Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -189,10 +189,15 @@
 static void annotateDereferenceableBytes(CallInst *CI,
                                          ArrayRef<unsigned> ArgNos,
                                          uint64_t DereferenceableBytes) {
+  Function *F = CI->getFunction();
+  if (!F)
+    return;
   for (unsigned ArgNo : ArgNos) {
-    uint64_t DerefBytes = std::max(
-        CI->getDereferenceableOrNullBytes(ArgNo + AttributeList::FirstArgIndex),
-        DereferenceableBytes);
+    uint64_t DerefBytes = DereferenceableBytes;
+    if (!F->nullPointerIsDefined())
+      std::max(CI->getDereferenceableOrNullBytes(ArgNo +
+                                                 AttributeList::FirstArgIndex),
+               DereferenceableBytes);
 
     if (CI->getDereferenceableBytes(ArgNo + AttributeList::FirstArgIndex) <
         DerefBytes) {
Index: test/Transforms/InstCombine/mem-deref-bytes.ll
===================================================================
--- test/Transforms/InstCombine/mem-deref-bytes.ll
+++ test/Transforms/InstCombine/mem-deref-bytes.ll
@@ -57,7 +57,7 @@
 
 define i32 @memcmp_const_size_update_deref5(i8* nocapture readonly %d, i8* nocapture readonly %s) {
 ; CHECK-LABEL: @memcmp_const_size_update_deref5(
-; CHECK-NEXT:    [[CALL:%.*]] = tail call i32 @memcmp(i8* dereferenceable(40) [[D:%.*]], i8* dereferenceable(16) [[S:%.*]], i64 16)
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i32 @memcmp(i8* dereferenceable(16) [[D:%.*]], i8* dereferenceable(16) [[S:%.*]], i64 16)
 ; CHECK-NEXT:    ret i32 [[CALL]]
 ;
   %call = tail call i32 @memcmp(i8* dereferenceable_or_null(40) %d, i8* %s, i64 16)
@@ -66,6 +66,15 @@
 
 define i32 @memcmp_const_size_no_update_deref(i8* nocapture readonly %d, i8* nocapture readonly %s) {
 ; CHECK-LABEL: @memcmp_const_size_no_update_deref(
+; CHECK-NEXT:    [[CALL:%.*]] = tail call i32 @memcmp(i8* dereferenceable(16) [[D:%.*]], i8* dereferenceable(16) [[S:%.*]], i64 16) #1
+; CHECK-NEXT:    ret i32 [[CALL]]
+;
+  %call = tail call i32 @memcmp(i8* dereferenceable_or_null(40) %d, i8* %s, i64 16) #0
+  ret i32 %call
+}
+
+define i32 @memcmp_const_size_no_update_deref2(i8* nocapture readonly %d, i8* nocapture readonly %s) {
+; CHECK-LABEL: @memcmp_const_size_no_update_deref2(
 ; CHECK-NEXT:    [[CALL:%.*]] = tail call i32 @memcmp(i8* dereferenceable(40) [[D:%.*]], i8* dereferenceable(20) [[S:%.*]], i64 16)
 ; CHECK-NEXT:    ret i32 [[CALL]]
 ;
@@ -143,3 +152,5 @@
   call void @llvm.memset.p0i8.i64(i8* align 1 %s, i8 %c, i64 16, i1 false)
   ret i8* %s
 }
+
+attributes #0 = { "null-pointer-is-valid"="true" }