当使用 int64 而不是 int32 时，循环性能要慢得多

Question

我正在尝试找出有关性能的最佳实践。我注意到为 for 循环指定整数类型可能会极大地影响性能（在我的例子中是 x2 倍）。

我的问题是，使用 int64 是否应该比使用 int32 慢得多，或者我的代码中遗漏了什么？

我使用的代码：

a.go

package main

import (
    "fmt"
    "time"
    "runtime"
    "strconv"
)

func main() {
    start := time.Now()
    var x1 int // later change all int to int32 or int64 

    for i := int(0); i <= int(1000000000); i++  {
        x1 = x1 + i
    }
        t := time.Now()
        elapsed := t.Sub(start)

    fmt.Println(x1)
    fmt.Println(elapsed)
    fmt.Println(runtime.Compiler, runtime.GOARCH, runtime.GOOS)
    fmt.Println(strconv.IntSize)
}

x1 使用 int32 输出

C:\...>go build a.go
C:\...>a

-243309312
238.3333ms
gc amd64 windows
64

使用 int64 输出 x1

C:\...>go build a.go
C:\...>a

500000000500000000
467.7835ms
gc amd64 windows
64

更新

我尝试了@Giulio Micheloni 的建议并获得了更准确的基准。

goos: windows
goarch: amd64
BenchmarkInt64-12       1000000000           0.234 ns/op           0 B/op          0 allocs/op
PASS
ok      _/c_/.../.../Desktop    0.402s
Success: Benchmarks passed.

goos: windows
goarch: amd64
BenchmarkInt32-12       1000000000           0.231 ns/op           0 B/op          0 allocs/op
PASS
ok      _/c_/.../.../Desktop    0.403s
Success: Benchmarks passed.

Answer 1

无意义的微基准测试产生无意义的结果。

The Go Programming Language Specification

Numeric types
int32   set of all signed 32-bit integers 
        (-2147483648 to 2147483647)
int64   set of all signed 64-bit integers 
        (-9223372036854775808 to 9223372036854775807)
Arithmetic operators

Integer overflow

For signed integers, the operations +, -, *, /, and << may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. Overflow does not cause a run-time panic.

溢出！

package main

import (
    "fmt"
    "math"
    "runtime"
    "strconv"
    "time"
)

func main() {
    start := time.Now()
    var x1 int32 // later change all int to int32 or int64

    for i := int32(0); i <= int32(1000000000); i++ {

        if int64(x1)+int64(i) > math.MaxInt32 {
            fmt.Println("Overflow:", x1, "+", i, "=", x1+i)
            break
        }

        x1 = x1 + i
    }
    t := time.Now()
    elapsed := t.Sub(start)

    fmt.Println(x1)
    fmt.Println(elapsed)
    fmt.Println(runtime.Compiler, runtime.GOARCH, runtime.GOOS)
    fmt.Println(strconv.IntSize)
}

游乐场：https://play.golang.org/p/bdhB4ABf7jY

输出：

Overflow: 2147450880 + 65536 = -2147450880
gc amd64 linux
64

当使用 int64 而不是 int32 时，循环性能要慢得多

For loop performance much slower when using int64 instead of int32

int

performance

for-loop

go