[ADT][DebugInfo][RemoveDIs] Add extra bits to ilist_iterator for debug-info

...behind an experimental CMAKE option that's off by default.

This patch adds a new ilist-iterator-like class that can carry two extra bits
as well as the usual node pointer. This is part of the project to remove
debug-intrinsics from LLVM: see the rationale here [0], they're needed to
signal whether a "position" in a BasicBlock includes any debug-info before or
after the iterator.

This entirely duplicates ilist_iterator, attempting re-use showed it to be a
false economy. It's enable-able through the existing ilist_node options
interface, hence a few sites where the instruction-list type needs to be
updated. The actual main feature, the extra bits in the class, aren't part of
the class unless the cmake flag is given: this is because there's a
compile-time cost associated with it, and I'd like to get everything in-tree
but off-by-default so that we can do proper comparisons.

Nothing actually makes use of this yet, but will do soon, see the Phab patch
stack.

[0] https://discourse.llvm.org/t/rfc-instruction-api-changes-needed-to-eliminate-debug-intrinsics-from-ir/68939

Differential Revision: https://reviews.llvm.org/D153777

1 files changed, 13 insertions, 2 deletions

diff --git a/llvm/lib/IR/BasicBlock.cpp b/llvm/lib/IR/BasicBlock.cpp
index d6677aa..46b1a3b 100644
--- a/llvm/lib/IR/BasicBlock.cpp
+++ b/llvm/lib/IR/BasicBlock.cpp
@@ -42,7 +42,8 @@ template <> void llvm::invalidateParentIListOrdering(BasicBlock *BB) {
 
 // Explicit instantiation of SymbolTableListTraits since some of the methods
 // are not in the public header file...
-template class llvm::SymbolTableListTraits<Instruction>;
+template class llvm::SymbolTableListTraits<Instruction,
+                                           ilist_iterator_bits<true>>;
 
 BasicBlock::BasicBlock(LLVMContext &C, const Twine &Name, Function *NewParent,
                        BasicBlock *InsertBefore)
@@ -221,7 +222,13 @@ const Instruction* BasicBlock::getFirstNonPHI() const {
 }
 
 BasicBlock::const_iterator BasicBlock::getFirstNonPHIIt() const {
-  return getFirstNonPHI()->getIterator();
+  const Instruction *I = getFirstNonPHI();
+  BasicBlock::const_iterator It = I->getIterator();
+  // Set the head-inclusive bit to indicate that this iterator includes
+  // any debug-info at the start of the block. This is a no-op unless the
+  // appropriate CMake flag is set.
+  It.setHeadBit(true);
+  return It;
 }
 
 const Instruction *BasicBlock::getFirstNonPHIOrDbg(bool SkipPseudoOp) const {
@@ -261,6 +268,10 @@ BasicBlock::const_iterator BasicBlock::getFirstInsertionPt() const {
 
   const_iterator InsertPt = FirstNonPHI->getIterator();
   if (InsertPt->isEHPad()) ++InsertPt;
+  // Set the head-inclusive bit to indicate that this iterator includes
+  // any debug-info at the start of the block. This is a no-op unless the
+  // appropriate CMake flag is set.
+  InsertPt.setHeadBit(true);
   return InsertPt;
 }