construct/include/ircd/simd/tzcnt.h

// The Construct
//
// Copyright (C) The Construct Developers, Authors & Contributors
// Copyright (C) 2016-2020 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

#pragma once
#define HAVE_IRCD_SIMD_TZCNT_H

// This suite is for counting trailing zero bits of a word T. It is not for
// per-lane ctz'ing; for this reason all overloads are explicitly instantiated
// and optimal conversions are performed.
namespace ircd::simd
{
	template<class T> uint _tzcnt(const T) noexcept;
	template<class T> uint tzcnt(const T) noexcept = delete;
	template<> uint tzcnt(const u512x1) noexcept;
	template<> uint tzcnt(const u256x1) noexcept;
	template<> uint tzcnt(const u128x1) noexcept;
	template<> uint lzcnt(const u64x8) noexcept;
	template<> uint lzcnt(const u64x4) noexcept;
	template<> uint lzcnt(const u64x2) noexcept;
	template<> uint lzcnt(const u32x16) noexcept;
	template<> uint lzcnt(const u32x8) noexcept;
	template<> uint lzcnt(const u32x4) noexcept;
	template<> uint lzcnt(const u16x32) noexcept;
	template<> uint lzcnt(const u16x16) noexcept;
	template<> uint lzcnt(const u16x8) noexcept;
	template<> uint lzcnt(const u8x64) noexcept;
	template<> uint lzcnt(const u8x32) noexcept;
	template<> uint lzcnt(const u8x16) noexcept;
}

/// Convenience template. Unfortunately this drops to scalar until specific
/// targets and specializations are created. The behavior of can differ among
/// platforms; we use lzcnt when available, otherwise we account for bsr.
template<class T>
[[gnu::always_inline]]
inline uint
ircd::simd::_tzcnt(const T a)
noexcept
{
	uint ret(0), i(lanes<T>() - 1), mask(-1U); do
	{
		if constexpr(sizeof_lane<T>() <= sizeof(u8))
			ret += mask & __builtin_ctz(a[i] | 0xffffff00U);

		else if constexpr(sizeof_lane<T>() <= sizeof(u16))
			ret += mask & __builtin_ctz(__builtin_bswap16(a[i]) | 0xffff0000U);

		else if constexpr(sizeof_lane<T>() <= sizeof(u32))
			ret += mask &
			(
				(boolmask(uint(a[i] != 0)) & __builtin_ctz(__builtin_bswap32(a[i])))
				| (boolmask(uint(a[i] == 0)) & 32U)
			);

		else if constexpr(sizeof_lane<T>() <= sizeof(u64))
			ret += mask &
			(
				(boolmask(uint(a[i] != 0)) & __builtin_ctzl(__builtin_bswap64(a[i])))
				| (boolmask(uint(a[i] == 0)) & 64U)
			);

		static const auto lane_bits(sizeof_lane<T>() * 8);
		mask &= boolmask(uint(ret % lane_bits == 0));
		mask &= boolmask(uint(ret != 0));
	}
	while(i--);
	return ret;
}

template<>
inline uint
ircd::simd::tzcnt(const u512x1 a)
noexcept
{
	return _tzcnt(u64x8(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u256x1 a)
noexcept
{
	return _tzcnt(u64x4(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u128x1 a)
noexcept
{
	return _tzcnt(u64x2(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u64x8 a)
noexcept
{
	return _tzcnt(u64x8(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u64x4 a)
noexcept
{
	return _tzcnt(u64x4(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u64x2 a)
noexcept
{
	return _tzcnt(u64x2(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u32x16 a)
noexcept
{
	return _tzcnt(u64x8(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u32x8 a)
noexcept
{
	return _tzcnt(u64x4(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u32x4 a)
noexcept
{
	return _tzcnt(u64x2(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u16x32 a)
noexcept
{
	return _tzcnt(u64x8(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u16x16 a)
noexcept
{
	return _tzcnt(u64x4(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u16x8 a)
noexcept
{
	return _tzcnt(u64x2(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u8x64 a)
noexcept
{
	return _tzcnt(u64x8(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u8x32 a)
noexcept
{
	return _tzcnt(u64x4(a));
}

template<>
inline uint
ircd::simd::tzcnt(const u8x16 a)
noexcept
{
	return _tzcnt(u64x2(a));
}