[llvm-dev] LLVM and heap-allocated thread stacks

[Demi M. Obenour via llvm-dev]

In some language implementations, such as the Glasgow Haskell Compiler
(GHC) and the reference implementation of Go, a thread’s stack is
allocated as a data structure on the garbage-collected heap.  The
garbage collector is free to move this data structure whenever it is
invoked.

Currently, GHC’s LLVM backend does not use the C stack.  However, there
have been discussions about whether using the C stack could lead to a
performance gain.  I think it could.  The elephant in the room, however,
is that */any call into the RTS may then change the stack pointer*./ 
LLVM presumably has no support for this.  Without such support, however,
GHC must spill all locals to memory at every call into the RTS.  It
seems to me that this is why GHC cannot transform its output into SSA
form: GHC must reify its stack.

It would be nice (both for GHC and for llgo) if LLVM could be made to
treat the stack pointer as a volatile register that may be changed by
any function call.  In this model, the stack pointer needs to be treated
the same as any other GC’d object — stack maps need to be emitted for
it, and the RTS is allowed to relocate it.

Would this be practical?  If so, it would be a major boon to the
implementation of lightweight threading in languages that compile to
LLVM IR.

Sincerely,

Demi Obenour
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