[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prevent debug info from falling out-of-date.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, 42 insertions, 17 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 79d4857c2c8..0e74a26987c 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -13,6 +13,7 @@
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/MapVector.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SetOperations.h"
@@ -38,6 +39,7 @@
 #include "llvm/IR/ConstantRange.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalValue.h"
@@ -1250,14 +1252,38 @@ static bool HoistThenElseCodeToIf(BranchInst *BI,
 
   Instruction *I1 = &*BB1_Itr++, *I2 = &*BB2_Itr++;
   // Skip debug info if it is not identical.
-  DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
-  DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
-  if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
-    while (isa<DbgInfoIntrinsic>(I1))
-      I1 = &*BB1_Itr++;
-    while (isa<DbgInfoIntrinsic>(I2))
-      I2 = &*BB2_Itr++;
-  }
+
+  // If the terminator instruction is hoisted, and any variable locations have
+  // non-identical debug intrinsics, then those variable locations must be set
+  // as undef.
+  // FIXME: If each block contains identical debug variable intrinsics in a
+  // different order, they will be considered non-identical and be dropped.
+  MapVector<DebugVariable, DbgVariableIntrinsic *> UndefDVIs;
+
+  auto SkipDbgInfo = [&UndefDVIs](Instruction *&I,
+                                  BasicBlock::iterator &BB_Itr) {
+    while (isa<DbgInfoIntrinsic>(I)) {
+      if (DbgVariableIntrinsic *DVI = dyn_cast<DbgVariableIntrinsic>(I))
+        UndefDVIs.insert(
+            {DebugVariable(DVI->getVariable(), DVI->getExpression(),
+                           DVI->getDebugLoc()->getInlinedAt()),
+             DVI});
+      I = &*BB_Itr++;
+    }
+  };
+
+  auto SkipNonIdenticalDbgInfo =
+      [&BB1_Itr, &BB2_Itr, &SkipDbgInfo](Instruction *&I1, Instruction *&I2) {
+        DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
+        DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
+        if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
+          SkipDbgInfo(I1, BB1_Itr);
+          SkipDbgInfo(I2, BB2_Itr);
+        }
+      };
+
+  SkipNonIdenticalDbgInfo(I1, I2);
+
   // FIXME: Can we define a safety predicate for CallBr?
   if (isa<PHINode>(I1) || !I1->isIdenticalToWhenDefined(I2) ||
       (isa<InvokeInst>(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) ||
@@ -1330,15 +1356,7 @@ static bool HoistThenElseCodeToIf(BranchInst *BI,
 
     I1 = &*BB1_Itr++;
     I2 = &*BB2_Itr++;
-    // Skip debug info if it is not identical.
-    DbgInfoIntrinsic *DBI1 = dyn_cast<DbgInfoIntrinsic>(I1);
-    DbgInfoIntrinsic *DBI2 = dyn_cast<DbgInfoIntrinsic>(I2);
-    if (!DBI1 || !DBI2 || !DBI1->isIdenticalToWhenDefined(DBI2)) {
-      while (isa<DbgInfoIntrinsic>(I1))
-        I1 = &*BB1_Itr++;
-      while (isa<DbgInfoIntrinsic>(I2))
-        I2 = &*BB2_Itr++;
-    }
+    SkipNonIdenticalDbgInfo(I1, I2);
   } while (I1->isIdenticalToWhenDefined(I2));
 
   return true;
@@ -1374,6 +1392,13 @@ HoistTerminator:
   }
 
   // Okay, it is safe to hoist the terminator.
+  for (auto DIVariableInst : UndefDVIs) {
+    DbgVariableIntrinsic *DVI = DIVariableInst.second;
+    DVI->moveBefore(BI);
+    if (Value *V = DVI->getVariableLocation())
+      setDbgVariableUndef(DVI);
+  }
+
   Instruction *NT = I1->clone();
   BIParent->getInstList().insert(BI->getIterator(), NT);
   if (!NT->getType()->isVoidTy()) {