runtime: remove implementation restriction on channel element size

[bradfitz]

It appears that gc and the runtime currently disable channel elements to be over 64kB:

    http://play.golang.org/p/_aVWt-qiud

But I see nothing about this (allowed?) implementation restriction in the spec, despite
the spec calling out a number of other implementation-allowed restrictions.

[minux]

Comment 1:

ideally the spec should also set a minimum requirement on the item size limit.

[griesemer]

Comment 2:

What's the implementation issue requiring a limit in the first place (besides memory
availability)? (Array don't have a limit, for instance).

[rsc]

Comment 3:

Ken put the limit there. I have always assumed it was something like the "stupid shift"
errors, where if you have that big an element size you are probably doing something
wrong. The fact that it took this long for someone to report a problem suggests he was
right. (And even Brad doesn't seem like he's actually run into this.)
I can take it out for 1.5.

[bradfitz]

Comment 4:

Correct. I was just checking something in the runtime and noticed in chan.go:
func makechan(t *chantype, size int64) *hchan {
        elem := t.elem
        // compiler checks this but be safe.
        if elem.size >= 1<<16 {
                gothrow("makechan: invalid channel element type")
        }
 
And it just surprised me to see it in either place, much less both places.

[robpike]

Comment 5:

Let's just remove the restriction from the compiler and replace it with a
overall buffer size check.
-rob

[griesemer]

Comment 6:

Labels changed: removed documentation.

Owner changed to ---.