add constexpr_strcmp()

Summary: Add a constexpr implementation of `strcmp()`

Reviewed By: yfeldblum

Differential Revision: D6881816

fbshipit-source-id: 08a9e5ab9ef4a422ff6d004ec13b8ec8c71b0d6f

folly/portability/Constexpr.h

@@ -33,6 +33,13 @@ constexpr size_t constexpr_strlen_internal(const Char* s, size_t len) {
 static_assert(
     constexpr_strlen_internal("123456789", 0) == 9,
     "Someone appears to have broken constexpr_strlen...");
+
+template <typename Char>
+constexpr int constexpr_strcmp_internal(const Char* s1, const Char* s2) {
+  return (*s1 == '\0' || *s1 != *s2)
+      ? (static_cast<int>(*s1 - *s2))
+      : constexpr_strcmp_internal(s1 + 1, s2 + 1);
+}
 } // namespace detail
 
 template <typename Char>
@@ -52,4 +59,22 @@ constexpr size_t constexpr_strlen(const char* s) {
   return detail::constexpr_strlen_internal(s, 0);
 #endif
 }
+
+template <typename Char>
+constexpr int constexpr_strcmp(const Char* s1, const Char* s2) {
+  return detail::constexpr_strcmp_internal(s1, s2);
+}
+
+template <>
+constexpr int constexpr_strcmp(const char* s1, const char* s2) {
+#if FOLLY_HAS_FEATURE(cxx_constexpr_string_builtins)
+  // clang provides a constexpr builtin
+  return __builtin_strcmp(s1, s2);
+#elif defined(__GNUC__) && !defined(__clang__)
+  // strcmp() happens to already be constexpr under gcc
+  return std::strcmp(s1, s2);
+#else
+  return detail::constexpr_strcmp_internal(s1, s2);
+#endif
+}
 } // namespace folly

folly/portability/test/ConstexprTest.cpp

@@ -18,12 +18,9 @@
 
 #include <folly/portability/GTest.h>
 
-namespace {
+using folly::constexpr_strcmp;
 
-class ConstexprTest : public testing::Test {};
-} // namespace
-
-TEST_F(ConstexprTest, constexpr_strlen_cstr) {
+TEST(ConstexprTest, constexpr_strlen_cstr) {
   constexpr auto v = "hello";
   constexpr auto a = folly::constexpr_strlen(v);
   EXPECT_EQ(5, a);
@@ -32,10 +29,26 @@ TEST_F(ConstexprTest, constexpr_strlen_cstr) {
 
 // gcc-4.9 cannot compile the following constexpr code correctly
 #if !(defined(__GNUC__) && !defined(__clang__) && __GNUC__ < 5)
-TEST_F(ConstexprTest, constexpr_strlen_ints) {
+TEST(ConstexprTest, constexpr_strlen_ints) {
   constexpr int v[] = {5, 3, 4, 0, 7};
   constexpr auto a = folly::constexpr_strlen(v);
   EXPECT_EQ(3, a);
   EXPECT_TRUE((std::is_same<const size_t, decltype(a)>::value));
 }
+
+TEST(ConstexprTest, constexpr_strcmp_ints) {
+  constexpr int v[] = {5, 3, 4, 0, 7};
+  constexpr int v1[] = {6, 4};
+  static_assert(constexpr_strcmp(v1, v) > 0, "constexpr_strcmp is broken");
+  static_assert(constexpr_strcmp(v, v) == 0, "constexpr_strcmp is broken");
+}
 #endif
+
+static_assert(
+    constexpr_strcmp("abc", "abc") == 0,
+    "constexpr_strcmp is broken");
+static_assert(constexpr_strcmp("", "") == 0, "constexpr_strcmp is broken");
+static_assert(constexpr_strcmp("abc", "def") < 0, "constexpr_strcmp is broken");
+static_assert(constexpr_strcmp("xyz", "abc") > 0, "constexpr_strcmp is broken");
+static_assert(constexpr_strcmp("a", "abc") < 0, "constexpr_strcmp is broken");
+static_assert(constexpr_strcmp("abc", "a") > 0, "constexpr_strcmp is broken");