math.go

     1  // Copyright 2018 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  package math
     6  
     7  import "internal/goarch"
     8  
     9  const MaxUintptr = ^uintptr(0)
    10  
    11  // MulUintptr returns a * b and whether the multiplication overflowed.
    12  // On supported platforms this is an intrinsic lowered by the compiler.
    13  func MulUintptr(a, b uintptr) (uintptr, bool) {
    14  	if a|b < 1<<(4*goarch.PtrSize) || a == 0 {
    15  		return a * b, false
    16  	}
    17  	overflow := b > MaxUintptr/a
    18  	return a * b, overflow
    19  }
    20  
    21  // Mul64 returns the 128-bit product of x and y: (hi, lo) = x * y
    22  // with the product bits' upper half returned in hi and the lower
    23  // half returned in lo.
    24  // This is a copy from math/bits.Mul64
    25  // On supported platforms this is an intrinsic lowered by the compiler.
    26  func Mul64(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  
    42  // Add64 returns the sum with carry of x, y and carry: sum = x + y + carry.
    43  // The carry input must be 0 or 1; otherwise the behavior is undefined.
    44  // The carryOut output is guaranteed to be 0 or 1.
    45  //
    46  // This function's execution time does not depend on the inputs.
    47  // On supported platforms this is an intrinsic lowered by the compiler.
    48  func Add64(x, y, carry uint64) (sum, carryOut uint64) {
    49  	sum = x + y + carry
    50  	// The sum will overflow if both top bits are set (x & y) or if one of them
    51  	// is (x | y), and a carry from the lower place happened. If such a carry
    52  	// happens, the top bit will be 1 + 0 + 1 = 0 (&^ sum).
    53  	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
    54  	return
    55  }
    56
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