Add Frame.read_register to Python API

  I've fixed the problems you pointed out, please take another look.
Sasha


The ability to read registers is needed to use Frame Filter API to
display the frames created by JIT compilers.

gdb/Changelog
2014-08-26  Sasha Smundak  <asmundak@google.com>

* python/py-frame.c (frapy_read_register): New function.

2014-08-26  Sasha Smundak  <asmundak@google.com>

* python.texi (Frames in Python): Add read_register description.

2014-08-26  Sasha Smundak  <asmundak@google.com>

* gdb.python/py-frame.exp: Test Frame.read_register.
  

Alexander Smundak writes:
 > I've fixed the problems you pointed out, please take another look.
 > Sasha
 > 
 > 
 > The ability to read registers is needed to use Frame Filter API to
 > display the frames created by JIT compilers.
 > 
 > gdb/Changelog
 > 2014-08-26  Sasha Smundak  <asmundak@google.com>
 > 
 > * python/py-frame.c (frapy_read_register): New function.
 > 
 > 2014-08-26  Sasha Smundak  <asmundak@google.com>
 > 
 > * python.texi (Frames in Python): Add read_register description.
 > 
 > 2014-08-26  Sasha Smundak  <asmundak@google.com>
 >
 > * gdb.python/py-frame.exp: Test Frame.read_register.

Hi.

Nit: See
https://sourceware.org/gdb/wiki/ContributionChecklist#Properly_formatted_commit_messages
for new guidelines on how commit messages should be formatted.
The Changelog part of it is a little different than what's
written above.
[I don't want to be excessively picky about such things
during review. Just want to make sure that when this gets checked
in the format is right: the above is missing directory names for
each section.]

 > diff --git a/gdb/NEWS b/gdb/NEWS
 > index d603cf7..46c6a87 100644
 > --- a/gdb/NEWS
 > +++ b/gdb/NEWS
 > @@ -3,6 +3,9 @@
 >  
 >  *** Changes since GDB 7.8
 >  
 > +* Python Scripting
 > +  You can now access frame registers from Python scripts.
 > +
 >  * On resume, GDB now always passes the signal the program had stopped
 >    for to the thread the signal was sent to, even if the user changed
 >    threads before resuming.  Previously GDB would often (but not
 > diff --git a/gdb/doc/python.texi b/gdb/doc/python.texi
 > index 4688783..3cb6bf8 100644
 > --- a/gdb/doc/python.texi
 > +++ b/gdb/doc/python.texi
 > @@ -3589,6 +3589,13 @@ Return the frame's symtab and line object.
 >  @xref{Symbol Tables In Python}.
 >  @end defun
 >  
 > +@defun Frame.read_register (register)
 > +Return the value of @var{register} in this frame.  The @var{register}
 > +argument must be a string (e.g., @code{'sp'} or @code{'rax'}).
 > +Returns a @code{Gdb.Value} object.  Throws an exception if @var{register}
 > +does not exist.
 > +@end defun
 > +
 >  @defun Frame.read_var (variable @r{[}, block@r{]})
 >  Return the value of @var{variable} in this frame.  If the optional
 >  argument @var{block} is provided, search for the variable from that
 > diff --git a/gdb/python/py-frame.c b/gdb/python/py-frame.c
 > index 120e147..88e9114 100644
 > --- a/gdb/python/py-frame.c
 > +++ b/gdb/python/py-frame.c
 > @@ -28,6 +28,7 @@
 >  #include "python-internal.h"
 >  #include "symfile.h"
 >  #include "objfiles.h"
 > +#include "user-regs.h"
 >  
 >  typedef struct {
 >    PyObject_HEAD
 > @@ -235,6 +236,40 @@ frapy_pc (PyObject *self, PyObject *args)
 >    return gdb_py_long_from_ulongest (pc);
 >  }
 >  
 > +/* Implementation of gdb.Frame.read_register (self, register) -> gdb.Value.
 > +   Returns the value of a register in this frame.  */
 > +
 > +static PyObject *
 > +frapy_read_register (PyObject *self, PyObject *args)
 > +{
 > +  volatile struct gdb_exception except;
 > +  const char *regnum_str;
 > +  struct value *val = NULL;
 > +
 > +  if (!PyArg_ParseTuple (args, "s", &regnum_str))
 > +    return NULL;
 > +
 > +  TRY_CATCH (except, RETURN_MASK_ALL)
 > +    {
 > +      struct frame_info *frame;
 > +      int regnum = -1;

Nit: I suspect the "= -1" is no longer needed.

 > +
 > +      FRAPY_REQUIRE_VALID (self, frame);
 > +
 > +      regnum = user_reg_map_name_to_regnum (get_frame_arch (frame),
 > +                                            regnum_str,
 > +                                            strlen (regnum_str));
 > +      if (regnum >= 0)
 > +        val = value_of_register (regnum, frame);
 > +
 > +      if (val == NULL)
 > +        PyErr_SetString (PyExc_ValueError, _("Unknown register."));
 > +    }
 > +  GDB_PY_HANDLE_EXCEPTION (except);
 > +
 > +  return val == NULL ? NULL : value_to_value_object (val);
 > +}
 > +
 >  /* Implementation of gdb.Frame.block (self) -> gdb.Block.
 >     Returns the frame's code block.  */
 >  
 > @@ -674,6 +709,9 @@ Return the reason why it's not possible to find frames older than this." },
 >    { "pc", frapy_pc, METH_NOARGS,
 >      "pc () -> Long.\n\
 >  Return the frame's resume address." },
 > +  { "read_register", frapy_read_register, METH_VARARGS,
 > +    "read_register (register_name) -> gdb.Value\n\
 > +Return the value of the register in the frame." },
 >    { "block", frapy_block, METH_NOARGS,
 >      "block () -> gdb.Block.\n\
 >  Return the frame's code block." },
 > diff --git a/gdb/testsuite/gdb.python/py-frame.exp b/gdb/testsuite/gdb.python/py-frame.exp
 > index 3517824..e47f340 100644
 > --- a/gdb/testsuite/gdb.python/py-frame.exp
 > +++ b/gdb/testsuite/gdb.python/py-frame.exp
 > @@ -94,3 +94,20 @@ gdb_test "python print ('result = %s' % f0.read_var ('variable_which_surely_does
 >  gdb_test "python print ('result = %s' % f0.read_var ('a'))" " = 1" "test Frame.read_var - success"
 >  
 >  gdb_test "python print ('result = %s' % (gdb.selected_frame () == f1))" " = True" "test gdb.selected_frame"
 > +
 > +# Can read SP register.
 > +gdb_test "python print ('result = %s' % (gdb.selected_frame ().read_register ('sp') == gdb.parse_and_eval ('\$sp')))" \
 > +  " = True" \
 > +  "test Frame.read_register(sp)"
 > +
 > +# PC value obtained via read_register is as expected.
 > +gdb_test "python print ('result = %s' % (f0.read_register('pc') == f0.pc()))" \
 > +  " = True" \
 > +  "test Frame.read_register(pc)"
 > +
 > +# On x86-64, PC is in $rip register.
 > +if {[istarget x86_64-*]} {
 > +    gdb_test "python print ('result = %s' % (f0.read_register('pc') == f0.read_register('rip')))" \
 > +	" = True" \
 > +	"test Frame.read_register(rip)"
 > +}

LGTM with the above nits addressed.