[PowerPC][NFC] refactor eligible check for tail call optimization

[tingwang]

Summary

The check logic for TCO is scattered in two functions: IsEligibleForTailCallOptimization_64SVR4() IsEligibleForTailCallOptimization(), and serves instruction selection phase only at this moment.

There is one scenario (D131953) during transformation, we would like to determine if tailcall can be guaranteed or not. Existing logic cannot be directly reused due to dependency on DAG and SDValue etc.

This patch aims to refactor existing logic to export an API for TCO eligible query before instruction selection phase.

[shchenz]

LGTM. Please wait for some days for other reviewers.

nit: I think this patch should be NFC, right? If so please update the patch title.

[nemanjai]

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

5650–5660

This seems like a strange structure. You have 3 disjoint conditions that each simply set a Boolean variable that is then returned. Why not:

if (...)
  return false;
else if (...)
  return IsEligibleForTailCallOptimization_64SVR4(...);
else
  return IsEligibleForTailCallOptimization(...);

[tingwang]

Address comments.

[shchenz]

llvm/lib/Target/PowerPC/PPCISelLowering.cpp

5651

nit: no need for the two else.

[tingwang]

Address comment and removed the second else.

[tingwang]

Continue NIT: maybe remove the first else makes more sense...

[nemanjai]

LGTM. Thanks.