[llvm] [DebugInfo][RemoveDIs] Support maintaining DPValues in CodeGenPrepare (PR #73660)

[Orlando Cazalet-Hyams via llvm-commits]

================
@@ -8421,6 +8479,65 @@ bool CodeGenPrepare::placeDbgValues(Function &F) {
       }
     }
   }
+
+  return MadeChange;
+}
+
+bool CodeGenPrepare::placeDPValues(Instruction &I, DominatorTree &DT) {
+  bool MadeChange = false;
+
+  for (DPValue &DPV : llvm::make_early_inc_range(I.getDbgValueRange())) {
+    if (DPV.Type != DPValue::LocationType::Value)
+      continue;
+
+    SmallVector<Instruction *, 4> VIs;
+    for (Value *V : DPV.location_ops())
+      if (Instruction *VI = dyn_cast_or_null<Instruction>(V))
+        VIs.push_back(VI);
+
+    // This DPV may depend on multiple instructions, complicating any
+    // potential sink. This block takes the defensive approach, opting to
+    // "undef" the DVI if it has more than one instruction and any of them do
+    // not dominate DVI.
+    for (Instruction *VI : VIs) {
+      if (VI->isTerminator())
+        continue;
+
+      // If VI is a phi in a block with an EHPad terminator, we can't insert
+      // after it.
+      if (isa<PHINode>(VI) && VI->getParent()->getTerminator()->isEHPad())
+        continue;
+
+      // If the defining instruction dominates the position, we do not need
+      // to move the dbg.value.
+      if (DT.dominates(VI, &I))
+        continue;
+
+      // If we depend on multiple instructions and any of them doesn't
+      // dominate this DPV, we probably can't salvage it: moving it to
+      // after any of the instructions could cause us to lose the others.
+      if (VIs.size() > 1) {
+        LLVM_DEBUG(
+            dbgs()
+            << "Unable to find valid location for Debug Value, undefing:\n"
+            << DPV);
+        DPV.setKillLocation();
+        break;
+      }
+
+      LLVM_DEBUG(dbgs() << "Moving Debug Value before :\n"
+                        << DPV << ' ' << *VI);
+      DPV.removeFromParent();
+      BasicBlock *VIBB = VI->getParent();
+      if (isa<PHINode>(VI)) {
+        VIBB->insertDPValueBefore(&DPV, VIBB->getFirstInsertionPt());
+      } else {
+        VIBB->insertDPValueAfter(&DPV, VI);
+      }
+      MadeChange = true;
+      ++NumDbgValueMoved;
+    }
+  }
----------------
OCHyams wrote:

Can this be folded back into the `placeDbgValues` function using the "turn the loop into a lambda and then call for_each(DbgValues, Lambda), for_each(DPValues, Lambda)" idiom (workshopping the name still).

https://github.com/llvm/llvm-project/pull/73660