On Friday, April 21, 2017 at 1:33:57 AM UTC+8, Keith Randall wrote:
>
>
>
> On Wednesday, April 19, 2017 at 3:56:21 AM UTC-7, T L wrote:
>>
>>
>>
>> On Wednesday, April 19, 2017 at 3:37:29 AM UTC+8, Keith Randall wrote:
>>>
>>> This is a weird corner case in string concatenation optimization.
>>>
>>> runtime.concatstrings (what the + in this code gets rewritten to) has an
>>> optimization where if all the strings but one that it is concatenating are
>>> 0 length, then it returns the remaining one without doing any allocation.
>>> Because of that optimization, runtime.concatstrings might return its
>>> argument. Thus, we can't do the zero-copy conversion of []byte to string
>>> for arguments to runtime.concatstrings.
>>> Except when we know that at least one of the input strings has non-zero
>>> length. Then we can. That's what is happening in f2.
>>>
>>
>> Keith, thanks. But I haven't fully get your explanation.
>> Do you mean that compiler can't confirm whether or not the optimization
>> which might return its argument can be used for f1 at compile time,
>> but compiler can confirm it can't be used for f2 at compile time?
>>
>> That's correct. Or more precisely, if that optimization is used in f2,
> it can only be used for the string " ", it can't be used for any of the
> string(a) arguments.
>
ok, thanks.
but I feel the optimization used f2 is more general.
>
>>
>>>
>>> On Monday, April 17, 2017 at 11:06:26 PM UTC-7, T L wrote:
>>>>
>>>>
>>>> package main
>>>>
>>>> import "fmt"
>>>> import "testing"
>>>>
>>>> var s string
>>>> var a = make([]byte, 1000)
>>>>
>>>> func f0() {
>>>> x := string(a)
>>>> s = x + x + x +
>>>> x + x + x +
>>>> x + x + x +
>>>> x
>>>> }
>>>>
>>>> func f1() {
>>>> s = string(a) + string(a) + string(a) +
>>>> string(a) + string(a) + string(a) +
>>>> string(a) + string(a) + string(a) +
>>>> string(a)
>>>> }
>>>>
>>>> func f2() {
>>>> s = (" " +
>>>> string(a) + string(a) + string(a) +
>>>> string(a) + string(a) + string(a) +
>>>> string(a) + string(a) + string(a) +
>>>> string(a) )[1:]
>>>> }
>>>>
>>>> func main() {
>>>> fmt.Println(testing.AllocsPerRun(1, f0)) // 2
>>>> fmt.Println(testing.AllocsPerRun(1, f1)) // 11
>>>> fmt.Println(testing.AllocsPerRun(1, f2)) // 1
>>>> }
>>>>
>>>> why doesn't gc make optimization for f1?
>>>>
>>>
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