Re: libgo patch committed: Add precise stack scan support

[Ian Lance Taylor <iant@golang.org>]

On Fri, Dec 7, 2018 at 2:07 AM Rainer Orth <ro@cebitec.uni-bielefeld.de> wrote:
>
> > This libgo patch by Cherry Zhang adds support for precise stack
> > scanning to the Go runtime.  This uses per-function stack maps stored
> > in the exception tables in the language-specific data area.  The
> > compiler needs to generate these stack maps; currently this is only
> > done by a version of LLVM, not by GCC.  Each safepoint in a function
> > is associated with a (real or dummy) landing pad, and its "type info"
> > in the exception table is a pointer to the stack map. When a stack is
> > scanned, the stack map is found by the stack unwinding code.
> >
> > For precise stack scan we need to unwind the stack. There are three cases:
> >
> > - If a goroutine is scanning its own stack, it can unwind the stack
> > and scan the frames.
> >
> > - If a goroutine is scanning another, stopped, goroutine, it cannot
> > directly unwind the target stack. We handle this by switching
> > (runtime.gogo) to the target g, letting it unwind and scan the stack,
> > and switch back.
> >
> > - If we are scanning a goroutine that is blocked in a syscall, we send
> > a signal to the target goroutine's thread, and let the signal handler
> > unwind and scan the stack. Extra care is needed as this races with
> > enter/exit syscall.
> >
> > Currently this is only implemented on GNU/Linux.
> >
> > Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
> > to mainline.
>
> this broke Solaris (and other non-Linux) bootstrap:
>
> /vol/gcc/src/hg/trunk/local/libgo/go/runtime/stubs_nonlinux.go:20:1: error: missing return at end of function
>    20 | }
>       | ^
>
> Fixed by returning 0 for now, the return value is ignored in
> go/runtime/proc.go (scang) anyway.

Thanks.  Committed to mainline.

Ian