Finding the lowest set bit in a 512-bit array using SIMD

_mm512_tzcnt_si512.rs

#![feature(portable_simd)]
#![feature(stdsimd)]
use core::arch::x86_64::*;

unsafe fn _mm512_tzcnt_si512(v: __m512i) -> Option<usize> {
	let zeros = _mm512_setzero_si512();
	let mask = _mm512_cmpneq_epi64_mask(v, zeros);

	// 63 - lzcnt((v - 1) ^ v);
	let unweighted = _mm512_sub_epi64(
		_mm512_set1_epi64(63),
		// lzcnt((v - 1) ^ v)
		_mm512_lzcnt_epi64(
			// (v - 1) ^ v
			_mm512_xor_epi64(
				v,
				// v - 1
				_mm512_sub_epi64(
					v,
					_mm512_set1_epi64(1),
				),
			),
		)
	);

	let weights = _mm512_set_epi64(
		64 * 7,
		64 * 6,
		64 * 5,
		64 * 4,
		64 * 3,
		64 * 2,
		64 * 1,
		64 * 0,
	);
	let weighted = _mm512_mask_add_epi64(
		_mm512_set1_epi64(i64::MAX),
		mask,
		weights,
		unweighted,
	);

	let result = _mm512_reduce_min_epi64(weighted);
	(result < 512).then_some(result as usize)
}