Index: include/llvm/IR/IntrinsicInst.h
===================================================================
--- include/llvm/IR/IntrinsicInst.h
+++ include/llvm/IR/IntrinsicInst.h
@@ -93,6 +93,10 @@
       return cast<MetadataAsValue>(getArgOperand(2))->getMetadata();
     }
 
+    /// Get the size (in bits) of the variable, or fragment of the variable,
+    /// that is described.
+    Optional<uint64_t> getFragmentSizeInBits() const;
+
     /// \name Casting methods
     /// @{
     static bool classof(const IntrinsicInst *I) {
Index: lib/IR/IntrinsicInst.cpp
===================================================================
--- lib/IR/IntrinsicInst.cpp
+++ lib/IR/IntrinsicInst.cpp
@@ -24,6 +24,7 @@
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/ADT/StringSwitch.h"
 #include "llvm/IR/Constants.h"
+#include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/Metadata.h"
 #include "llvm/IR/Module.h"
@@ -48,6 +49,14 @@
   return nullptr;
 }
 
+Optional<uint64_t> DbgInfoIntrinsic::getFragmentSizeInBits() const {
+  if (auto Fragment = getExpression()->getFragmentInfo())
+    return Fragment->SizeInBits;
+  if (auto VarSize = getVariable()->getSizeInBits())
+    return *VarSize;
+  return None;
+}
+
 int llvm::Intrinsic::lookupLLVMIntrinsicByName(ArrayRef<const char *> NameTable,
                                                StringRef Name) {
   assert(Name.startswith("llvm."));
Index: lib/Transforms/Utils/Local.cpp
===================================================================
--- lib/Transforms/Utils/Local.cpp
+++ lib/Transforms/Utils/Local.cpp
@@ -1228,6 +1228,16 @@
   return false;
 }
 
+/// Check if the size of \p ValTy is large enough to cover the variable (or
+/// fragment of the variable) described by \p DII.
+static bool valueCoversEntireFragment(Type *ValTy, DbgInfoIntrinsic *DII) {
+  const DataLayout &DL = DII->getModule()->getDataLayout();
+  uint64_t ValueSize = DL.getTypeSizeInBits(ValTy);
+  if (auto FragmentSize = DII->getFragmentSizeInBits())
+    return ValueSize >= *FragmentSize;
+  return false;
+}
+
 /// Inserts a llvm.dbg.value intrinsic before a store to an alloca'd value
 /// that has an associated llvm.dbg.declare or llvm.dbg.addr intrinsic.
 void llvm::ConvertDebugDeclareToDebugValue(DbgInfoIntrinsic *DII,
@@ -1238,6 +1248,21 @@
   auto *DIExpr = DII->getExpression();
   Value *DV = SI->getOperand(0);
 
+  if (!valueCoversEntireFragment(SI->getValueOperand()->getType(), DII)) {
+    // FIXME: If storing to a part of the variable described by the dbg.declare,
+    // then we want to insert a dbg.value for the corresponding fragment.
+    LLVM_DEBUG(dbgs() << "Failed to convert dbg.declare to dbg.value: "
+                      << *DII << '\n');
+    // For now, when there is a store to parts of the variable (but we do not
+    // know which part) we insert an dbg.value instrinsic to indicate that we
+    // know nothing about the variables content.
+    DV = UndefValue::get(DV->getType());
+    if (!LdStHasDebugValue(DIVar, DIExpr, SI))
+      Builder.insertDbgValueIntrinsic(DV, DIVar, DIExpr, DII->getDebugLoc(),
+                                      SI);
+    return;
+  }
+
   // If an argument is zero extended then use argument directly. The ZExt
   // may be zapped by an optimization pass in future.
   Argument *ExtendedArg = nullptr;
@@ -1281,6 +1306,9 @@
   if (LdStHasDebugValue(DIVar, DIExpr, LI))
     return;
 
+  assert(valueCoversEntireFragment(LI->getType(), DII) &&
+         "Load is not loading the full variable fragment.");
+
   // We are now tracking the loaded value instead of the address. In the
   // future if multi-location support is added to the IR, it might be
   // preferable to keep tracking both the loaded value and the original
@@ -1301,6 +1329,9 @@
   if (PhiHasDebugValue(DIVar, DIExpr, APN))
     return;
 
+  assert(valueCoversEntireFragment(APN->getType(), DII) &&
+         "PHI node is not describing the full variable.");
+
   BasicBlock *BB = APN->getParent();
   auto InsertionPt = BB->getFirstInsertionPt();
 
Index: test/Transforms/InstCombine/debuginfo.ll
===================================================================
--- test/Transforms/InstCombine/debuginfo.ll
+++ test/Transforms/InstCombine/debuginfo.ll
@@ -71,9 +71,11 @@
 ; NOLOWER-NOT: alloca
 ; NOLOWER-NOT: store
 ; NOLOWER-NOT: call void @llvm.dbg.declare
-; NOLOWER: call void @llvm.dbg.value(metadata i64 %o.coerce0, {{.*}})
+; Here we want to find:  call void @llvm.dbg.value(metadata i64 %o.coerce0, metadata [[VARIABLE_O]], metadata !DIExpression(DW_OP_LLVM_fragment, 0, 64))
+; NOLOWER: call void @llvm.dbg.value(metadata i64 undef, {{.*}})
 ; NOLOWER-NOT: store
-; NOLOWER: call void @llvm.dbg.value(metadata i64 %o.coerce1, {{.*}})
+; Here we want to find:  call void @llvm.dbg.value(metadata i64 %o.coerce1, metadata [[VARIABLE_O]], metadata !DIExpression(DW_OP_LLVM_fragment, 64, 64))
+; NOLOWER: call void @llvm.dbg.value(metadata i64 undef, {{.*}})
 ; NOLOWER-NOT: store
 ; NOLOWER: call void @tworegs_callee(i64 %o.coerce0, i64 %o.coerce1)