On Thu, Mar 18, 2021 at 9:55 AM Paul Zhang <tianxin8...@gmail.com> wrote:
>
> I was reading the source code of makechan(), and these questions confused me.
> I would appreciate it if someone could help me.
>
> Q1. What does Elements do not contain pointers. mean? Does that means that
> the type of channel is not a pointer type (like chan int and chan *int)?
It means that the element type of the channel is not a pointer and
also does not contain any pointers. For example "struct { a, b int }"
does not contain any pointers, but "struct { a int; b []byte }" does
contain pointers.
> Q2. Why does the allocation strategy of memory allocation differ? It seems
> like the buf and the hchan should be allocated into one piece of continuous
> memory, if the "elements contains pointers", so what's the point? The
> logically continuous memory might not physically continuous.
The garbage collector has to always know exactly where pointers are
stored in memory. We could in principle use contiguous allocation for
the case where the element type contains pointers, but we would have
to build a description that tells the garbage collector exactly where
those pointers are. Those descriptions are built by the compiler (on
tip, the code is in cmd/compile/internal/reflectdata/reflect.go), so
the compiler would have to build a new descriptor for every channel
type with an element type that contains pointers. And the descriptor
would have to vary based on the channel size, so it would be based not
just on the channel type but also on the argument passed to "make".
Of course the argument passed to "make" can be a variable, so that
adds another complication.
So it's probably possible to use a contiguous buffer here, but it's
not simple, and it's not the common case, and it's not clear that it
would be worth it.
Ian
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