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math/bits: use common mask constants #73419

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math/bits: use common mask constants #73419

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38 changes: 16 additions & 22 deletions src/math/bits/bits.go
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Original file line number Diff line number Diff line change
Expand Up @@ -113,6 +113,10 @@ const m2 = 0x0f0f0f0f0f0f0f0f // 00001111 ...
const m3 = 0x00ff00ff00ff00ff // etc.
const m4 = 0x0000ffff0000ffff

const two32 = 1 << 32
const mask32 = 1<<32 - 1
const mask64 = 1<<64 - 1

// OnesCount returns the number of one bits ("population count") in x.
func OnesCount(x uint) int {
if UintSize == 32 {
Expand Down Expand Up @@ -157,10 +161,9 @@ func OnesCount64(x uint64) int {
// Per "Hacker's Delight", the first line can be simplified
// more, but it saves at best one instruction, so we leave
// it alone for clarity.
const m = 1<<64 - 1
x = x>>1&(m0&m) + x&(m0&m)
x = x>>2&(m1&m) + x&(m1&m)
x = (x>>4 + x) & (m2 & m)
x = x>>1&(m0&mask64) + x&(m0&mask64)
x = x>>2&(m1&mask64) + x&(m1&mask64)
x = (x>>4 + x) & (m2 & mask64)
x += x >> 8
x += x >> 16
x += x >> 32
Expand Down Expand Up @@ -242,19 +245,17 @@ func Reverse16(x uint16) uint16 {

// Reverse32 returns the value of x with its bits in reversed order.
func Reverse32(x uint32) uint32 {
const m = 1<<32 - 1
x = x>>1&(m0&m) | x&(m0&m)<<1
x = x>>2&(m1&m) | x&(m1&m)<<2
x = x>>4&(m2&m) | x&(m2&m)<<4
x = x>>1&(m0&mask32) | x&(m0&mask32)<<1
x = x>>2&(m1&mask32) | x&(m1&mask32)<<2
x = x>>4&(m2&mask32) | x&(m2&mask32)<<4
return ReverseBytes32(x)
}

// Reverse64 returns the value of x with its bits in reversed order.
func Reverse64(x uint64) uint64 {
const m = 1<<64 - 1
x = x>>1&(m0&m) | x&(m0&m)<<1
x = x>>2&(m1&m) | x&(m1&m)<<2
x = x>>4&(m2&m) | x&(m2&m)<<4
x = x>>1&(m0&mask64) | x&(m0&mask64)<<1
x = x>>2&(m1&mask64) | x&(m1&mask64)<<2
x = x>>4&(m2&mask64) | x&(m2&mask64)<<4
return ReverseBytes64(x)
}

Expand All @@ -281,18 +282,16 @@ func ReverseBytes16(x uint16) uint16 {
//
// This function's execution time does not depend on the inputs.
func ReverseBytes32(x uint32) uint32 {
const m = 1<<32 - 1
x = x>>8&(m3&m) | x&(m3&m)<<8
x = x>>8&(m3&mask32) | x&(m3&mask32)<<8
return x>>16 | x<<16
}

// ReverseBytes64 returns the value of x with its bytes in reversed order.
//
// This function's execution time does not depend on the inputs.
func ReverseBytes64(x uint64) uint64 {
const m = 1<<64 - 1
x = x>>8&(m3&m) | x&(m3&m)<<8
x = x>>16&(m4&m) | x&(m4&m)<<16
x = x>>8&(m3&mask64) | x&(m3&mask64)<<8
x = x>>16&(m4&mask64) | x&(m4&mask64)<<16
return x>>32 | x<<32
}

Expand Down Expand Up @@ -468,7 +467,6 @@ func Mul32(x, y uint32) (hi, lo uint32) {
//
// This function's execution time does not depend on the inputs.
func Mul64(x, y uint64) (hi, lo uint64) {
const mask32 = 1<<32 - 1
x0 := x & mask32
x1 := x >> 32
y0 := y & mask32
Expand Down Expand Up @@ -531,10 +529,6 @@ func Div64(hi, lo, y uint64) (quo, rem uint64) {
s := uint(LeadingZeros64(y))
y <<= s

const (
two32 = 1 << 32
mask32 = two32 - 1
)
yn1 := y >> 32
yn0 := y & mask32
un32 := hi<<s | lo>>(64-s)
Expand Down