runtime: grow stack by copying
On stack overflow, if all frames on the stack are
copyable, we copy the frames to a new stack twice
as large as the old one. During GC, if a G is using
less than 1/4 of its stack, copy the stack to a stack
half its size.
TODO
- Do something about C frames. When a C frame is in the
stack segment, it isn't copyable. We allocate a new segment
in this case.
- For idempotent C code, we can abort it, copy the stack,
then retry. I'm working on a separate CL for this.
- For other C code, we can raise the stackguard
to the lowest Go frame so the next call that Go frame
makes triggers a copy, which will then succeed.
- Pick a starting stack size?
The plan is that eventually we reach a point where the
stack contains only copyable frames.
LGTM=rsc
R=dvyukov, rsc
CC=golang-codereviews
https://golang.org/cl/
54650044