Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -432,9 +432,7 @@
   if (Len == 0)
     return Dst; // strncpy(x, y, 0) -> x
 
-  // Let strncpy handle the zero padding
-  if (Len > SrcLen + 1)
-    return nullptr;
+  Len = std::min(Len, SrcLen + 1);
 
   Type *PT = Callee->getFunctionType()->getParamType(0);
   // strncpy(x, s, c) -> memcpy(align 1 x, align 1 s, c) [s and c are constant]
Index: test/Transforms/InstCombine/strncpy-1.ll
===================================================================
--- test/Transforms/InstCombine/strncpy-1.ll
+++ test/Transforms/InstCombine/strncpy-1.ll
@@ -1,3 +1,4 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; Test that the strncpy library call simplifier works correctly.
 ;
 ; RUN: opt < %s -instcombine -S | FileCheck %s
@@ -17,8 +18,15 @@
 
 define i32 @test_simplify1() {
 ; CHECK-LABEL: @test_simplify1(
-; CHECK-NOT: call i8* @strncpy
-; CHECK: call i32 @puts
+; CHECK-NEXT:    [[TARGET:%.*]] = alloca [1024 x i8], align 1
+; CHECK-NEXT:    [[ARG1:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[TARGET]], i32 0, i32 0
+; CHECK-NEXT:    store i8 0, i8* [[ARG1]], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* nonnull align 1 [[ARG1]], i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 6, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* nonnull align 1 [[ARG1]], i8 0, i32 42, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* nonnull align 1 [[ARG1]], i8 0, i32 42, i1 false)
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @puts(i8* nonnull [[ARG1]])
+; CHECK-NEXT:    ret i32 0
+;
   %target = alloca [1024 x i8]
   %arg1 = getelementptr [1024 x i8], [1024 x i8]* %target, i32 0, i32 0
   store i8 0, i8* %arg1
@@ -40,11 +48,13 @@
 
 define void @test_simplify2() {
 ; CHECK-LABEL: @test_simplify2(
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8 0, i32 32, i1 false)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [1 x i8], [1 x i8]* @null, i32 0, i32 0
 
   call i8* @strncpy(i8* %dst, i8* %src, i32 32)
-; CHECK: call void @llvm.memset.p0i8.i32
   ret void
 }
 
@@ -52,44 +62,67 @@
 
 define i8* @test_simplify3() {
 ; CHECK-LABEL: @test_simplify3(
+; CHECK-NEXT:    ret i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0)
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
 
   %ret = call i8* @strncpy(i8* %dst, i8* %src, i32 0)
   ret i8* %ret
-; CHECK: ret i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0)
 }
 
-; Check  strncpy(x, s, c) -> memcpy(x, s, c, 1) [s and c are constant].
+; Check strncpy(x, s, c) -> memcpy(x, s, c, 1) [s and c are constant].
 
 define void @test_simplify4() {
 ; CHECK-LABEL: @test_simplify4(
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 6, i1 false)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
 
   call i8* @strncpy(i8* %dst, i8* %src, i32 6)
-; CHECK: call void @llvm.memcpy.p0i8.p0i8.i32
   ret void
 }
 
-; Check cases that shouldn't be simplified.
-
-define void @test_no_simplify1() {
-; CHECK-LABEL: @test_no_simplify1(
+; Check strncpy(x, s, c) -> memcpy(x, s, min(c, len(s)), 1) [s and c are constant].
+; Length: len(s)
+define void @test_simplify5() {
+; CHECK-LABEL: @test_simplify5(
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 6, i1 false)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
-  %src = getelementptr [32 x i8], [32 x i8]* @b, i32 0, i32 0
+  %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
 
-  call i8* @strncpy(i8* %dst, i8* %src, i32 32)
-; CHECK: call i8* @strncpy
+  call i8* @strncpy(i8* %dst, i8* %src, i32 12)
   ret void
 }
 
-define void @test_no_simplify2() {
-; CHECK-LABEL: @test_no_simplify2(
+; Check strncpy(x, s, c) -> memcpy(x, s, min(c, len(s)), 1) [s and c are constant].
+; Length: c
+define void @test_simplify6() {
+; CHECK-LABEL: @test_simplify6(
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i32(i8* align 1 getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* align 1 getelementptr inbounds ([6 x i8], [6 x i8]* @hello, i32 0, i32 0), i32 5, i1 false)
+; CHECK-NEXT:    ret void
+;
   %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
   %src = getelementptr [6 x i8], [6 x i8]* @hello, i32 0, i32 0
 
-  call i8* @strncpy(i8* %dst, i8* %src, i32 8)
-; CHECK: call i8* @strncpy
+  call i8* @strncpy(i8* %dst, i8* %src, i32 5)
+  ret void
+}
+
+; Check cases that shouldn't be simplified.
+
+define void @test_no_simplify1() {
+; CHECK-LABEL: @test_no_simplify1(
+; CHECK-NEXT:    [[TMP1:%.*]] = call i8* @strncpy(i8* getelementptr inbounds ([32 x i8], [32 x i8]* @a, i32 0, i32 0), i8* getelementptr inbounds ([32 x i8], [32 x i8]* @b, i32 0, i32 0), i32 32)
+; CHECK-NEXT:    ret void
+;
+  %dst = getelementptr [32 x i8], [32 x i8]* @a, i32 0, i32 0
+  %src = getelementptr [32 x i8], [32 x i8]* @b, i32 0, i32 0
+
+  call i8* @strncpy(i8* %dst, i8* %src, i32 32)
   ret void
 }