Index: llvm/trunk/include/llvm/IR/Value.h
===================================================================
--- llvm/trunk/include/llvm/IR/Value.h
+++ llvm/trunk/include/llvm/IR/Value.h
@@ -570,7 +570,7 @@
   ///
   /// If CanBeNull is set by this function the pointer can either be null or be
   /// dereferenceable up to the returned number of bytes.
-  unsigned getPointerDereferenceableBytes(const DataLayout &DL,
+  uint64_t getPointerDereferenceableBytes(const DataLayout &DL,
                                           bool &CanBeNull) const;
 
   /// \brief Returns an alignment of the pointer value.
Index: llvm/trunk/lib/IR/Value.cpp
===================================================================
--- llvm/trunk/lib/IR/Value.cpp
+++ llvm/trunk/lib/IR/Value.cpp
@@ -619,11 +619,11 @@
   return stripPointerCastsAndOffsets<PSK_InBounds>(this);
 }
 
-unsigned Value::getPointerDereferenceableBytes(const DataLayout &DL,
+uint64_t Value::getPointerDereferenceableBytes(const DataLayout &DL,
                                                bool &CanBeNull) const {
   assert(getType()->isPointerTy() && "must be pointer");
 
-  unsigned DerefBytes = 0;
+  uint64_t DerefBytes = 0;
   CanBeNull = false;
   if (const Argument *A = dyn_cast<Argument>(this)) {
     DerefBytes = A->getDereferenceableBytes();
@@ -655,8 +655,10 @@
       CanBeNull = true;
     }
   } else if (auto *AI = dyn_cast<AllocaInst>(this)) {
-    if (AI->getAllocatedType()->isSized()) {
-      DerefBytes = DL.getTypeStoreSize(AI->getAllocatedType());
+    const ConstantInt *ArraySize = dyn_cast<ConstantInt>(AI->getArraySize());
+    if (ArraySize && AI->getAllocatedType()->isSized()) {
+      DerefBytes = DL.getTypeStoreSize(AI->getAllocatedType()) *
+        ArraySize->getZExtValue();
       CanBeNull = false;
     }
   } else if (auto *GV = dyn_cast<GlobalVariable>(this)) {
Index: llvm/trunk/test/Analysis/ValueTracking/memory-dereferenceable.ll
===================================================================
--- llvm/trunk/test/Analysis/ValueTracking/memory-dereferenceable.ll
+++ llvm/trunk/test/Analysis/ValueTracking/memory-dereferenceable.ll
@@ -34,6 +34,23 @@
     %alloca = alloca i1
     %load2 = load i1, i1* %alloca
 
+    ; Load from empty array alloca
+; CHECK-NOT: %empty_alloca
+    %empty_alloca = alloca i8, i64 0
+    %empty_load = load i8, i8* %empty_alloca
+
+    ; Load from too small array alloca
+; CHECK-NOT: %small_array_alloca
+    %small_array_alloca = alloca i8, i64 2
+    %saa_cast = bitcast i8* %small_array_alloca to i32*
+    %saa_load = load i32, i32* %saa_cast
+
+    ; Load from array alloca
+; CHECK: %big_array_alloca{{.*}}(unaligned)
+    %big_array_alloca = alloca i8, i64 4
+    %baa_cast = bitcast i8* %big_array_alloca to i32*
+    %baa_load = load i32, i32* %baa_cast
+
 ; CHECK: %dparam{{.*}}(aligned)
     %load3 = load i32, i32 addrspace(1)* %dparam