cmd/compile: -l=4 triggers "write barrier prohibited by caller; oneNewExtraM"

[mdempsky]

$ go build -a -gcflags=-l=4 runtime
# runtime
../src/runtime/proc.go:1655:10: write barrier prohibited by caller; oneNewExtraM
        /home/mdempsky/wd/go/src/runtime/proc.go:1618:16: called by newextram
        /home/mdempsky/wd/go/src/runtime/proc.go:1200:11: called by mstart1
        /home/mdempsky/wd/go/src/runtime/proc.go:1170:9: called by mstart

It's complaining about the mp.curg = gp asignment in oneNewExtraM. This is a pointer assignment, and type runtime.g is not labeled go:notinheap.

mstart is labeled go:nowritebarrierrec, and none of mstart1, newextram, or oneNewExtraM are labeled go:yeswritebarrierrec.

I suspect there's also a compiler bug here that this isn't being detected at -l=0, but filing as a runtime bug first for investigation.

/cc @aclements

[mdempsky]

Oh, nevermind, there's an mstartm0 call in between, which is labeled yeswritebarrierrec.

This is a compiler bug.

[aclements]

I attempted, and failed, to fix the general issue with losing function annotations during inlining in CL 41152 by disallowing inlining that would break function-level analysis. Besides not working, this was sort of an unfortunate approach. It would be much nicer if we could push the function annotations into inlining points and still analyze them correctly.

[gopherbot]

Change https://golang.org/cl/72132 mentions this issue: cmd/compile: prevent inlining go:yeswritebarrierrec functions