[libc][NFC] Simplify FPBits (#76835)

This patch reduces the scope of `FPBits` exported variables and
functions.
It also moves storage up into `FPRep` and tries to make the default and
specialized versions of `FPBits` more uniform.

The next step is to move the specialization from `FPBits` to `FPRep` so
we can manipulate floating point representations through `FPType` 
alone - that is - independently from the host architecture.

Author: gchatelet

libc/src/__support/FPUtil/FPBits.h

@@ -33,12 +33,6 @@ enum class FPType {
 
 namespace internal {
 
-// The type of encoding for supported floating point types.
-enum class FPEncoding {
-  IEEE754,
-  X86_ExtendedPrecision,
-};
-
 // Defines the layout (sign, exponent, significand) of a floating point type in
 // memory. It also defines its associated StorageType, i.e., the unsigned
 // integer type used to manipulate its representation.
@@ -49,47 +43,41 @@ template <> struct FPLayout<FPType::IEEE754_Binary16> {
   LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
   LIBC_INLINE_VAR static constexpr int EXP_LEN = 5;
   LIBC_INLINE_VAR static constexpr int SIG_LEN = 10;
-  LIBC_INLINE_VAR static constexpr auto ENCODING = FPEncoding::IEEE754;
 };
 
 template <> struct FPLayout<FPType::IEEE754_Binary32> {
   using StorageType = uint32_t;
   LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
   LIBC_INLINE_VAR static constexpr int EXP_LEN = 8;
   LIBC_INLINE_VAR static constexpr int SIG_LEN = 23;
-  LIBC_INLINE_VAR static constexpr auto ENCODING = FPEncoding::IEEE754;
 };
 
 template <> struct FPLayout<FPType::IEEE754_Binary64> {
   using StorageType = uint64_t;
   LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
   LIBC_INLINE_VAR static constexpr int EXP_LEN = 11;
   LIBC_INLINE_VAR static constexpr int SIG_LEN = 52;
-  LIBC_INLINE_VAR static constexpr auto ENCODING = FPEncoding::IEEE754;
 };
 
 template <> struct FPLayout<FPType::IEEE754_Binary128> {
   using StorageType = UInt128;
   LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
   LIBC_INLINE_VAR static constexpr int EXP_LEN = 15;
   LIBC_INLINE_VAR static constexpr int SIG_LEN = 112;
-  LIBC_INLINE_VAR static constexpr auto ENCODING = FPEncoding::IEEE754;
 };
 
 template <> struct FPLayout<FPType::X86_Binary80> {
   using StorageType = UInt128;
   LIBC_INLINE_VAR static constexpr int SIGN_LEN = 1;
   LIBC_INLINE_VAR static constexpr int EXP_LEN = 15;
   LIBC_INLINE_VAR static constexpr int SIG_LEN = 64;
-  LIBC_INLINE_VAR static constexpr auto ENCODING =
-      FPEncoding::X86_ExtendedPrecision;
 };
 
 } // namespace internal
 
-// FPBaseMasksAndShifts derives useful constants from the FPLayout.
+// FPRepBase derives useful constants from the FPLayout.
 template <FPType fp_type>
-struct FPBaseMasksAndShifts : public internal::FPLayout<fp_type> {
+struct FPRepBase : public internal::FPLayout<fp_type> {
 private:
   using UP = internal::FPLayout<fp_type>;
 
@@ -149,95 +137,67 @@ struct FPBaseMasksAndShifts : public internal::FPLayout<fp_type> {
     return StorageType(1) << position;
   }
 
-public:
+  // Merge bits from 'a' and 'b' values according to 'mask'.
+  // Use 'a' bits when corresponding 'mask' bits are zeroes and 'b' bits when
+  // corresponding bits are ones.
+  LIBC_INLINE static constexpr StorageType merge(StorageType a, StorageType b,
+                                                 StorageType mask) {
+    // https://graphics.stanford.edu/~seander/bithacks.html#MaskedMerge
+    return a ^ ((a ^ b) & mask);
+  }
+
+protected:
   // The number of bits after the decimal dot when the number is in normal form.
   LIBC_INLINE_VAR static constexpr int FRACTION_LEN =
-      UP::ENCODING == internal::FPEncoding::X86_ExtendedPrecision ? SIG_LEN - 1
-                                                                  : SIG_LEN;
+      fp_type == FPType::X86_Binary80 ? SIG_LEN - 1 : SIG_LEN;
   LIBC_INLINE_VAR static constexpr uint32_t MANTISSA_PRECISION =
       FRACTION_LEN + 1;
   LIBC_INLINE_VAR static constexpr StorageType FRACTION_MASK =
       mask_trailing_ones<StorageType, FRACTION_LEN>();
 
-protected:
   // If a number x is a NAN, then it is a quiet NAN if:
   //   QUIET_NAN_MASK & bits(x) != 0
   LIBC_INLINE_VAR static constexpr StorageType QUIET_NAN_MASK =
-      UP::ENCODING == internal::FPEncoding::X86_ExtendedPrecision
+      fp_type == FPType::X86_Binary80
           ? bit_at(SIG_LEN - 1) | bit_at(SIG_LEN - 2) // 0b1100...
           : bit_at(SIG_LEN - 1);                      // 0b1000...
 
   // If a number x is a NAN, then it is a signalling NAN if:
   //   SIGNALING_NAN_MASK & bits(x) != 0
   LIBC_INLINE_VAR static constexpr StorageType SIGNALING_NAN_MASK =
-      UP::ENCODING == internal::FPEncoding::X86_ExtendedPrecision
+      fp_type == FPType::X86_Binary80
           ? bit_at(SIG_LEN - 1) | bit_at(SIG_LEN - 3) // 0b1010...
           : bit_at(SIG_LEN - 2);                      // 0b0100...
-};
-
-namespace internal {
 
-// This is a temporary class to unify common methods and properties between
-// FPBits and FPBits<long double>.
-template <FPType fp_type> struct FPRep : private FPBaseMasksAndShifts<fp_type> {
-  using UP = FPBaseMasksAndShifts<fp_type>;
-  using typename UP::StorageType;
-  using UP::TOTAL_LEN;
-
-protected:
-  using UP::EXP_SIG_MASK;
-  using UP::QUIET_NAN_MASK;
+  // The floating point number representation as an unsigned integer.
+  StorageType bits = 0;
 
 public:
-  using UP::EXP_BIAS;
-  using UP::EXP_LEN;
-  using UP::EXP_MASK;
-  using UP::EXP_MASK_SHIFT;
-  using UP::FP_MASK;
-  using UP::FRACTION_LEN;
-  using UP::FRACTION_MASK;
-  using UP::MANTISSA_PRECISION;
-  using UP::SIGN_MASK;
-  using UP::STORAGE_LEN;
-
-  // Reinterpreting bits as an integer value and interpreting the bits of an
-  // integer value as a floating point value is used in tests. So, a convenient
-  // type is provided for such reinterpretations.
-  StorageType bits;
-
-  LIBC_INLINE constexpr FPRep() : bits(0) {}
-  LIBC_INLINE explicit constexpr FPRep(StorageType bits) : bits(bits) {}
-
-  LIBC_INLINE constexpr void set_mantissa(StorageType mantVal) {
-    mantVal &= FRACTION_MASK;
-    bits &= ~FRACTION_MASK;
-    bits |= mantVal;
-  }
-
-  LIBC_INLINE constexpr StorageType get_mantissa() const {
-    return bits & FRACTION_MASK;
+  LIBC_INLINE constexpr bool get_sign() const {
+    return (bits & SIGN_MASK) != 0;
   }
 
   LIBC_INLINE constexpr void set_sign(bool signVal) {
     if (get_sign() != signVal)
       bits ^= SIGN_MASK;
   }
 
-  LIBC_INLINE constexpr bool get_sign() const {
-    return (bits & SIGN_MASK) != 0;
+  LIBC_INLINE constexpr StorageType get_mantissa() const {
+    return bits & FRACTION_MASK;
   }
 
-  LIBC_INLINE constexpr void set_biased_exponent(StorageType biased) {
-    // clear exponent bits
-    bits &= ~EXP_MASK;
-    // set exponent bits
-    bits |= (biased << EXP_MASK_SHIFT) & EXP_MASK;
+  LIBC_INLINE constexpr void set_mantissa(StorageType mantVal) {
+    bits = merge(bits, mantVal, FRACTION_MASK);
   }
 
   LIBC_INLINE constexpr uint16_t get_biased_exponent() const {
     return uint16_t((bits & EXP_MASK) >> EXP_MASK_SHIFT);
   }
 
+  LIBC_INLINE constexpr void set_biased_exponent(StorageType biased) {
+    bits = merge(bits, biased << EXP_MASK_SHIFT, EXP_MASK);
+  }
+
   LIBC_INLINE constexpr int get_exponent() const {
     return int(get_biased_exponent()) - EXP_BIAS;
   }
@@ -266,6 +226,23 @@ template <FPType fp_type> struct FPRep : private FPBaseMasksAndShifts<fp_type> {
   }
 };
 
+namespace internal {
+
+// Manipulates the representation of a floating point number defined by its
+// FPType. This layer is architecture agnostic and does not handle C++ floating
+// point types directly ('float', 'double' and 'long double'). Use the FPBits
+// below if needed.
+//
+// TODO: Specialize this class for FPType::X86_Binary80 and remove ad-hoc logic
+// from FPRepBase.
+template <FPType fp_type> struct FPRep : public FPRepBase<fp_type> {
+  using UP = FPRepBase<fp_type>;
+  using typename UP::StorageType;
+  using UP::FRACTION_LEN;
+  using UP::FRACTION_MASK;
+  using UP::MANTISSA_PRECISION;
+};
+
 } // namespace internal
 
 // Returns the FPType corresponding to C++ type T on the host.
@@ -311,14 +288,16 @@ template <typename T> struct FPBits : public internal::FPRep<get_fp_type<T>()> {
   static_assert(cpp::is_floating_point_v<T>,
                 "FPBits instantiated with invalid type.");
   using UP = internal::FPRep<get_fp_type<T>()>;
-  using StorageType = typename UP::StorageType;
-  using UP::bits;
 
 private:
   using UP::EXP_SIG_MASK;
   using UP::QUIET_NAN_MASK;
+  using UP::SIG_LEN;
+  using UP::SIG_MASK;
 
 public:
+  using StorageType = typename UP::StorageType;
+  using UP::bits;
   using UP::EXP_BIAS;
   using UP::EXP_LEN;
   using UP::EXP_MASK;
@@ -327,46 +306,47 @@ template <typename T> struct FPBits : public internal::FPRep<get_fp_type<T>()> {
   using UP::FRACTION_MASK;
   using UP::SIGN_MASK;
   using UP::TOTAL_LEN;
+  using UP::UP;
 
   using UP::get_biased_exponent;
   using UP::is_zero;
-
-  // The function return mantissa with the implicit bit set iff the current
-  // value is a valid normal number.
-  LIBC_INLINE constexpr StorageType get_explicit_mantissa() {
-    return ((get_biased_exponent() > 0 && !is_inf_or_nan())
-                ? (FRACTION_MASK + 1)
-                : 0) |
-           (FRACTION_MASK & bits);
-  }
-
+  // Constants.
   static constexpr int MAX_BIASED_EXPONENT = (1 << EXP_LEN) - 1;
   static constexpr StorageType MIN_SUBNORMAL = StorageType(1);
   static constexpr StorageType MAX_SUBNORMAL = FRACTION_MASK;
   static constexpr StorageType MIN_NORMAL = (StorageType(1) << FRACTION_LEN);
   static constexpr StorageType MAX_NORMAL =
-      ((StorageType(MAX_BIASED_EXPONENT) - 1) << FRACTION_LEN) | MAX_SUBNORMAL;
-
-  // We don't want accidental type promotions/conversions, so we require exact
-  // type match.
-  template <typename XType, cpp::enable_if_t<cpp::is_same_v<T, XType>, int> = 0>
-  LIBC_INLINE constexpr explicit FPBits(XType x)
-      : UP(cpp::bit_cast<StorageType>(x)) {}
+      (StorageType(MAX_BIASED_EXPONENT - 1) << SIG_LEN) | SIG_MASK;
 
-  template <typename XType,
-            cpp::enable_if_t<cpp::is_same_v<XType, StorageType>, int> = 0>
-  LIBC_INLINE constexpr explicit FPBits(XType x) : UP(x) {}
+  // Constructors.
+  LIBC_INLINE constexpr FPBits() = default;
 
-  LIBC_INLINE constexpr FPBits() : UP() {}
-
-  LIBC_INLINE constexpr void set_val(T value) {
-    bits = cpp::bit_cast<StorageType>(value);
+  template <typename XType> LIBC_INLINE constexpr explicit FPBits(XType x) {
+    using Unqual = typename cpp::remove_cv_t<XType>;
+    if constexpr (cpp::is_same_v<Unqual, T>) {
+      bits = cpp::bit_cast<StorageType>(x);
+    } else if constexpr (cpp::is_same_v<Unqual, StorageType>) {
+      bits = x;
+    } else {
+      // We don't want accidental type promotions/conversions, so we require
+      // exact type match.
+      static_assert(cpp::always_false<XType>);
+    }
   }
-
+  // Floating-point conversions.
   LIBC_INLINE constexpr T get_val() const { return cpp::bit_cast<T>(bits); }
 
   LIBC_INLINE constexpr explicit operator T() const { return get_val(); }
 
+  // The function return mantissa with the implicit bit set iff the current
+  // value is a valid normal number.
+  LIBC_INLINE constexpr StorageType get_explicit_mantissa() {
+    return ((get_biased_exponent() > 0 && !is_inf_or_nan())
+                ? (FRACTION_MASK + 1)
+                : 0) |
+           (FRACTION_MASK & bits);
+  }
+
   LIBC_INLINE constexpr bool is_inf() const {
     return (bits & EXP_SIG_MASK) == EXP_MASK;
   }
@@ -387,14 +367,22 @@ template <typename T> struct FPBits : public internal::FPRep<get_fp_type<T>()> {
     return FPBits(bits & EXP_SIG_MASK);
   }
 
+  // Methods below this are used by tests.
+
   LIBC_INLINE static constexpr T zero(bool sign = false) {
-    return FPBits(sign ? SIGN_MASK : StorageType(0)).get_val();
+    StorageType rep = (sign ? SIGN_MASK : StorageType(0)) // sign
+                      | 0                                 // exponent
+                      | 0;                                // mantissa
+    return FPBits(rep).get_val();
   }
 
   LIBC_INLINE static constexpr T neg_zero() { return zero(true); }
 
   LIBC_INLINE static constexpr T inf(bool sign = false) {
-    return FPBits((sign ? SIGN_MASK : StorageType(0)) | EXP_MASK).get_val();
+    StorageType rep = (sign ? SIGN_MASK : StorageType(0)) // sign
+                      | EXP_MASK                          // exponent
+                      | 0;                                // mantissa
+    return FPBits(rep).get_val();
   }
 
   LIBC_INLINE static constexpr T neg_inf() { return inf(true); }
@@ -416,15 +404,24 @@ template <typename T> struct FPBits : public internal::FPRep<get_fp_type<T>()> {
   }
 
   LIBC_INLINE static constexpr T build_nan(StorageType v) {
-    FPBits<T> bits(inf());
-    bits.set_mantissa(v);
-    return T(bits);
+    StorageType rep = 0                      // sign
+                      | EXP_MASK             // exponent
+                      | (v & FRACTION_MASK); // mantissa
+    return FPBits(rep).get_val();
   }
 
   LIBC_INLINE static constexpr T build_quiet_nan(StorageType v) {
     return build_nan(QUIET_NAN_MASK | v);
   }
 
+  LIBC_INLINE static constexpr FPBits<T>
+  create_value(bool sign, StorageType biased_exp, StorageType mantissa) {
+    StorageType rep = (sign ? SIGN_MASK : StorageType(0))           // sign
+                      | ((biased_exp << EXP_MASK_SHIFT) & EXP_MASK) // exponent
+                      | (mantissa & FRACTION_MASK);                 // mantissa
+    return FPBits(rep);
+  }
+
   // The function convert integer number and unbiased exponent to proper float
   // T type:
   //   Result = number * 2^(ep+1 - exponent_bias)
@@ -452,15 +449,6 @@ template <typename T> struct FPBits : public internal::FPRep<get_fp_type<T>()> {
     }
     return result;
   }
-
-  LIBC_INLINE static constexpr FPBits<T>
-  create_value(bool sign, StorageType biased_exp, StorageType mantissa) {
-    FPBits<T> result;
-    result.set_sign(sign);
-    result.set_biased_exponent(biased_exp);
-    result.set_mantissa(mantissa);
-    return result;
-  }
 };
 
 } // namespace fputil

libc/src/__support/FPUtil/fpbits_str.h

@@ -45,7 +45,7 @@ template <typename T> LIBC_INLINE cpp::string str(fputil::FPBits<T> x) {
 
   cpp::string s;
 
-  const details::ZeroPaddedHexFmt<StorageType> bits(x.bits);
+  const details::ZeroPaddedHexFmt<StorageType> bits(x.uintval());
   s += bits.view();
 
   s += " = (S: ";