[llvm-dev] Purpose of glue nodes in call lowering

[Jon Chesterfield via llvm-dev]

Unfortunately our backend is not yet in experimental. I think AArch64
LowerCall is representative.

  // Build a sequence of copy-to-reg nodes chained together with token chain
  // and flag operands which copy the outgoing args into the appropriate
regs.
  SDValue InFlag;
  for (auto &RegToPass : RegsToPass) {
    Chain = DAG.getCopyToReg(Chain, DL, RegToPass.first,
                             RegToPass.second, InFlag);
    InFlag = Chain.getValue(1);
  }

I have two questions about this. Primarily, what is the purpose of
threading InFlag through the copyToReg nodes? It seems to be a common
choice across multiple backends.

My understanding is that glue nodes are used to "stick" other nodes
together, increasing the probability that they end up near each other at
scheduling. I think that would make them optional in the sense that codegen
would be correct in their absence. If so, I would like to remove them from
our backend as the scheduler will probably do a reasonable job without
hints.

The second query is whether each copy needs to be dependent on the previous
argument. I expected each copy to take the same chain node, with the the
chain nodes returned by getCopyToReg merged using a TokenFactor. The goal
would be to allow the register allocator / scheduler to rearrange the
instructions used to pass arguments.

If each copy can indeed be passed the same chain node to indicate that they
are independent of one another then I should be able to get better codegen
out of the register allocator.

Thanks!

Jon
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180725/4511fb91/attachment.html>