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 (
    10  	MaxUint32  = ^uint32(0)
    11  	MaxUint64  = ^uint64(0)
    12  	MaxUintptr = ^uintptr(0)
    13  
    14  	MaxInt64 = int64(MaxUint64 >> 1)
    15  )
    16  
    17  // MulUintptr returns a * b and whether the multiplication overflowed.
    18  // On supported platforms this is an intrinsic lowered by the compiler.
    19  func MulUintptr(a, b uintptr) (uintptr, bool) {
    20  	if a|b < 1<<(4*goarch.PtrSize) || a == 0 {
    21  		return a * b, false
    22  	}
    23  	overflow := b > MaxUintptr/a
    24  	return a * b, overflow
    25  }
    26  
    27  // Mul64 returns the 128-bit product of x and y: (hi, lo) = x * y
    28  // with the product bits' upper half returned in hi and the lower
    29  // half returned in lo.
    30  // This is a copy from math/bits.Mul64
    31  // On supported platforms this is an intrinsic lowered by the compiler.
    32  func Mul64(x, y uint64) (hi, lo uint64) {
    33  	const mask32 = 1<<32 - 1
    34  	x0 := x & mask32
    35  	x1 := x >> 32
    36  	y0 := y & mask32
    37  	y1 := y >> 32
    38  	w0 := x0 * y0
    39  	t := x1*y0 + w0>>32
    40  	w1 := t & mask32
    41  	w2 := t >> 32
    42  	w1 += x0 * y1
    43  	hi = x1*y1 + w2 + w1>>32
    44  	lo = x * y
    45  	return
    46  }
    47  
    48  // Add64 returns the sum with carry of x, y and carry: sum = x + y + carry.
    49  // The carry input must be 0 or 1; otherwise the behavior is undefined.
    50  // The carryOut output is guaranteed to be 0 or 1.
    51  //
    52  // This function's execution time does not depend on the inputs.
    53  // On supported platforms this is an intrinsic lowered by the compiler.
    54  func Add64(x, y, carry uint64) (sum, carryOut uint64) {
    55  	sum = x + y + carry
    56  	// The sum will overflow if both top bits are set (x & y) or if one of them
    57  	// is (x | y), and a carry from the lower place happened. If such a carry
    58  	// happens, the top bit will be 1 + 0 + 1 = 0 (&^ sum).
    59  	carryOut = ((x & y) | ((x | y) &^ sum)) >> 63
    60  	return
    61  }
    62
View as plain text