Re-architect MCCAS debug info section materialization to no longer be recursive or attach children DIEDataRefs to their parents

[rastogishubham]

With this change, the layout of the debug info section in MCCAS has been changed. Now, if there is a DIE that can be split off into it's own CAS block, we do not attach it to its parent CAS block as a reference, it gets added as a neighbor instead. This reduces the dependencies between CAS blocks and increases chances of deduplication if a DIEDataRef doesn't dedupe, by making sure it's parent will still dedupe.

This change also re-writes the materialization of the debug info section to be iterative, rather than recursive.

It also introduces the new cas block DIEDedupeTopLevelRef

The results of this work give us a cleaner instruments trace which enabled us to identify the function visitDIEAttrs as a place we could optimize MCCAS replay

[felipepiovezan]

So I still think we would benefit a lot from a simpler while loop, and I spent some time trying to figure out if this is feasiable. As you know better than I do, the devil is in the details, so I tried to be pretty thoughtful here. If you don't think this is possibe, that's fine, but what do you think of this algorithm?

(btw, you are right about the need to push the "data" into the stack as well as the reader, the visitDIEAttrs need to "peek" into the data, and the stream class doesn't give us access to it)

// DIEData == ArrayRef<StringRef>

BinaryStreamReader Reader(DIEData[0], /*offset*/ 0);
stack.push({Reader, DIEData[0]});
DIEData = DIEData.drop_front();

while(!stack.empty() {

  auto AbbrevIdx = DistinctReader.read_abbrev();

  if (AbbrevIdx == getDIEInAnotherBlockMarker()) {
    NewBlockCallback (...)
    BinaryStreamReader Reader(DIEData[0], /*offset*/ 0);
    stack.push(Reader, DIEData[0]);
    DIEData = DIEData.drop_front();
    continue;
  }

  auto CurrentReader, CurrentData = stack.pop_back_val();

  if (AbbrevIdx == getEndOfDIESiblingsMarker())
    EndTagCallback(true /*had children*/);
  else {
    AbbrevReader = get abbrev reader for AbbrevIdx;
    Tag = AbbrevReader.readTag();
    StartTagCallback(*MaybeTag, AbbrevIdx);
    visitDIEAttrs(AbbrevReader, CurrentReader, CurrentData))
    if (!AbbrevReader.HasChildren)
      EndTagCallback(false /*had children*/);
  }

  // Reached the end of a DIE, if this reader still has data in it, push it.
  if (!CurrentReader.empty())
    stack.push({CurrentReader, CurrentData});
}

[rastogishubham]

@swift-ci please test

[rastogishubham]

@swift-ci please test llvm

[rastogishubham]

@swift-ci please test llvm

[yoki31]

Ok thank you I go to home GitHub