiler-rt (or to a different "segmented-stacks" library
altogether). That way, __generic_morestack (the function in libgcc
that allocates space for a new stacklet) gets a cousin
__generic_morestack_with_context, which take a pointer to a "context"
holding the otherwise thread local variables use by libgcc's
__generic_morestack. That way, every coroutine can have its own
"context" which it passes to __generic_morestack_context.

__generic_morestack in compiler-rt can then call __generic_morestack
with a context which it holds in a TLS variable, to take care of the
most common case.

Does this sound sane?

Thanks!

--
Sanjoy Das
http://playingwithpointers.com