[DebugInfo][SimpleLoopUnswitch] Fix missing debug location updates (#97662)

Fix #97559 .

For the change at line 1253, I propagate the debug location of the
terminator (i.e., the insertion point) to the new phi. because `MergeBB`
is generated by splitting `ExitBB` several lines above, it only has the
terminator, which could provide a reasonable debug location.

For the change at line 2348, I switch the order of moving and cloning
`TI`. Because `NewTI` cloned from `TI` is inserted into the original
place where `TI` is, `NewTI` should preserve the origianl debug
location. At the same time, doing this allows us to propagate the debug
location to the new branch instruction replacing `NewTI` (the change at
line 2446).

1 files changed, 22 insertions, 11 deletions

diff --git a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
index fdb3211..6a3b0e1 100644
--- a/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
+++ b/llvm/lib/Transforms/Scalar/SimpleLoopUnswitch.cpp
@@ -1245,9 +1245,12 @@ static BasicBlock *buildClonedLoopBlocks(
       if (SE && isa<PHINode>(I))
         SE->forgetValue(&I);
 
+      BasicBlock::iterator InsertPt = MergeBB->getFirstInsertionPt();
+
       auto *MergePN =
           PHINode::Create(I.getType(), /*NumReservedValues*/ 2, ".us-phi");
-      MergePN->insertBefore(MergeBB->getFirstInsertionPt());
+      MergePN->insertBefore(InsertPt);
+      MergePN->setDebugLoc(InsertPt->getDebugLoc());
       I.replaceAllUsesWith(MergePN);
       MergePN->addIncoming(&I, ExitBB);
       MergePN->addIncoming(&ClonedI, ClonedExitBB);
@@ -1306,8 +1309,9 @@ static BasicBlock *buildClonedLoopBlocks(
   else if (auto *SI = dyn_cast<SwitchInst>(ClonedTerminator))
     ClonedConditionToErase = SI->getCondition();
 
+  Instruction *BI = BranchInst::Create(ClonedSuccBB, ClonedParentBB);
+  BI->setDebugLoc(ClonedTerminator->getDebugLoc());
   ClonedTerminator->eraseFromParent();
-  BranchInst::Create(ClonedSuccBB, ClonedParentBB);
 
   if (ClonedConditionToErase)
     RecursivelyDeleteTriviallyDeadInstructions(ClonedConditionToErase, nullptr,
@@ -2334,22 +2338,27 @@ static void unswitchNontrivialInvariants(
   // nuke the initial terminator placed in the split block.
   SplitBB->getTerminator()->eraseFromParent();
   if (FullUnswitch) {
-    // Splice the terminator from the original loop and rewrite its
-    // successors.
-    TI.moveBefore(*SplitBB, SplitBB->end());
-
     // Keep a clone of the terminator for MSSA updates.
     Instruction *NewTI = TI.clone();
     NewTI->insertInto(ParentBB, ParentBB->end());
 
+    // Splice the terminator from the original loop and rewrite its
+    // successors.
+    TI.moveBefore(*SplitBB, SplitBB->end());
+    TI.dropLocation();
+
     // First wire up the moved terminator to the preheaders.
     if (BI) {
       BasicBlock *ClonedPH = ClonedPHs.begin()->second;
       BI->setSuccessor(ClonedSucc, ClonedPH);
       BI->setSuccessor(1 - ClonedSucc, LoopPH);
       Value *Cond = skipTrivialSelect(BI->getCondition());
-      if (InsertFreeze)
+      if (InsertFreeze) {
+        // We don't give any debug location to the new freeze, because the
+        // BI (`dyn_cast<BranchInst>(TI)`) is an in-loop instruction hoisted
+        // out of the loop.
         Cond = new FreezeInst(Cond, Cond->getName() + ".fr", BI->getIterator());
+      }
       BI->setCondition(Cond);
       DTUpdates.push_back({DominatorTree::Insert, SplitBB, ClonedPH});
     } else {
@@ -2432,12 +2441,13 @@ static void unswitchNontrivialInvariants(
         DTUpdates.push_back({DominatorTree::Delete, ParentBB, SuccBB});
     }
 
-    // After MSSAU update, remove the cloned terminator instruction NewTI.
-    ParentBB->getTerminator()->eraseFromParent();
-
     // Create a new unconditional branch to the continuing block (as opposed to
     // the one cloned).
-    BranchInst::Create(RetainedSuccBB, ParentBB);
+    Instruction *NewBI = BranchInst::Create(RetainedSuccBB, ParentBB);
+    NewBI->setDebugLoc(NewTI->getDebugLoc());
+
+    // After MSSAU update, remove the cloned terminator instruction NewTI.
+    NewTI->eraseFromParent();
   } else {
     assert(BI && "Only branches have partial unswitching.");
     assert(UnswitchedSuccBBs.size() == 1 &&
@@ -2710,6 +2720,7 @@ static BranchInst *turnSelectIntoBranch(SelectInst *SI, DominatorTree &DT,
       PHINode::Create(SI->getType(), 2, "unswitched.select", SI->getIterator());
   Phi->addIncoming(SI->getTrueValue(), ThenBB);
   Phi->addIncoming(SI->getFalseValue(), HeadBB);
+  Phi->setDebugLoc(SI->getDebugLoc());
   SI->replaceAllUsesWith(Phi);
   SI->eraseFromParent();