Function mul_hi_u64

src/fl/stl/ieee754_string.cpp.hpp:242–253

Top 64 bits of the 128-bit product of two u64 values, computed via four 32x32->64 widening multiplies. Portable to every Cortex-M target -- uses only integer ops, never the libgcc soft-FP cascade.

// 32x32->64 widening multiplies. Portable to every Cortex-M target -- uses
// only integer ops, never the libgcc soft-FP cascade.
inline fl::u64 mul_hi_u64(fl::u64 a, fl::u64 b) FL_NOEXCEPT {
    const fl::u64 ah = a >> 32;
    const fl::u64 al = a & 0xFFFFFFFFull;
    const fl::u64 bh = b >> 32;
    const fl::u64 bl = b & 0xFFFFFFFFull;
    const fl::u64 ll = al * bl;
    const fl::u64 hl = ah * bl;
    const fl::u64 lh = al * bh;
    const fl::u64 hh = ah * bh;
    const fl::u64 mid = (ll >> 32) + (hl & 0xFFFFFFFFull) + (lh & 0xFFFFFFFFull);
    return hh + (hl >> 32) + (lh >> 32) + (mid >> 32);
}

constexpr fl::u32 kSignBitMask = 0x80000000u;
constexpr fl::u32 kInfBitsPos  = 0x7F800000u;