[libc][NFC] Decouple FP properties from C++ types (#73517)

We simplify the floating point properties by splitting concerns:
 - We define all distinct floating point formats in the `FPType` enum.
 - We map them to properties via the `FPProperties` trait.
- We map from C++ type to `FPType` in the `getFPType` function so logic
is easier to understand and extend.

Author: gchatelet

libc/src/__support/FPUtil/FloatProperties.h

@@ -17,12 +17,18 @@
 namespace LIBC_NAMESPACE {
 namespace fputil {
 
-template <typename T> struct FloatProperties {};
+// The supported floating point types.
+enum class FPType {
+  IEEE754_Binary16,
+  IEEE754_Binary32,
+  IEEE754_Binary64,
+  IEEE754_Binary128,
+  X86_Binary80,
+};
 
-template <> struct FloatProperties<float> {
+template <FPType> struct FPProperties {};
+template <> struct FPProperties<FPType::IEEE754_Binary32> {
   typedef uint32_t BitsType;
-  static_assert(sizeof(BitsType) == sizeof(float),
-                "Unexpected size of 'float' type.");
 
   static constexpr uint32_t BIT_WIDTH = sizeof(BitsType) * 8;
 
@@ -46,10 +52,8 @@ template <> struct FloatProperties<float> {
   static constexpr BitsType QUIET_NAN_MASK = 0x00400000U;
 };
 
-template <> struct FloatProperties<double> {
+template <> struct FPProperties<FPType::IEEE754_Binary64> {
   typedef uint64_t BitsType;
-  static_assert(sizeof(BitsType) == sizeof(double),
-                "Unexpected size of 'double' type.");
 
   static constexpr uint32_t BIT_WIDTH = sizeof(BitsType) * 8;
 
@@ -72,47 +76,10 @@ template <> struct FloatProperties<double> {
   static constexpr BitsType QUIET_NAN_MASK = 0x0008000000000000ULL;
 };
 
-#if defined(LONG_DOUBLE_IS_DOUBLE)
-// Properties for numbers represented in 64 bits long double on Windows
-// platform.
-template <> struct FloatProperties<long double> {
-  typedef uint64_t BitsType;
-  static_assert(sizeof(BitsType) == sizeof(double),
-                "Unexpected size of 'double' type.");
-
-  static constexpr uint32_t BIT_WIDTH = FloatProperties<double>::BIT_WIDTH;
-
-  static constexpr uint32_t MANTISSA_WIDTH =
-      FloatProperties<double>::MANTISSA_WIDTH;
-  static constexpr uint32_t MANTISSA_PRECISION = MANTISSA_WIDTH + 1;
-  static constexpr uint32_t EXPONENT_WIDTH =
-      FloatProperties<double>::EXPONENT_WIDTH;
-  static constexpr BitsType MANTISSA_MASK =
-      FloatProperties<double>::MANTISSA_MASK;
-  static constexpr BitsType SIGN_MASK = FloatProperties<double>::SIGN_MASK;
-  static constexpr BitsType EXPONENT_MASK =
-      FloatProperties<double>::EXPONENT_MASK;
-  static constexpr uint32_t EXPONENT_BIAS =
-      FloatProperties<double>::EXPONENT_BIAS;
-
-  static constexpr BitsType EXP_MANT_MASK =
-      FloatProperties<double>::EXP_MANT_MASK;
-  static_assert(EXP_MANT_MASK == ~SIGN_MASK,
-                "Exponent and mantissa masks are not as expected.");
-
-  // If a number x is a NAN, then it is a quiet NAN if:
-  //   QuietNaNMask & bits(x) != 0
-  // Else, it is a signalling NAN.
-  static constexpr BitsType QUIET_NAN_MASK =
-      FloatProperties<double>::QUIET_NAN_MASK;
-};
-#elif defined(SPECIAL_X86_LONG_DOUBLE)
 // Properties for numbers represented in 80 bits long double on non-Windows x86
 // platforms.
-template <> struct FloatProperties<long double> {
+template <> struct FPProperties<FPType::X86_Binary80> {
   typedef UInt128 BitsType;
-  static_assert(sizeof(BitsType) == sizeof(long double),
-                "Unexpected size of 'long double' type.");
 
   static constexpr uint32_t BIT_WIDTH = (sizeof(BitsType) * 8) - 48;
   static constexpr BitsType FULL_WIDTH_MASK = ((BitsType(1) << BIT_WIDTH) - 1);
@@ -143,13 +110,11 @@ template <> struct FloatProperties<long double> {
   static constexpr BitsType QUIET_NAN_MASK = BitsType(1)
                                              << (MANTISSA_WIDTH - 1);
 };
-#else
+
 // Properties for numbers represented in 128 bits long double on non x86
 // platform.
-template <> struct FloatProperties<long double> {
+template <> struct FPProperties<FPType::IEEE754_Binary128> {
   typedef UInt128 BitsType;
-  static_assert(sizeof(BitsType) == sizeof(long double),
-                "Unexpected size of 'long double' type.");
 
   static constexpr uint32_t BIT_WIDTH = sizeof(BitsType) << 3;
 
@@ -172,39 +137,39 @@ template <> struct FloatProperties<long double> {
   static constexpr BitsType QUIET_NAN_MASK = BitsType(1)
                                              << (MANTISSA_WIDTH - 1);
 };
-#endif
 
-#if (defined(LIBC_COMPILER_HAS_FLOAT128) &&                                    \
-     !defined(LIBC_FLOAT128_IS_LONG_DOUBLE))
-// Properties for numbers represented in 128 bits long double on non x86
-// platform.
-template <> struct FloatProperties<float128> {
-  typedef UInt128 BitsType;
-  static_assert(sizeof(BitsType) == sizeof(float128),
-                "Unexpected size of 'float128' type.");
-
-  static constexpr uint32_t BIT_WIDTH = sizeof(BitsType) << 3;
-
-  static constexpr uint32_t MANTISSA_WIDTH = 112;
-  static constexpr uint32_t MANTISSA_PRECISION = MANTISSA_WIDTH + 1;
-  static constexpr uint32_t EXPONENT_WIDTH = 15;
-  static constexpr BitsType MANTISSA_MASK = (BitsType(1) << MANTISSA_WIDTH) - 1;
-  static constexpr BitsType SIGN_MASK = BitsType(1)
-                                        << (EXPONENT_WIDTH + MANTISSA_WIDTH);
-  static constexpr BitsType EXPONENT_MASK = ~(SIGN_MASK | MANTISSA_MASK);
-  static constexpr uint32_t EXPONENT_BIAS = 16383;
-
-  static constexpr BitsType EXP_MANT_MASK = MANTISSA_MASK | EXPONENT_MASK;
-  static_assert(EXP_MANT_MASK == ~SIGN_MASK,
-                "Exponent and mantissa masks are not as expected.");
+//-----------------------------------------------------------------------------
+template <typename FP> static constexpr FPType get_fp_type() {
+  if constexpr (cpp::is_same_v<FP, float> && __FLT_MANT_DIG__ == 24)
+    return FPType::IEEE754_Binary32;
+  else if constexpr (cpp::is_same_v<FP, double> && __DBL_MANT_DIG__ == 53)
+    return FPType::IEEE754_Binary64;
+  else if constexpr (cpp::is_same_v<FP, long double>) {
+    if constexpr (__LDBL_MANT_DIG__ == 53)
+      return FPType::IEEE754_Binary64;
+    else if constexpr (__LDBL_MANT_DIG__ == 64)
+      return FPType::X86_Binary80;
+    else if constexpr (__LDBL_MANT_DIG__ == 113)
+      return FPType::IEEE754_Binary128;
+  }
+#if defined(LIBC_COMPILER_HAS_C23_FLOAT16)
+  else if constexpr (cpp::is_same_v<FP, _Float16>)
+    return FPType::IEEE754_Binary16;
+#endif
+#if defined(LIBC_COMPILER_HAS_C23_FLOAT128)
+  else if constexpr (cpp::is_same_v<FP, _Float128>)
+    return FPType::IEEE754_Binary128;
+#endif
+#if defined(LIBC_COMPILER_HAS_FLOAT128_EXTENSION)
+  else if constexpr (cpp::is_same_v<FP, __float128>)
+    return FPType::IEEE754_Binary128;
+#endif
+  else
+    static_assert(cpp::always_false<FP>, "Unsupported type");
+}
 
-  // If a number x is a NAN, then it is a quiet NAN if:
-  //   QuietNaNMask & bits(x) != 0
-  // Else, it is a signalling NAN.
-  static constexpr BitsType QUIET_NAN_MASK = BitsType(1)
-                                             << (MANTISSA_WIDTH - 1);
-};
-#endif // LIBC_COMPILER_HAS_FLOAT128
+template <typename FP>
+struct FloatProperties : public FPProperties<get_fp_type<FP>()> {};
 
 } // namespace fputil
 } // namespace LIBC_NAMESPACE

libc/src/__support/FPUtil/dyadic_float.h

@@ -38,13 +38,11 @@ template <size_t Bits> struct DyadicFloat {
   int exponent = 0;
   MantissaType mantissa = MantissaType(0);
 
-  DyadicFloat() = default;
+  constexpr DyadicFloat() = default;
 
-  template <typename T,
-            cpp::enable_if_t<cpp::is_floating_point_v<T> &&
-                                 (FloatProperties<T>::MANTISSA_WIDTH < Bits),
-                             int> = 0>
+  template <typename T, cpp::enable_if_t<cpp::is_floating_point_v<T>, int> = 0>
   DyadicFloat(T x) {
+    static_assert(FloatProperties<T>::MANTISSA_WIDTH < Bits);
     FPBits<T> x_bits(x);
     sign = x_bits.get_sign();
     exponent = x_bits.get_exponent() - FloatProperties<T>::MANTISSA_WIDTH;