[LLVMdev] Fixing segmented stacks

[Jakob Stoklund Olesen]

On Oct 18, 2011, at 2:02 PM, Sanjoy Das wrote:

> This about fixing the issue with having a the BB ending with a
> non-terminating instruction when compiling with segmented stacks.  I'm
> not sure if having an isel pseudo instruction which is lowered into a
> RET and then a MOV would work better.
> LLVMTargetMachine::addCommonCodeGenPasses adds the
> ExpandISelPseudosPass before the PEI pass (so it boils down to the
> same thing, I think -- the verifier will still complain).  This is, of
> course, assuming I understood Duncan correctly.
> 
> The only fix I can think of is splitting the thing into two separate
> basic blocks.  I already have sent Duncan patches which does this. Of
> course, then I'll have a BB that ends with a RET, but has successors.
> As it was pointed out, this may not be desirable.
> 
> Is there some third way, better way?  I think the best solution would
> involve delaying the addition of this extra bit of code till the very
> last moment.  How about adding something to
> X86TargetMachine::addPreEmitPass?  Will that work?

If I understand correctly, you need to insert:

  call _morestack
  ret

in the middle of a basic block. There are two problems:

1. RET is a terminator, and can only appear at the end of a basic block.
2. The CALL and RET instructions must be adjacent.

The best way to handle this is to create a pseudo-instruction representing the call+ret pair.

Since the two instructions must stay together, it should be expanded late: In lib/Target/X86/X86MCInstLower.cpp.

/jakob