[DebugInfo] Recover debug intrinsics when killing duplicated/empty basic blocks

When basic blocks are killed, either due to being empty or to being an if.then
or if.else block whose complement contains identical instructions, some of the
debug intrinsics in that block are lost. This patch sinks those intrinsics
into the single successor block, setting them Undef if necessary to
prevent debug info from falling out-of-date.

Differential Revision: https://reviews.llvm.org/D70318

1 files changed, 22 insertions, 5 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index eaccaa1..94edf8c 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -485,15 +485,19 @@ void llvm::RecursivelyDeleteTriviallyDeadInstructions(
     I.eraseFromParent();
   }
 }
+void llvm::setDbgVariableUndef(DbgVariableIntrinsic *DVI) {
+  Value *DbgValue = DVI->getVariableLocation(false);
+  Value *Undef = UndefValue::get(DbgValue ? DbgValue->getType()
+                                          : Type::getInt1Ty(DVI->getContext()));
+  DVI->setOperand(
+      0, MetadataAsValue::get(DVI->getContext(), ValueAsMetadata::get(Undef)));
+}
 
 bool llvm::replaceDbgUsesWithUndef(Instruction *I) {
   SmallVector<DbgVariableIntrinsic *, 1> DbgUsers;
   findDbgUsers(DbgUsers, I);
-  for (auto *DII : DbgUsers) {
-    Value *Undef = UndefValue::get(I->getType());
-    DII->setOperand(0, MetadataAsValue::get(DII->getContext(),
-                                            ValueAsMetadata::get(Undef)));
-  }
+  for (auto *DII : DbgUsers)
+    setDbgVariableUndef(DII);
   return !DbgUsers.empty();
 }
 
@@ -1040,6 +1044,19 @@ bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB,
       assert(PN->use_empty() && "There shouldn't be any uses here!");
       PN->eraseFromParent();
     }
+    // If Succ has multiple predecessors, each debug intrinsic in BB may or may
+    // not be valid when we reach Succ, so the debug variable should be set
+    // undef since its value is unknown.
+    Instruction *DbgInsertPoint = Succ->getFirstNonPHI();
+    while (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(&BB->front())) {
+      if (auto DVI = cast<DbgVariableIntrinsic>(DI)) {
+        if (!isa<DbgDeclareInst>(DVI))
+          setDbgVariableUndef(DVI);
+        DVI->moveBefore(DbgInsertPoint);
+      } else {
+        break;
+      }
+    }
   }
 
   // If the unconditional branch we replaced contains llvm.loop metadata, we