[llvm-dev] Advise to implement return barriar

[Chuan Qiu via llvm-dev]

Hi llvm-dev,
I'm trying to build a managed language that requires function call with
return barrier using LLVM IR.

During execution of a function call, if a special event during the call
happens, it will overwrite the return address in the stack frame to another
block, then continue the execution in the original function until it's
actually returned, then execute a different block.

It will be equivalent to the following pseudo code:

flag, X = doSomeCall()
if (!flag) {
use1:
  // something use X
} else {
use2:
  // something else use X
}

While I'd like to make the flag implicit by changing the return address in
stack frame from use1 to use2 and duplicate the post-call part, because
such flag check could be added to almost every call site for my language
and the probability returning other branch will be extremely unlikely.

May I get some advise on how to implement the return barrier like this,
even just locally?

Introducing a new type of invoke inst that allows 2 normal destinations
plus 2 unwind destination seems too much changes to make.

Currently, I'm trying to add a new intrinsic IMPLICIT_FORK that returns a
bool that can be used for conditional branch to introduce an extra
successor in IR without emitting an code to trick the CFG passes, and that
successor block must start with another intrinsic call CONTINUE which
continues the call: it just emit a NOP but returns the value, and PC for
the NOP will be stored by some global metadata value so the runtime knows
which is the new PC value to overwrite, and address of NOP inst can still
be used to unwind the stack properly.

I've also tried using a general trampoline to be new return address, and
based on return value from calling convention to record the return value,
and throw an exception to go back to original method.

Cheers
Chuan
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