[DebugInfo][RemoveDIs] "Final" cleanup for non-instr debug-info (#79121)

Here's a raft of minor fixes for the RemoveDIs project that's replacing
dbg.value intrinsics with DPValue objects, all IMO trivial:
 * When inserting functions or blocks and calling setIsNewDbgInfoFormat,
   do that after setting the Parent pointer, just in case conversion from
   (or to) dbg.value mode is triggered.
 * When transferring DPValues from an empty range in a splice call, don't
   transfer if there are no DPValues attached to the source block at all.
 * stripNonLineTableDebugInfo should drop DPValues.
 * In insertBefore, don't try to transfer DPValues if there aren't any.

llvm/include/llvm/IR/Function.h

@@ -722,8 +722,9 @@ public:
   /// Insert \p BB in the basic block list at \p Position. \Returns an iterator
   /// to the newly inserted BB.
   Function::iterator insert(Function::iterator Position, BasicBlock *BB) {
+    Function::iterator FIt = BasicBlocks.insert(Position, BB);
     BB->setIsNewDbgInfoFormat(IsNewDbgInfoFormat);
-    return BasicBlocks.insert(Position, BB);
+    return FIt;
   }
 
   /// Transfer all blocks from \p FromF to this function at \p ToIt.

llvm/lib/IR/BasicBlock.cpp

@@ -237,12 +237,12 @@ void BasicBlock::insertInto(Function *NewParent, BasicBlock *InsertBefore) {
   assert(NewParent && "Expected a parent");
   assert(!Parent && "Already has a parent");
 
-  setIsNewDbgInfoFormat(NewParent->IsNewDbgInfoFormat);
-
   if (InsertBefore)
     NewParent->insert(InsertBefore->getIterator(), this);
   else
     NewParent->insert(NewParent->end(), this);
+
+  setIsNewDbgInfoFormat(NewParent->IsNewDbgInfoFormat);
 }
 
 BasicBlock::~BasicBlock() {
@@ -821,9 +821,15 @@ void BasicBlock::spliceDebugInfoEmptyBlock(BasicBlock::iterator Dest,
 
   // There are instructions in this block; if the First iterator was
   // with begin() / getFirstInsertionPt() then the caller intended debug-info
-  // at the start of the block to be transferred.
-  if (!Src->empty() && First == Src->begin() && ReadFromHead)
-    Dest->DbgMarker->absorbDebugValues(*First->DbgMarker, InsertAtHead);
+  // at the start of the block to be transferred. Return otherwise.
+  if (Src->empty() || First != Src->begin() || !ReadFromHead)
+    return;
+
+  // Is there actually anything to transfer?
+  if (!First->hasDbgValues())
+    return;
+
+  createMarker(Dest)->absorbDebugValues(*First->DbgMarker, InsertAtHead);
 
   return;
 }

llvm/lib/IR/DebugInfo.cpp

@@ -892,6 +892,9 @@ bool llvm::stripNonLineTableDebugInfo(Module &M) {
         // Strip heapallocsite attachments, they point into the DIType system.
         if (I.hasMetadataOtherThanDebugLoc())
           I.setMetadata("heapallocsite", nullptr);
+
+        // Strip any DPValues attached.
+        I.dropDbgValues();
       }
     }
   }

llvm/lib/IR/Instruction.cpp

@@ -146,9 +146,9 @@ void Instruction::insertBefore(BasicBlock &BB,
   bool InsertAtHead = InsertPos.getHeadBit();
   if (!InsertAtHead) {
     DPMarker *SrcMarker = BB.getMarker(InsertPos);
-    if (!SrcMarker)
-      SrcMarker = BB.createMarker(InsertPos);
-    DbgMarker->absorbDebugValues(*SrcMarker, false);
+    // If there's no source marker, InsertPos is very likely end().
+    if (SrcMarker)
+      DbgMarker->absorbDebugValues(*SrcMarker, false);
   }
 
   // If we're inserting a terminator, check if we need to flush out

llvm/test/DebugInfo/salvage-limit-expr-size.ll

@@ -1,4 +1,5 @@
 ; RUN: opt %s -passes=dce -S | FileCheck %s
+; RUN: opt %s -passes=dce -S --try-experimental-debuginfo-iterators | FileCheck %s
 
 ;; Tests that a DIExpression will only be salvaged up to a certain length, and
 ;; will produce an undef value if an expression would need to exceed that length.

llvm/test/Transforms/Util/strip-nonlinetable-debuginfo-localvars.ll

@@ -1,4 +1,5 @@
 ; RUN: opt -S -passes=strip-nonlinetable-debuginfo %s -o - | FileCheck %s
+; RUN: opt -S -passes=strip-nonlinetable-debuginfo %s -o - --try-experimental-debuginfo-iterators | FileCheck %s
 ; CHECK: define void @f() !dbg ![[F:[0-9]+]]
 define void @f() !dbg !4 {
 entry: