Implement `clz` using floating point math if `clz` instruction is not available

[Kmeakin]

In https://graphics.stanford.edu/~seander/bithacks.html#IntegerLogIEEE64Float we can find a neat trick to calculate the ilog2 of a u32 with floating point math:

u32 ilog2_u32(u32 x) {
    // zexto extend to 64 bits
    u64 x_u64 = x;

    // set exponent to 52
    x_u64 |= (52 + 1023ULL) << 52;

    // move into f64 register
    double f = __builtin_bit_cast(double, x_u64);

    // subtract 2^52
    f -= 0x1.0p52;

    // move back into u64 register
    x_u64 = __builtin_bit_cast(u64, f);

    // extract exponent
    return (x_u64 >> 52) - 1023;
}

Rearrange the equation:

ilog2(x) == 31 - clz(x)
clz(x) + ilog2(x) == 31
clz(x) == 31 - ilog2(x)

and we get:

u32 src32(u32 x) { return __builtin_clzg(x); }
u32 tgt32(u32 x) { return 31 - ilog2_u32(x); }

This generalizes to u8 and u16 using float instead of double

https://godbolt.org/z/cW8js3MK9
https://alive2.llvm.org/ce/z/-bK_nC

[RKSimon]

@asb @topperc Is this useful in general for risc-v or only if certain ISAs are missing?

[topperc]

@asb @topperc Is this useful in general for risc-v or only if certain ISAs are missing?

For scalars, only if the Zbb(basic bit manipulation) extension isn't present. For vectors, we already do this trick. All SiFive cores implement Zbb today so I haven't been motivated to do anything with scalar.