StringRef: Extend constexpr capabilities and introduce ConstStringRef

(1) Add llvm_expect(), an asserting macro that can be evaluated as a constexpr
    expression as well as a runtime assert or compiler hint in release builds. This
    technique can be used to construct functions that are both unevaluated and
    compiled depending on usage.

(2) Update StringRef using llvm_expect() to preserve runtime assertions while
    extending the same checks to static asserts in C++11 builds that support the
    feature.

(3) Introduce ConstStringRef, a strong subclass of StringRef that references
    compile-time constant strings. It's convertible to, but not from, ordinary
    StringRef and thus can be used to add compile-time safety to various interfaces
    in LLVM and clang that only accept fixed inputs such as diagnostic format
    strings that tend to get misused.

llvm-svn: 200187

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/StringRefTest.cpp b/llvm/unittests/ADT/StringRefTest.cpp
index 0ab8fcf..b240a87 100644
--- a/llvm/unittests/ADT/StringRefTest.cpp
+++ b/llvm/unittests/ADT/StringRefTest.cpp
@@ -531,4 +531,22 @@ TEST(StringRefTest, joinStrings) {
   EXPECT_TRUE(v2_join3);
 }
 
+static void fn_stringref(StringRef str) {
+  EXPECT_TRUE(str == "hello");
+}
+static void fn_conststringref(ConstStringRef str) {
+  fn_stringref(str);
+}
+
+TEST(StringRefTest, constStringRef) {
+  LLVM_CONSTEXPR ConstStringRef csr("hello");
+#if __has_feature(cxx_constexpr) || defined(__GXX_EXPERIMENTAL_CXX0X__)
+  LLVM_STATIC_ASSERT(csr[0] != csr[1], "");
+  LLVM_STATIC_ASSERT(csr[2] == csr[3], "");
+  LLVM_STATIC_ASSERT(csr.size() == 5, "");
+#endif
+  llvm_expect(csr[2] == csr[3]);
+  fn_conststringref(csr);
+}
+
 } // end anonymous namespace