compiler: move allocations > 256 bytes to the heap

[dgryski]

No description provided.

[aykevl]

What's the rationale for this change?
(I'm not against it, but didn't expect this to be an issue anywhere).

[dgryski]

In the snappy encoder on wasi, it tries to allocate a 32kb object on a 16k stack and the subsequent memset() call wipes out some globals, including metadataStart, corrupting the garbage collector: https://github.com/golang/snappy/blob/master/encode_other.go#L130

[aykevl]

I see. I just read the #tinygo-dev discussion :)

It does seem reasonable to me to do this: a 32kB stack object is just way too large. 256 bytes is maybe a little bit on the low side? I think it depends on the target and the amount of stack/heap allocated for it (256 bytes is a lot on AVR, but 1kB would be perfectly fine on x86).

[aykevl]

Let's just merge this as it does fix a bug.

[deadprogram]

Thank you @dgryski now merging.