1 files changed, 118 insertions, 0 deletions
diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index 31eaf260735..d5faa4b5c33 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -324,6 +324,124 @@ bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, DomTreeUpdater *DTU,
   return true;
 }
 
+/// Remove redundant instructions within sequences of consecutive dbg.value
+/// instructions. This is done using a backward scan to keep the last dbg.value
+/// describing a specific variable/fragment.
+///
+/// BackwardScan strategy:
+/// ----------------------
+/// Given a sequence of consecutive DbgValueInst like this
+///
+///   dbg.value ..., "x", FragmentX1  (*)
+///   dbg.value ..., "y", FragmentY1
+///   dbg.value ..., "x", FragmentX2
+///   dbg.value ..., "x", FragmentX1  (**)
+///
+/// then the instruction marked with (*) can be removed (it is guaranteed to be
+/// obsoleted by the instruction marked with (**) as the latter instruction is
+/// describing the same variable using the same fragment info).
+///
+/// Possible improvements:
+/// - Check fully overlapping fragments and not only identical fragments.
+/// - Support dbg.addr, dbg.declare. dbg.label, and possibly other meta
+///   instructions being part of the sequence of consecutive instructions.
+static bool removeRedundantDbgInstrsUsingBackwardScan(BasicBlock *BB) {
+  SmallVector<DbgValueInst *, 8> ToBeRemoved;
+  SmallDenseSet<DebugVariable> VariableSet;
+  for (auto &I : reverse(*BB)) {
+    if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(&I)) {
+      DebugVariable Key(DVI->getVariable(),
+                        DVI->getExpression(),
+                        DVI->getDebugLoc()->getInlinedAt());
+      auto R = VariableSet.insert(Key);
+      // If the same variable fragment is described more than once it is enough
+      // to keep the last one (i.e. the first found since we for reverse
+      // iteration).
+      if (!R.second)
+        ToBeRemoved.push_back(DVI);
+      continue;
+    }
+    // Sequence with consecutive dbg.value instrs ended. Clear the map to
+    // restart identifying redundant instructions if case we find another
+    // dbg.value sequence.
+    VariableSet.clear();
+  }
+
+  for (auto &Instr : ToBeRemoved)
+    Instr->eraseFromParent();
+
+  return !ToBeRemoved.empty();
+}
+
+/// Remove redundant dbg.value instructions using a forward scan. This can
+/// remove a dbg.value instruction that is redundant due to indicating that a
+/// variable has the same value as already being indicated by an earlier
+/// dbg.value.
+///
+/// ForwardScan strategy:
+/// ---------------------
+/// Given two identical dbg.value instructions, separated by a block of
+/// instructions that isn't describing the same variable, like this
+///
+///   dbg.value X1, "x", FragmentX1  (**)
+///   <block of instructions, none being "dbg.value ..., "x", ...">
+///   dbg.value X1, "x", FragmentX1  (*)
+///
+/// then the instruction marked with (*) can be removed. Variable "x" is already
+/// described as being mapped to the SSA value X1.
+///
+/// Possible improvements:
+/// - Keep track of non-overlapping fragments.
+static bool removeRedundantDbgInstrsUsingForwardScan(BasicBlock *BB) {
+  SmallVector<DbgValueInst *, 8> ToBeRemoved;
+  DenseMap<DebugVariable, std::pair<Value *, DIExpression *> > VariableMap;
+  for (auto &I : *BB) {
+    if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(&I)) {
+      DebugVariable Key(DVI->getVariable(),
+                        NoneType(),
+                        DVI->getDebugLoc()->getInlinedAt());
+      auto VMI = VariableMap.find(Key);
+      // Update the map if we found a new value/expression describing the
+      // variable, or if the variable wasn't mapped already.
+      if (VMI == VariableMap.end() ||
+          VMI->second.first != DVI->getValue() ||
+          VMI->second.second != DVI->getExpression()) {
+        VariableMap[Key] = { DVI->getValue(), DVI->getExpression() };
+        continue;
+      }
+      // Found an identical mapping. Remember the instruction for later removal.
+      ToBeRemoved.push_back(DVI);
+    }
+  }
+
+  for (auto &Instr : ToBeRemoved)
+    Instr->eraseFromParent();
+
+  return !ToBeRemoved.empty();
+}
+
+bool llvm::RemoveRedundantDbgInstrs(BasicBlock *BB) {
+  bool MadeChanges = false;
+  // By using the "backward scan" strategy before the "forward scan" strategy we
+  // can remove both dbg.value (2) and (3) in a situation like this:
+  //
+  //   (1) dbg.value V1, "x", DIExpression()
+  //       ...
+  //   (2) dbg.value V2, "x", DIExpression()
+  //   (3) dbg.value V1, "x", DIExpression()
+  //
+  // The backward scan will remove (2), it is made obsolete by (3). After
+  // getting (2) out of the way, the foward scan will remove (3) since "x"
+  // already is described as having the value V1 at (1).
+  MadeChanges |= removeRedundantDbgInstrsUsingBackwardScan(BB);
+  MadeChanges |= removeRedundantDbgInstrsUsingForwardScan(BB);
+
+  if (MadeChanges)
+    LLVM_DEBUG(dbgs() << "Removed redundant dbg instrs from: "
+                      << BB->getName() << "\n");
+  return MadeChanges;
+}
+
 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
                                 BasicBlock::iterator &BI, Value *V) {
   Instruction &I = *BI;