[llvm] [libc] [libc][NFC] Refactor FPBits and remove LongDoubleBits specialization (PR #78192)

[Nick Desaulniers via llvm-commits]

================
@@ -132,11 +166,94 @@ struct FPRepBase : public internal::FPLayout<fp_type> {
   static_assert((SIG_MASK & EXP_MASK & SIGN_MASK) == 0, "masks disjoint");
   static_assert((SIG_MASK | EXP_MASK | SIGN_MASK) == FP_MASK, "masks cover");
 
-private:
+protected:
   LIBC_INLINE static constexpr StorageType bit_at(int position) {
     return StorageType(1) << position;
   }
 
+  // An opaque type to store a floating point exponent.
+  // We define special values but it is valid to create arbitrary values as long
+  // as they are in the range [MIN, MAX].
+  enum class Exponent : int32_t {
+    MIN = 1 - EXP_BIAS,
+    ZERO = 0,
+    MAX = EXP_BIAS,
+  };
+
+  // An opaque type to store a floating point biased exponent.
+  // We define special values but it is valid to create arbitrary values as long
+  // as they are in the range [BITS_ALL_ZEROES, BITS_ALL_ONES].
+  // Values greater than BITS_ALL_ONES are truncated.
+  enum class BiasedExponent : uint32_t {
+    // The exponent value for denormal numbers.
+    BITS_ALL_ZEROES = 0,
+    // The exponent value for infinity.
+    BITS_ALL_ONES = 2 * EXP_BIAS + 1,
+  };
+
+  LIBC_INLINE static constexpr BiasedExponent biased(Exponent value) {
+    return static_cast<BiasedExponent>(static_cast<int32_t>(value) + EXP_BIAS);
+  }
+
+  // An opaque type to store a floating point significand.
+  // We define special values but it is valid to create arbitrary values as long
+  // as they are in the range [BITS_ALL_ZEROES, BITS_ALL_ONES].
+  // Note that the semantics of the Significand are implementation dependent.
+  // Values greater than BITS_ALL_ONES are truncated.
+  enum class Significand : StorageType {
+    ZERO = 0,
+    LSB = 1,
+    MSB = bit_at(SIG_LEN - 1),
+    // Aliases
+    BITS_ALL_ZEROES = ZERO,
+    BITS_ALL_ONES = SIG_MASK,
+  };
+
+  template <typename T>
+  LIBC_INLINE static constexpr auto storage_cast(T value) {
+    return static_cast<StorageType>(value);
----------------
nickdesaulniers wrote:

32b arm build bot failure here:
```
/llvm/ndesaulniers/llvm-project/libc/src/__support/FPUtil/FPBits.h:214:12: error: no matching conversion for static_cast from '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent' to '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::StorageType' (aka 'BigInt<128U, false>')
    return static_cast<StorageType>(value);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/llvm/ndesaulniers/llvm-project/libc/src/__support/FPUtil/FPBits.h:231:13: note: in instantiation of function template specialization '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::storage_cast<__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent>' requested here
    return (storage_cast(exp) << SIG_LEN) & EXP_MASK;
            ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/FPUtil/FPBits.h:472:23: note: in instantiation of member function '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::encode' requested here
    if (exp_bits() == encode(BiasedExponent::BITS_ALL_ONES))
                      ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/UInt.h:44:25: note: candidate constructor not viable: no known conversion from '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent' to 'const BigInt<128U, false>' for 1st argument
  LIBC_INLINE constexpr BigInt(const BigInt<Bits, Signed> &other) = default;
                        ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/UInt.h:100:34: note: candidate constructor not viable: no known conversion from '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent' to 'const cpp::array<uint64_t, WORDCOUNT>' (aka 'const array<unsigned long long, WORDCOUNT>') for 1st argument
  LIBC_INLINE constexpr explicit BigInt(
                                 ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/UInt.h:47:25: note: candidate template ignored: could not match 'BigInt<OtherBits, OtherSigned>' against '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent'
  LIBC_INLINE constexpr BigInt(const BigInt<OtherBits, OtherSigned> &other) {
                        ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/UInt.h:67:25: note: candidate template ignored: could not match 'uint64_t[N]' (aka 'unsigned long long[N]') against '__llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent'
  LIBC_INLINE constexpr BigInt(const uint64_t (&nums)[N]) {
                        ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/UInt.h:81:25: note: candidate template ignored: substitution failure [with T = __llvm_libc_18_0_0_git::fputil::FPRepBase<__llvm_libc_18_0_0_git::fputil::FPType::X86_Binary80>::BiasedExponent]: implicit instantiation of undefined template '__llvm_libc_18_0_0_git::cpp::enable_if<false>'
  LIBC_INLINE constexpr BigInt(T v) {
                        ^
/llvm/ndesaulniers/llvm-project/libc/src/__support/UInt.h:42:25: note: candidate constructor not viable: requires 0 arguments, but 1 was provided
  LIBC_INLINE constexpr BigInt() = default;
                        ^
```

https://github.com/llvm/llvm-project/pull/78192