[gofrontend-dev] Re: libgo patch committed: Add precise stack scan support

[Ian Lance Taylor]

On Tue, Dec 11, 2018 at 6:52 AM Matthias Klose <doko@ubuntu.com> wrote:
>
> On 10.12.18 16:54, Cherry Zhang wrote:
> > On Mon, Dec 10, 2018 at 1:41 AM Matthias Klose <doko@ubuntu.com> wrote:
> >
> >> 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'
> >>
> >>
> > Hell Matthias,
> >
> > Thank you for the report. And sorry about the breakage. Does
> > https://go-review.googlesource.com/c/gofrontend/+/153417 (or the patch
> > below) fix ARM32 build? I don't have an ARM32 machine at hand to test.
>
> this fixes the build.
>
> currently running the testsuite, almost every test case core dumps on
> arm-linux-gnueabihf

I committed Cherry's patch to trunk, since it looks reasonable to me.
Cherry, Matthias, let me know if you figure out why programs are
failing.

Ian