llvm::isInTailCallPosition TailCall Debug

/// Test if the given instruction is in a position to be optimized
/// with a tail-call. This roughly means that it's in a block with
/// a return and there's nothing that needs to be scheduled
/// between it and the return.
///
/// This function only tests target-independent requirements.
bool llvm::isInTailCallPosition(const CallBase &Call, const TargetMachine &TM) {
  const BasicBlock *ExitBB = Call.getParent();
  const Instruction *Term = ExitBB->getTerminator();
  const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);

  // The block must end in a return statement or unreachable.
  //
  // FIXME: Decline tailcall if it's not guaranteed and if the block ends in
  // an unreachable, for now. The way tailcall optimization is currently
  // implemented means it will add an epilogue followed by a jump. That is
  // not profitable. Also, if the callee is a special function (e.g.
  // longjmp on x86), it can end up causing miscompilation that has not
  // been fully understood.
  if (!Ret &&
      ((!TM.Options.GuaranteedTailCallOpt &&
        Call.getCallingConv() != CallingConv::Tail) || !isa<UnreachableInst>(Term)))
    return false;

  // If I will have a chain, make sure no other instruction that will have a
  // chain interposes between I and the return.
  if (Call.mayHaveSideEffects() || Call.mayReadFromMemory() ||
      !isSafeToSpeculativelyExecute(&Call))
    for (BasicBlock::const_iterator BBI = std::prev(ExitBB->end(), 2);; --BBI) {
      if (&*BBI == &Call)
        break;
      // Debug info intrinsics do not get in the way of tail call optimization.
      if (isa<DbgInfoIntrinsic>(BBI))
        continue;
      // A lifetime end or assume intrinsic should not stop tail call
      // optimization.
      if (const IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI))
        if (II->getIntrinsicID() == Intrinsic::lifetime_end ||
            II->getIntrinsicID() == Intrinsic::assume)
          continue;
      if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
          !isSafeToSpeculativelyExecute(&*BBI))
        return false;
    }

  const Function *F = ExitBB->getParent();
  return returnTypeIsEligibleForTailCall(
      F, &Call, Ret, *TM.getSubtargetImpl(*F)->getTargetLowering());
}