[clang][bytecode] Implement bitcasts to floating-point values

clang/lib/AST/ByteCode/Interp.h

@@ -3062,7 +3062,8 @@ inline bool BitCast(InterpState &S, CodePtr OpPC, bool TargetIsUCharOrByte,
     return false;
 
   if constexpr (std::is_same_v<T, Floating>) {
-    assert(false && "Implement bitcasting to a floating type");
+    assert(Sem);
+    S.Stk.push<Floating>(T::bitcastFromMemory(Buff.data(), *Sem));
   } else {
     assert(!Sem);
     S.Stk.push<T>(T::bitcastFromMemory(Buff.data(), ResultBitWidth));

clang/test/AST/ByteCode/builtin-bit-cast-long-double.cpp

@@ -0,0 +1,87 @@
+// RUN: %clang_cc1 -verify=ref,both -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s
+// RUN: %clang_cc1 -verify=ref,both -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fno-signed-char
+// RUN: %clang_cc1 -verify=ref,both -std=c++2a -fsyntax-only -triple aarch64_be-linux-gnu %s
+
+// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -triple x86_64-apple-macosx10.14.0 %s -fno-signed-char -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 -verify=expected,both -std=c++2a -fsyntax-only -triple aarch64_be-linux-gnu %s -fexperimental-new-constant-interpreter
+
+// both-no-diagnostics
+
+typedef decltype(nullptr) nullptr_t;
+typedef __INTPTR_TYPE__ intptr_t;
+
+static_assert(sizeof(int) == 4);
+static_assert(sizeof(long long) == 8);
+
+template <class To, class From>
+constexpr To bit_cast(const From &from) {
+  static_assert(sizeof(To) == sizeof(From));
+  return __builtin_bit_cast(To, from);
+}
+
+template <class Intermediate, class Init>
+constexpr bool check_round_trip(const Init &init) {
+  return bit_cast<Init>(bit_cast<Intermediate>(init)) == init;
+}
+
+template <class Intermediate, class Init>
+constexpr Init round_trip(const Init &init) {
+  return bit_cast<Init>(bit_cast<Intermediate>(init));
+}
+
+
+
+
+namespace test_long_double {
+#if __x86_64
+#if 0
+constexpr __int128_t test_cast_to_int128 = bit_cast<__int128_t>((long double)0); // expected-error{{must be initialized by a constant expression}}\
+                                                                                 // expected-note{{in call}}
+#endif
+constexpr long double ld = 3.1425926539;
+
+struct bytes {
+  unsigned char d[16];
+};
+
+// static_assert(round_trip<bytes>(ld), "");
+
+static_assert(round_trip<long double>(10.0L));
+
+#if 0
+constexpr bool f(bool read_uninit) {
+  bytes b = bit_cast<bytes>(ld);
+  unsigned char ld_bytes[10] = {
+    0x0,  0x48, 0x9f, 0x49, 0xf0,
+    0x3c, 0x20, 0xc9, 0x0,  0x40,
+  };
+
+  for (int i = 0; i != 10; ++i)
+    if (ld_bytes[i] != b.d[i])
+      return false;
+
+  if (read_uninit && b.d[10]) // expected-note{{read of uninitialized object is not allowed in a constant expression}}
+    return false;
+
+  return true;
+}
+
+static_assert(f(/*read_uninit=*/false), "");
+static_assert(f(/*read_uninit=*/true), ""); // expected-error{{static assertion expression is not an integral constant expression}} \
+                                            // expected-note{{in call to 'f(true)'}}
+#endif
+constexpr bytes ld539 = {
+  0x0, 0x0,  0x0,  0x0,
+  0x0, 0x0,  0xc0, 0x86,
+  0x8, 0x40, 0x0,  0x0,
+  0x0, 0x0,  0x0,  0x0,
+};
+
+constexpr long double fivehundredandthirtynine = 539.0;
+
+static_assert(bit_cast<long double>(ld539) == fivehundredandthirtynine, "");
+#else
+static_assert(round_trip<__int128_t>(34.0L));
+#endif
+}

clang/test/AST/ByteCode/builtin-bit-cast.cpp

@@ -76,6 +76,8 @@ static_assert(check_round_trip<__int128_t>((__int128_t)34));
 static_assert(check_round_trip<__int128_t>((__int128_t)-34));
 #endif
 
+static_assert(check_round_trip<double>(17.0));
+
 
 namespace simple {
   constexpr int A = __builtin_bit_cast(int, 10);
@@ -108,7 +110,7 @@ namespace simple {
   static_assert(check_round_trip<unsigned>((int)0x12345678));
   static_assert(check_round_trip<unsigned>((int)0x87654321));
   static_assert(check_round_trip<unsigned>((int)0x0C05FEFE));
-  // static_assert(round_trip<float>((int)0x0C05FEFE));
+  static_assert(round_trip<float>((int)0x0C05FEFE));
 
 
   /// This works in GCC and in the bytecode interpreter, but the current interpreter