poly1305/bits_compat.go

// Copyright 2019 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build !go1.13
// +build !go1.13

package poly1305

// Generic fallbacks for the math/bits intrinsics, copied from
// src/math/bits/bits.go. They were added in Go 1.12, but Add64 and Sum64 had
// variable time fallbacks until Go 1.13.

func bitsAdd64(x, y, carry uint64) (sum, carryOut uint64) {
	sum = x + y + carry
	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
	return
}

func bitsSub64(x, y, borrow uint64) (diff, borrowOut uint64) {
	diff = x - y - borrow
	borrowOut = ((^x & y) | (^(x ^ y) & diff)) >> 63
	return
}

func bitsMul64(x, y uint64) (hi, lo uint64) {
	const mask32 = 1<<32 - 1
	x0 := x & mask32
	x1 := x >> 32
	y0 := y & mask32
	y1 := y >> 32
	w0 := x0 * y0
	t := x1*y0 + w0>>32
	w1 := t & mask32
	w2 := t >> 32
	w1 += x0 * y1
	hi = x1*y1 + w2 + w1>>32
	lo = x * y
	return
}