Re: libgo patch committed: Add precise stack scan support

[Matthias Klose <doko at ubuntu dot com>]

On 06.12.18 00:09, Ian Lance Taylor 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 the libgo build on ARM32:

../../../src/libgo/runtime/go-unwind.c: In function 'scanstackwithmap_callback':
../../../src/libgo/runtime/go-unwind.c:754:18: error: '_URC_NORMAL_STOP'
undeclared (first use in this function)
  754 |           return _URC_NORMAL_STOP;
      |                  ^~~~~~~~~~~~~~~~
../../../src/libgo/runtime/go-unwind.c:754:18: note: each undeclared identifier
is reported only once for each function i
t appears in
../../../src/libgo/runtime/go-unwind.c: In function 'probestackmaps_callback':
../../../src/libgo/runtime/go-unwind.c:802:10: error: '_URC_NORMAL_STOP'
undeclared (first use in this function)
  802 |   return _URC_NORMAL_STOP;
      |          ^~~~~~~~~~~~~~~~
../../../src/libgo/runtime/go-unwind.c:803:1: warning: control reaches end of
non-void function [-Wreturn-type]
  803 | }
      | ^
make[6]: *** [Makefile:1474: runtime/go-unwind.lo] Error 1
make[6]: Leaving directory '/<<PKGBUILDDIR>>/build/arm-linux-gnueabihf/libgo'