libgo patch committed: Add precise stack scan support

Rainer Orth ro@CeBiTec.Uni-Bielefeld.DE
Fri Dec 7 10:07:00 GMT 2018

Hi Ian,

> 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.

The Solaris equivalents would be thr_self (for gettid) and thr_kill (for
tgkill).  It were probably better to use non-Linux specific names


Rainer Orth, Center for Biotechnology, Bielefeld University

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