[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast

This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.

The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.

rdar://44987528

Differential revision: https://reviews.llvm.org/D62825

llvm-svn: 364954

1 files changed, 39 insertions, 0 deletions

diff --git a/clang/test/SemaCXX/builtin-bit-cast.cpp b/clang/test/SemaCXX/builtin-bit-cast.cpp
new file mode 100644
index 00000000000..3dddadc5617
--- /dev/null
+++ b/clang/test/SemaCXX/builtin-bit-cast.cpp
@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s
+// RUN: %clang_cc1 -verify -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fno-signed-char
+
+#if !__has_builtin(__builtin_bit_cast)
+#error
+#endif
+
+template <class T, T v>
+T instantiate() {
+  return __builtin_bit_cast(T, v);
+}
+
+int x = instantiate<int, 32>();
+
+struct secret_ctor {
+  char member;
+
+private: secret_ctor() = default;
+};
+
+void test1() {
+  secret_ctor c = __builtin_bit_cast(secret_ctor, (char)0);
+}
+
+void test2() {
+  constexpr int i = 0;
+  // expected-error@+1{{__builtin_bit_cast source size does not equal destination size (4 vs 1)}}
+  constexpr char c = __builtin_bit_cast(char, i);
+}
+
+struct not_trivially_copyable {
+  virtual void foo() {}
+};
+
+// expected-error@+1{{__builtin_bit_cast source type must be trivially copyable}}
+constexpr unsigned long ul = __builtin_bit_cast(unsigned long, not_trivially_copyable{});
+
+// expected-error@+1 {{__builtin_bit_cast destination type must be trivially copyable}}
+constexpr long us = __builtin_bit_cast(unsigned long &, 0L);