On Sat, Jan 26, 2019 at 5:37 AM <mountain...@gmail.com> wrote:
>
> Why the garbage collector won't know how to find the pointers?
> I looked at mallocgc and decided if the GC needs to scan this object based
> on the noscan flag.
What version of Go are you looking at? There was a flagNoScan as late
as Go 1.6, but there is no such flag in current versions of Go.
Ian
> 在 2019年1月25日星期五 UTC+8下午10:58:44，Ian Lance Taylor写道：
>>
>> On Thu, Jan 24, 2019 at 11:58 PM <mount...@gmail.com> wrote:
>> >
>> > go 1.11.1 source code is below:
>> >
>> > Generally speaking, make chan just pay attention to the presence or
>> > absence of buf.
>> >
>> > When I saw the source code of make chan, I can understand case 1: chan
>> > buf is 0, but can't understand case 2 & default.
>> >
>> > Who knows this principle?
>> >
>> > Thanks!
>> >
>> > var c *hchan
>> > switch {
>> > case size == 0 || elem.size == 0:
>> > // Queue or element size is zero.
>> > c = (*hchan)(mallocgc(hchanSize, nil, true))
>> > // Race detector uses this location for synchronization.
>> > c.buf = c.raceaddr()
>> > case elem.kind&kindNoPointers != 0:
>> > // Elements do not contain pointers.
>> > // Allocate hchan and buf in one call.
>> > c = (*hchan)(mallocgc(hchanSize+uintptr(size)*elem.size, nil,
>> > true))
>> > c.buf = add(unsafe.Pointer(c), hchanSize)
>> > default:
>> > // Elements contain pointers.
>> > c = new(hchan)
>> > c.buf = mallocgc(uintptr(size)*elem.size, elem, true)
>> > }
>>
>>
>> First, let me say: please don't post screen shots. They are much
>> harder to read than ordinary text. I don't understand why anybody
>> ever posts screenshots of code, and I would be grateful for an
>> explanation. Thanks.
>>
>> What is happening in that code is that if the channel has a non-zero
>> buffer size, we need to allocate space to hold the elements in the
>> buffer. If the elements in the buffer do not contain any pointers, we
>> can optimize by allocating the channel structure (hchan) and the
>> buffer in a single memory allocation. If the elements do contain
>> pointers, then that won't work, because the garbage collector won't
>> know how to find the pointers. So in that case we allocate the hchan
>> struct and the buffer separately. Note the second argument to
>> mallocgc, which is the type of the memory being allocated. When there
>> are no pointers, we pass nil, which tells the garbage collector that
>> the allocation contains no pointers. In the pointer case, we pass the
>> element type.
>>
>> Ian
>
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