diff --git a/mace/codegen/BUILD b/mace/codegen/BUILD
index 29504c9b48255bc3ff543358662136ca1f38b801..8d7374d15df123c54615cfd059cc1db442c9f39b 100644
--- a/mace/codegen/BUILD
+++ b/mace/codegen/BUILD
@@ -10,7 +10,7 @@ load("//mace:mace.bzl", "if_embed_binary_program")
 cc_library(
     name = "generated_models_lib",
     srcs = glob(["models/*/*.cc"]),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
     deps = [
         "//mace/core",
@@ -25,21 +25,21 @@ cc_library(
 cc_library(
     name = "generated_opencl_lib",
     srcs = glob(["opencl/*.cc"]),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
 )
 
 cc_library(
     name = "generated_tuning_lib",
     srcs = glob(["tuning/*.cc"]),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
 )
 
 cc_library(
     name = "version_lib",
     srcs = glob(["version/*.cc"]),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
     deps = [
         "//mace/core",
diff --git a/mace/core/BUILD b/mace/core/BUILD
index 252982f005128b1670224c60d327331d819e46ff..4714a0ebee50f86c7ee162e7cbf149cdeba8a8b6 100644
--- a/mace/core/BUILD
+++ b/mace/core/BUILD
@@ -21,7 +21,7 @@ cc_library(
         "runtime/opencl/cl2.hpp",
         "runtime/opencl/*.h",
     ]),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"] +
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"] +
             if_profiling_enabled(["-DMACE_OPENCL_PROFILING"]) +
             if_embed_binary_program(["-DMACE_EMBED_BINARY_PROGRAM"]),
     linkopts = if_android(["-pie", "-ldl"]),
@@ -43,7 +43,7 @@ cc_library(
         "testing/test_benchmark.cc",
         "testing/test_benchmark_main.cc",
     ],
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     alwayslink = 1,
     deps = [
         ":core",
diff --git a/mace/core/operator.cc b/mace/core/operator.cc
index ae1770b135d54a20081af51577313c3a8eef3ff4..341493fcadfad04d099e183b426a0368df45d288 100644
--- a/mace/core/operator.cc
+++ b/mace/core/operator.cc
@@ -14,7 +14,10 @@ OperatorBase::OperatorBase(const OperatorDef &operator_def, Workspace *ws)
 
 OpKeyBuilder::OpKeyBuilder(const char *op_name) : op_name_(op_name) {}
 
-OpKeyBuilder &OpKeyBuilder::Device(DeviceType device) { device_type_ = device; }
+OpKeyBuilder &OpKeyBuilder::Device(DeviceType device) {
+  device_type_ = device;
+  return *this;
+}
 
 OpKeyBuilder &OpKeyBuilder::TypeConstraint(const char *attr_name,
                                            const DataType allowed) {
diff --git a/mace/examples/BUILD b/mace/examples/BUILD
index 9db67631e1cf877c60c96f151ee1ed8a193f4345..45063538648f7dd06b23d1820ed0095adcc0ce1e 100644
--- a/mace/examples/BUILD
+++ b/mace/examples/BUILD
@@ -6,7 +6,7 @@ cc_binary(
     srcs = [
         "helloworld.cc",
     ],
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = if_enable_neon(["-fopenmp"]),
     deps = [
         "//mace/core",
@@ -18,7 +18,7 @@ cc_test(
     name = "benchmark_example",
     testonly = 1,
     srcs = ["benchmark_example.cc"],
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = if_enable_neon(["-fopenmp"]),
     linkstatic = 1,
     deps = [
@@ -30,7 +30,7 @@ cc_test(
 cc_binary(
     name = "mace_run",
     srcs = ["mace_run.cc"],
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = if_enable_neon(["-fopenmp"]),
     linkstatic = 1,
     deps = [
diff --git a/mace/kernels/BUILD b/mace/kernels/BUILD
index a2ad37dbe93a1f00a7875cd244f2ecdf3d922f5e..1a3486ee0b624b7f14b04f9dda571e984fab13d2 100644
--- a/mace/kernels/BUILD
+++ b/mace/kernels/BUILD
@@ -20,6 +20,7 @@ cc_library(
     copts = [
         "-std=c++11",
         "-D_GLIBCXX_USE_C99_MATH_TR1",
+        "-Werror=return-type",
     ] +
     if_embed_binary_program(["-DMACE_EMBED_BINARY_PROGRAM"]) +
     if_enable_neon(["-fopenmp"]),
@@ -34,7 +35,7 @@ cc_test(
     name = "kernel_test",
     testonly = 1,
     srcs = glob(["test/*.cc"]),
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = if_android(["-pie"]),
     linkstatic = 1,
     deps = [
@@ -48,7 +49,7 @@ cc_test(
     name = "benchmark",
     testonly = 1,
     srcs = glob(["benchmark/*.cc"]),
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
     deps = [
         ":kernels",
diff --git a/mace/ops/BUILD b/mace/ops/BUILD
index 2395ded6179dc8c7102f1ca4c0096aeaedbc9e79..890c46b9f1a273e2246d995ca643a51a138f10fb 100644
--- a/mace/ops/BUILD
+++ b/mace/ops/BUILD
@@ -34,7 +34,7 @@ cc_library(
         ["*.h"],
         exclude = ["ops_test_util.h"],
     ),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"] +
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"] +
             if_enable_neon(["-DMACE_ENABLE_NEON"]),
     deps = [
         "//mace/kernels",
@@ -48,7 +48,7 @@ cc_test(
     srcs = glob(
         ["*_test.cc"],
     ),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = ["-fopenmp"],
     linkstatic = 1,
     deps = [
@@ -62,7 +62,7 @@ cc_test(
     name = "ops_benchmark",
     testonly = 1,
     srcs = glob(["*_benchmark.cc"]),
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = ["-fopenmp"],
     linkstatic = 1,
     deps = [
diff --git a/mace/tools/benchmark/BUILD b/mace/tools/benchmark/BUILD
index 321f451de2aaf9b141338e716440e507827465e5..5e026997a2d3c0d3ddb558e29ec7b70043f22772 100644
--- a/mace/tools/benchmark/BUILD
+++ b/mace/tools/benchmark/BUILD
@@ -7,7 +7,7 @@ cc_library(
     name = "stat_summarizer",
     srcs = ["stat_summarizer.cc"],
     hdrs = ["stat_summarizer.h"],
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
     deps = [
         "//mace/core",
@@ -19,7 +19,7 @@ cc_binary(
     srcs = [
         "benchmark_model.cc",
     ],
-    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkstatic = 1,
     deps = [
         ":stat_summarizer",
diff --git a/mace/utils/BUILD b/mace/utils/BUILD
index d98c167532e0c511fc4775eace32e24e3edbd99a..2eb252ff7df9b85d3c9ffa29cc5f1dffd647f7d0 100644
--- a/mace/utils/BUILD
+++ b/mace/utils/BUILD
@@ -17,7 +17,7 @@ cc_library(
     hdrs = [
         "logging.h",
     ],
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = if_android([
         "-llog",
     ]),
@@ -31,7 +31,7 @@ cc_library(
     hdrs = [
         "command_line_flags.h",
     ],
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     deps = [
         ":logging",
     ],
@@ -43,7 +43,7 @@ cc_library(
         "tuner.h",
         "timer.h",
     ],
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     deps = [
         ":utils_hdrs",
         ":logging",
@@ -56,7 +56,7 @@ cc_test(
     srcs = [
         "tuner_test.cc",
     ],
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
     linkopts = if_android(["-pie", "-lm"]),
     linkstatic = 1,
     deps = [
@@ -72,7 +72,7 @@ cc_library(
         "utils.h",
         "env_time.h",
     ],
-    copts = ["-std=c++11"],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
 )
 
 cc_library(
@@ -83,4 +83,5 @@ cc_library(
         ":command_line_flags",
         ":logging",
     ],
+    copts = ["-std=c++11", "-D_GLIBCXX_USE_C99_MATH_TR1", "-Werror=return-type"],
 )