runtime: cgo check of []byte checks every element uselessly

[ianlancetaylor]

This program

package main

// void f(void* p) {}
import "C"

import (
    "fmt"
    "time"
    "unsafe"
)

func main() {
    b := make([]byte, 1e6)
    t := time.Now()
    for i := 0; i < 1e3; i++ {
        C.f(unsafe.Pointer(&b[0]))
    }
    fmt.Println(time.Since(t))
}

takes 4 seconds on my laptop. Setting GODEBUG=cgocheck=0 it takes 430 microseconds. This is a []byte that should not take any time to check.

I have a simple fix. Opening this issue so that the fix goes into a future 1.6.1 release, if there is one.

[gopherbot]

CL https://golang.org/cl/19621 mentions this issue.

[gopherbot]

CL https://golang.org/cl/22036 mentions this issue.