bits_compat.go

     1  // Copyright 2019 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build !go1.13
     6  // +build !go1.13
     7  
     8  package poly1305
     9  
    10  // Generic fallbacks for the math/bits intrinsics, copied from
    11  // src/math/bits/bits.go. They were added in Go 1.12, but Add64 and Sum64 had
    12  // variable time fallbacks until Go 1.13.
    13  
    14  func bitsAdd64(x, y, carry uint64) (sum, carryOut uint64) {
    15  	sum = x + y + carry
    16  	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
    17  	return
    18  }
    19  
    20  func bitsSub64(x, y, borrow uint64) (diff, borrowOut uint64) {
    21  	diff = x - y - borrow
    22  	borrowOut = ((^x & y) | (^(x ^ y) & diff)) >> 63
    23  	return
    24  }
    25  
    26  func bitsMul64(x, y uint64) (hi, lo uint64) {
    27  	const mask32 = 1<<32 - 1
    28  	x0 := x & mask32
    29  	x1 := x >> 32
    30  	y0 := y & mask32
    31  	y1 := y >> 32
    32  	w0 := x0 * y0
    33  	t := x1*y0 + w0>>32
    34  	w1 := t & mask32
    35  	w2 := t >> 32
    36  	w1 += x0 * y1
    37  	hi = x1*y1 + w2 + w1>>32
    38  	lo = x * y
    39  	return
    40  }
    41
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