What is 4*PtrSize extra space on goroutine stack for?

[peng.g...@gmail.com]

In the code https://github.com/golang/go/blob/master/src/runtime/proc.go#L4317, there is a 4*PtrSize extra space. What is the purpose?

[Keith Randall]

I'm not sure it is necessary. The intent is to make sure we don't access off the top of the stack. sys.MinFrameSize should really handle that, I think. My guess is that the 4*sys.PtrSize was from before we had the MinFrameSize concept defined.