[llvm] [DebugInfo][RemoveDIs] Handle a debug-info splicing corner case (PR #73810)

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -829,6 +829,90 @@ void BasicBlock::spliceDebugInfoEmptyBlock(BasicBlock::iterator Dest,
 void BasicBlock::spliceDebugInfo(BasicBlock::iterator Dest, BasicBlock *Src,
                                  BasicBlock::iterator First,
                                  BasicBlock::iterator Last) {
+  /* Do a quick normalisation before calling the real splice implementation. We
+     might be operating on a degenerate basic block that has no instructions
+     in it, a legitimate transient state. In that case, Dest will be end() and
+     any DPValues temporarily stored in the TrailingDPValues map in LLVMContext.
+     We might illustrate it thus:
+
+                         Dest
+                           |
+     this-block:    ~~~~~~~~
+      Src-block             ++++B---B---B---B:::C
+                                |               |
+                               First           Last
+
+     However: does the caller expect the "~" DPValues to end up before or after
+     the spliced segment? This is communciated in the "Head" bit of Dest, which
+     signals whether the caller called begin() or end() on this block.
+
+     If the head bit is set, then all is well, we leave DPValues trailing just
+     like how dbg.value instructions would trail after instructions spliced to
+     the beginning of this block.
+
+     If the head bit isn't set, then try to jam the "~" DPValues onto the front
+     of the First instruction, then splice like normal, which joins the "~"
+     DPValues with the "+" DPValues. However if the "+" DPValues are supposed to
+     be left behind in Src, then:
+      * detach the "+" DPValues,
+      * move the "~" DPValues onto First,
+      * splice like normal,
+      * replace the "+" DPValues onto the Last position.
+     Complicated, but gets the job done. */
+
+  // If we're inserting at end(), and not in front of dangling DPValues, then
+  // move the DPValues onto "First". They'll then be moved naturally in the
+  // splice process.
+  DPMarker *MoreDanglingDPValues = nullptr;
+  DPMarker *OurTrailingDPValues = getTrailingDPValues();
+  if (Dest == end() && !Dest.getHeadBit() && OurTrailingDPValues) {
+    // Are the "+" DPValues not supposed to move? If so, detach them
+    // temporarily.
+    if (!First.getHeadBit() && Src->getMarker(First) &&
+        !Src->getMarker(First)->empty()) {
+      MoreDanglingDPValues = Src->getMarker(First);
+      MoreDanglingDPValues->removeFromParent();
+    }
+
+    if (DPMarker *CurMarker = Src->getMarker(First);
+        CurMarker && !CurMarker->empty()) {
----------------
OCHyams wrote:

```suggestion
    if (Src->hasDbgValues()) {
        DPMarker *CurMarker = Src->getMarker();
```
nit

https://github.com/llvm/llvm-project/pull/73810