ircd::simd: Add pack convenience template funcro.

include/ircd/simd/pack.h

@@ -0,0 +1,49 @@
+// Matrix Construct
+//
+// Copyright (C) Matrix Construct Developers, Authors & Contributors
+// Copyright (C) 2016-2021 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_PACK_H
+
+// Pack values left overwriting unmasked lanes.
+namespace ircd::simd
+{
+	template<class T,
+	         class U>
+	T pack(const T vals, const U mask) noexcept;
+}
+
+/// Left-pack values to eliminate unmasked lanes.
+/// TODO: WIP
+template<class T,
+         class U>
+inline T
+ircd::simd::pack(const T val,
+                 const U mask)
+noexcept
+{
+	static_assert
+	(
+		lanes<T>() == lanes<U>()
+	);
+
+	static_assert
+	(
+		std::is_integral<lane_type<U>>()
+	);
+
+	U idx{0}, add(mask & 1);
+	for(uint i(0); i < lanes<U>(); ++i)
+	{
+		add = shl<sizeof_lane<U>() * 8L>(add);
+		idx += add;
+	}
+
+	return shuf(val, idx);
+}

include/ircd/simd/simd.h

@@ -37,6 +37,7 @@
 #include "any.h"
 #include "all.h"
 #include "shuf.h"
+#include "pack.h"
 #include "stream.h"
 #include "for_each.h"
 #include "transform.h"