Index: llvm/trunk/include/llvm/IR/IntrinsicInst.h
===================================================================
--- llvm/trunk/include/llvm/IR/IntrinsicInst.h
+++ llvm/trunk/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: llvm/trunk/lib/IR/IntrinsicInst.cpp
===================================================================
--- llvm/trunk/lib/IR/IntrinsicInst.cpp
+++ llvm/trunk/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 +49,12 @@
   return nullptr;
 }
 
+Optional<uint64_t> DbgInfoIntrinsic::getFragmentSizeInBits() const {
+  if (auto Fragment = getExpression()->getFragmentInfo())
+    return Fragment->SizeInBits;
+  return getVariable()->getSizeInBits();
+}
+
 int llvm::Intrinsic::lookupLLVMIntrinsicByName(ArrayRef<const char *> NameTable,
                                                StringRef Name) {
   assert(Name.startswith("llvm."));
Index: llvm/trunk/lib/Transforms/Utils/Local.cpp
===================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp
@@ -1228,6 +1228,23 @@
   return false;
 }
 
+/// Check if the store size of \p ValTy is large enough to cover the variable
+/// (or fragment of the variable) described by \p DII.
+///
+/// This is primarily intended as a helper for the different
+/// ConvertDebugDeclareToDebugValue functions. The dbg.declare/dbg.addr that is
+/// converted describes an alloca'd variable, so we need to use the
+/// store size of the value when doing the comparison. E.g. an i1 value will be
+/// identified as covering an n-bit fragment, if the store size of i1 is at
+/// least n bits.
+static bool valueCoversEntireFragment(Type *ValTy, DbgInfoIntrinsic *DII) {
+  const DataLayout &DL = DII->getModule()->getDataLayout();
+  uint64_t ValueSize = DL.getTypeStoreSizeInBits(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 +1255,21 @@
   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 variable's 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 +1313,9 @@
   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 +1336,9 @@
   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: llvm/trunk/test/Transforms/InstCombine/debuginfo.ll
===================================================================
--- llvm/trunk/test/Transforms/InstCombine/debuginfo.ll
+++ llvm/trunk/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)
 
Index: llvm/trunk/test/Transforms/Mem2Reg/debug-alloca-phi-2.ll
===================================================================
--- llvm/trunk/test/Transforms/Mem2Reg/debug-alloca-phi-2.ll
+++ llvm/trunk/test/Transforms/Mem2Reg/debug-alloca-phi-2.ll
@@ -0,0 +1,46 @@
+; RUN: opt < %s -mem2reg -S | FileCheck %s
+source_filename = "bugpoint-output.bc"
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.12.0"
+
+define void @scan() #0 !dbg !12 {
+entry:
+  %entry1 = alloca i1, align 8
+  call void @llvm.dbg.declare(metadata i1* %entry1, metadata !18, metadata !19), !dbg !20
+  store i1 0, i1* %entry1, align 8, !dbg !20
+  br label %for.cond, !dbg !20
+
+for.cond:
+; CHECK: %[[PHI:.*]] = phi i1 [ false, %entry ], [ %0, %for.cond ]
+  %entryN = load i1, i1* %entry1, align 8, !dbg !20
+; CHECK: call void @llvm.dbg.value(metadata i1 %[[PHI]],
+; CHECK-SAME:                      metadata !DIExpression())
+  %0 = add i1 %entryN, 1
+; CHECK: %0 = add i1 %[[PHI]], true
+; CHECK: call void @llvm.dbg.value(metadata i1 %0,
+; CHECK-SAME:                      metadata !DIExpression())
+  store i1 %0, i1* %entry1, align 8, !dbg !20
+  br label %for.cond, !dbg !20
+}
+
+declare void @llvm.dbg.declare(metadata, metadata, metadata) #1
+
+attributes #0 = { nounwind ssp uwtable }
+attributes #1 = { nounwind readnone }
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!10, !11}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, producer: "adrian", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug)
+!1 = !DIFile(filename: "<stdin>", directory: "/")
+!2 = !{}
+!4 = !DIBasicType(name: "char", size: 8, encoding: DW_ATE_signed_char)
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !{i32 1, !"PIC Level", i32 2}
+!12 = distinct !DISubprogram(name: "scan", scope: !1, file: !1, line: 4, type: !13, isLocal: false, isDefinition: true, scopeLine: 5, flags: DIFlagPrototyped, isOptimized: true, unit: !0, retainedNodes: !15)
+!13 = !DISubroutineType(types: !14)
+!14 = !{null, !4, !4}
+!15 = !{!18}
+!18 = !DILocalVariable(name: "entry", scope: !12, file: !1, line: 6, type: !4)
+!19 = !DIExpression()
+!20 = !DILocation(line: 6, scope: !12)