[libc][nfc] update get_explicit_mantissa

[michaelrj]

Summary

The get_explicit_mantissa function returns the mantissa of an FPBits
floating point value with the implicit leading 1, if appropriate. This
function existed previously, but did not handle non-normal numbers
properly.

libc/src/__support/FPUtil/FPBits.h

template <typename T> struct FPBits {
   void set_mantissa(UIntType mantVal) {
     mantVal &= (FloatProp::MANTISSA_MASK);
     bits &= ~(FloatProp::MANTISSA_MASK);
     bits |= mantVal;
   }
 
   UIntType get_mantissa() const { return bits & FloatProp::MANTISSA_MASK; }
 
-  // The function return mantissa with implicit bit set for normal values.
-  constexpr UIntType get_explicit_mantissa() {
-    return (FloatProp::MANTISSA_MASK + 1) | (FloatProp::MANTISSA_MASK & bits);
-  }
-
   void set_unbiased_exponent(UIntType expVal) {
     expVal = (expVal << (FloatProp::MANTISSA_WIDTH)) & FloatProp::EXPONENT_MASK;
     bits &= ~(FloatProp::EXPONENT_MASK);
     bits |= expVal;
   }
 
   uint16_t get_unbiased_exponent() const {
     return uint16_t((bits & FloatProp::EXPONENT_MASK) >>
                     (FloatProp::MANTISSA_WIDTH));
   }
 
+  // The function return mantissa with the implicit bit set iff the current
+  // value is a valid normal number.
+  constexpr UIntType get_explicit_mantissa() {
+    return ((get_unbiased_exponent() > 0 && !is_inf_or_nan())
+                ? (FloatProp::MANTISSA_MASK + 1)
+                : 0) |
+           (FloatProp::MANTISSA_MASK & bits);
+  }
+
   void set_sign(bool signVal) {
     bits |= FloatProp::SIGN_MASK;
     if (!signVal)
       bits -= FloatProp::SIGN_MASK;
   }
 
   bool get_sign() const { return (bits & FloatProp::SIGN_MASK) != 0; }
 

libc/src/__support/FPUtil/x86_64/LongDoubleBits.h

template <> struct FPBits<long double> {
   void set_mantissa(UIntType mantVal) {
     mantVal &= (FloatProp::MANTISSA_MASK);
     bits &= ~(FloatProp::MANTISSA_MASK);
     bits |= mantVal;
   }
 
   UIntType get_mantissa() const { return bits & FloatProp::MANTISSA_MASK; }
 
+  UIntType get_explicit_mantissa() const {
+    return bits & (FloatProp::MANTISSA_MASK | FloatProp::EXPLICIT_BIT_MASK);
+  }
+
   void set_unbiased_exponent(UIntType expVal) {
     expVal =
         (expVal << (FloatProp::BIT_WIDTH - 1 - FloatProp::EXPONENT_WIDTH)) &
         FloatProp::EXPONENT_MASK;
     bits &= ~(FloatProp::EXPONENT_MASK);
     bits |= expVal;
   }