cgo: getting unsafe.Pointer of a slice slower in go 1.6 vs 1.5.3

[piotrnar]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version go1.6 windows/amd64
2. What operating system and processor architecture are you using (go env)?
   I tried on Windows 7 and all kinds of Linux OS.
   I only tried on 64-bit hosts.
3. What did you do?
   Just run this program with go 1.6 and see how slow it is, comparing to running it with 1.5.3

package main

/*
void alloc_ptr(void *c) {
    return;
}

*/
import "C"

import (
    "unsafe"
    "time"
)

type data_ptr_t unsafe.Pointer

func gcc_AllocPtr(v []byte) {
    C.alloc_ptr(unsafe.Pointer(&v[0]))
}

func main() {
    source_buf := make([]byte, 1024)
    start := time.Now()
    for i:=0; i<1e6; i++ {
        gcc_AllocPtr(source_buf)
    }
    println("done in", time.Now().Sub(start).String())
}

1. What did you expect to see?

done in 228.0131ms

1. What did you see instead?

done in 6.0343452s

[piotrnar]

I also want to add that there is an easy workaround:

package main

/*
void alloc_ptr(void *c) {
    return;
}

*/
import "C"

import (
    "unsafe"
    "time"
)

type data_ptr_t unsafe.Pointer

func gcc_AllocPtr(v []byte) {
    ptr := unsafe.Pointer(&v[0])
    C.alloc_ptr(ptr)
}

func main() {
    source_buf := make([]byte, 1024)
    start := time.Now()
    for i:=0; i<1e6; i++ {
        gcc_AllocPtr(source_buf)
    }
    println("done in", time.Now().Sub(start).String())
}

[bradfitz]

/cc @ianlancetaylor

[ianlancetaylor]

Dup of #14387 .