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

[Cherry Zhang via gcc-patches]

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.

Thanks,
Cherry

diff --git a/libgo/runtime/go-unwind.c b/libgo/runtime/go-unwind.c
index a1a95585..c44755f9 100644
--- a/libgo/runtime/go-unwind.c
+++ b/libgo/runtime/go-unwind.c
@@ -392,6 +392,12 @@ parse_lsda_header (struct _Unwind_Context *context,
const unsigned char *p,
 #define CONTINUE_UNWINDING return _URC_CONTINUE_UNWIND
 #endif

+#ifdef __ARM_EABI_UNWINDER__
+#define STOP_UNWINDING _URC_FAILURE
+#else
+#define STOP_UNWINDING _URC_NORMAL_STOP
+#endif
+
 #ifdef __USING_SJLJ_EXCEPTIONS__
 #define PERSONALITY_FUNCTION    __gccgo_personality_sj0
 #define __builtin_eh_return_data_regno(x) x
@@ -751,7 +757,7 @@ scanstackwithmap_callback (struct _Unwind_Context
*context, void *arg)
               // TODO: print gp, pc, sp
               runtime_throw ("no stack map");
             }
-          return _URC_NORMAL_STOP;
+          return STOP_UNWINDING;
         }
       case FOUND:
         break;
@@ -799,7 +805,7 @@ probestackmaps_callback (struct _Unwind_Context
*context,

   // Found a stack map. No need to keep unwinding.
   runtime_usestackmaps = true;
-  return _URC_NORMAL_STOP;
+  return STOP_UNWINDING;
 }

 // Try to find a stack map, store the result in global variable
runtime_usestackmaps.