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.
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 <javascript:>>
> 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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