alternate pns() debug function

[Thomas Stüfe]

Hi Chris, Ioi,

On Tue, Nov 7, 2017 at 8:20 AM, Ioi Lam <ioi.lam at oracle.com> wrote:

>
>
> On 11/6/17 9:05 PM, Chris Plummer wrote:
>
>> Hi,
>>
>> I'd like to add an alternate version of pns() to debug.cpp, and would
>> like some initial comments first before filing a bug and sending out an
>> RFR. pns() is used to print out the native stack:
>>
>> extern "C" void pns(void* sp, void* fp, void* pc) { // print native stack
>>   Command c("pns");
>>   static char buf[O_BUFLEN];
>>   Thread* t = Thread::current_or_null();
>>   // Call generic frame constructor (certain arguments may be ignored)
>>   frame fr(sp, fp, pc);
>>   VMError::print_native_stack(tty, fr, t, buf, sizeof(buf));
>> }
>>
>> And here's the help output for it:
>>
>>   pns(void* sp, void* fp, void* pc)  - print native (i.e. mixed) stack
>> trace. E.g.");
>>                    pns($sp, $rbp, $pc) on Linux/amd64 and Solaris/amd64
>> or");
>>                    pns($sp, $ebp, $pc) on Linux/x86 or");
>>                    pns($sp, 0, $pc)    on Linux/ppc64 or");
>>                    pns($sp + 0x7ff, 0, $pc) on Solaris/SPARC");
>>                  - in gdb do 'set overload-resolution off' before calling
>> pns()");
>>                  - in dbx do 'frame 1' before calling pns()");
>>
>> The intent is to call it from gdb, but it is potentially useful to call
>> it from within hotspot when debugging. I'd like to propose the following
>> alternative, since it does away with needing to pass in register values
>> (not easy to do from C):
>>
>> extern "C" void pns2() { // print native stack
>>   Command c("pns2");
>>   static char buf[O_BUFLEN];
>>   Thread* t = Thread::current_or_null();
>>   frame fr = os::current_frame();
>>   VMError::print_native_stack(tty, fr, t, buf, sizeof(buf));
>> }
>>
>> I actually used this quite a bit with some recent debugging. There were
>> places in hotspot where I wanted to know what the native stack looked like
>> when called, and it was a lot easier to just add a call to pns2() than to
>> setup the test to run in gdb. This seems to work on Linux/x64, macosx/64,
>> and solaris/sparc. For some reason it crashes on Windows/x64, but I don't
>> see any indication from the above help for pns() that it works on
>> Windows/x64 anyway.
>>
>> #
>> # A fatal error has been detected by the Java Runtime Environment:
>> #
>> #  EXCEPTION_ACCESS_VIOLATION (0xc0000005) at pc=0x0000003bcfae1466,
>> pid=9872, tid=12212
>> #
>> # JRE version: Java(TM) SE Runtime Environment (10.0) (fastdebug build
>> 10-internal+0-2017-11-07-0256309.chris.plummer.jdk10-hs)
>> # Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug
>> 10-internal+0-2017-11-07-0256309.chris.plummer.jdk10-hs, mixed mode,
>> tiered, compressed oops, g1 gc, windows-amd64)
>> # Problematic frame:
>> # v  ~StubRoutines::get_previous_fp
>> #
>>
>> I'm starting to suspect that os::current_frame() normally is never even
>> called on Windows (there are calls to it in the VmError code, but I suspect
>> they are never triggered on Windows) and that StubRoutines::get_previous_fp()
>> is not working on Windows, but I could be wrong.
>>
>> Hi Chris,
>
> I like this new pns2() function.
>
> To make this work on Windows, you probably need to follow this pattern in
> vmError.cpp.
>
>
>      if (os::platform_print_native_stack(st, _context, buf, sizeof(buf)))
> {
>        // We have printed the native stack in platform-specific code
>        // Windows/x64 needs special handling.
>      } else {
>        frame fr = _context ? os::fetch_frame_from_context(_context)
>                            : os::current_frame();
>
>        print_native_stack(st, fr, _thread, buf, sizeof(buf));
>      }
>
> Thanks
> - Ioi
>
>
Note that os::platform_print_native_stack() on Windows and AIX only print
native C/C++ frames. So, calling this from the debugger may not be that
useful. On AIX, it probably will not even be possible to invoke a function
in the debugger - never tried, but given that not much else works in that
thing, I am pessimistic.

But pns() may still be useful as a tracing option, though.

On Windows x64, we do not have frame pointers, so we cannot walk the frame
VM-Style, so os::platform_print_native_stack() is the only way to print a
stack. On AIX, both ways work - VMError::print_native_stack() and
os::platform_print_native_stack() - the former gives a mixed stack, the
latter the pure C/C++ stack, but with many more information.

In the end, for consistency, I'd suggest the same as Ioi, to follow the
pattern in VMError(). And we could probably factor that out to an own
function?

Kind Regards, Thomas



>
> Let me know what you think.
>>
>> thanks,
>>
>> Chris
>>
>>
>