Patchwork v3 of GDB hooks for debugging GCC

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Submitter David Malcolm
Date Aug. 26, 2013, 6:42 p.m.
Message ID <1377542578.15461.51.camel@surprise>
Download mbox | patch
Permalink /patch/269947/
State New
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Comments

David Malcolm - Aug. 26, 2013, 6:42 p.m.
On Wed, 2013-08-21 at 15:01 -0600, Tom Tromey wrote:
> >>>>> "David" == David Malcolm <dmalcolm@redhat.com> writes:
> 
> Tom> Naughty.
> 
> David> We chatted about this at Cauldron; I haven't yet had a chance to
> David> implement the magic bullet approach we discussed there.  In the
> David> meantime, is there a API I can call to determine how safe this kludge
> David> is?
> 
> Not right now.  You can just call the function and catch the exception
> that occurs if it can't be done.
> 
> I think you can still run into trouble sometimes.  For example if the
> user puts a breakpoint in one of the functions used by the
> pretty-printer, and then does "bt", hitting the breakpoint while
> printing the backtrace... not sure what happens then, maybe a crash.
> 
> Tom> I think you could set up the safe-path in the gcc .gdbinit.
> 
> David> Interesting idea - but .gdbinit itself seems to get declined, so I don't
> David> think this can help.
> 
> Haha, I didn't think of that :-)

But you were on the right track... if one marks gcc's .gdbinit as
loadable, then gdb can happily *import* (rather than autoload) the
python hooks without needing the user to grant extra permission.

I'm attaching a revised patch that reworks things to use this scheme, by
adding a python import line into the configure[.ac] hook that generates
builddir/gcc/.gdbinit : all that a gcc hacker has to do to use the
python hooks now is to ensure that their ~/.gdbinit script contains a
line like this:

   add-auto-load-safe-path /absolute/path/to/build/gcc

and it all should just work.  You need this already to get the
pre-existing gdbinit hooks to work with a recent gdb that has the
autoload protection.  [I renamed the file from gdb-hooks.py to
gdbhooks.py so that it's importable as a python module.  Doing it from
the srcdir avoids having to copy the file to the builddir].

So in this scheme, the Python hooks piggyback on top of the older gdb
hooks.  Hope that's OK.  As noted earlier in the thread, the Python
hooks can be disabled by running:

 (gdb) disable pretty-printer .* gcc
 7 printers disabled

The patch also adds me a maintainer of gdbhooks.py into the MAINTAINERS
file.  (There doesn't seem to be any sort order to the maintainer part
of that file, should there be?)

Finally, I added a copyright header to the new file ("part of GCC", FSF
assignee, GPLv3 or later).

OK for trunk?

Dave
Jeff Law - Aug. 27, 2013, 3:58 p.m.
On 08/26/2013 12:42 PM, David Malcolm wrote:
> The patch also adds me a maintainer of gdbhooks.py into the MAINTAINERS
> file.  (There doesn't seem to be any sort order to the maintainer part
> of that file, should there be?)
>
> Finally, I added a copyright header to the new file ("part of GCC", FSF
> assignee, GPLv3 or later).
>
> OK for trunk?
Yes.  This is good.

Thanks,

jeff
David Malcolm - Aug. 28, 2013, 3:02 a.m.
On Tue, 2013-08-27 at 09:58 -0600, Jeff Law wrote:
> On 08/26/2013 12:42 PM, David Malcolm wrote:
> > The patch also adds me a maintainer of gdbhooks.py into the MAINTAINERS
> > file.  (There doesn't seem to be any sort order to the maintainer part
> > of that file, should there be?)
> >
> > Finally, I added a copyright header to the new file ("part of GCC", FSF
> > assignee, GPLv3 or later).
> >
> > OK for trunk?
> Yes.  This is good.

Thanks; committed to trunk as r202040.

Patch

commit 9ef4a9c7474b56f19bfa49905944931e52e95514
Author: David Malcolm <dmalcolm@redhat.com>
Date:   Wed Aug 21 15:45:55 2013 -0400

    initial version of gdb hooks
    
    	* MAINTAINERS (gdbhooks.py): Add myself as maintainer
    
    gcc/
    	* gdbhooks.py: New.
    	* configure.ac (gdbinit.in): Add import of gcc/gdbhooks.py.
    	* configure: Regenerate.

diff --git a/MAINTAINERS b/MAINTAINERS
index 78b288f..50ede75 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -256,6 +256,7 @@  testsuite		Rainer Orth		ro@CeBiTec.Uni-Bielefeld.DE
 testsuite		Mike Stump		mikestump@comcast.net
 testsuite		Janis Johnson		janisjo@codesourcery.com
 register allocation	Vladimir Makarov	vmakarov@redhat.com
+gdbhooks.py		David Malcolm		dmalcolm@redhat.com
 
 Note that individuals who maintain parts of the compiler need approval to
 check in changes outside of the parts of the compiler they maintain.
diff --git a/gcc/configure b/gcc/configure
index ec662f5..c6bc3a6 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -27397,6 +27397,7 @@  if test "x$subdirs" != x; then
 	done
 fi
 echo "source ${srcdir}/gdbinit.in" >> .gdbinit
+echo "python import sys; sys.path.append('${srcdir}'); import gdbhooks" >> .gdbinit
 
 gcc_tooldir='$(libsubdir)/$(libsubdir_to_prefix)$(target_noncanonical)'
 
diff --git a/gcc/configure.ac b/gcc/configure.ac
index 62d3053..5d3e5ad 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -5181,6 +5181,7 @@  if test "x$subdirs" != x; then
 	done
 fi
 echo "source ${srcdir}/gdbinit.in" >> .gdbinit
+echo "python import sys; sys.path.append('${srcdir}'); import gdbhooks" >> .gdbinit
 
 gcc_tooldir='$(libsubdir)/$(libsubdir_to_prefix)$(target_noncanonical)'
 AC_SUBST(gcc_tooldir)
diff --git a/gcc/gdbhooks.py b/gcc/gdbhooks.py
new file mode 100644
index 0000000..3d69b11
--- /dev/null
+++ b/gcc/gdbhooks.py
@@ -0,0 +1,397 @@ 
+# Python hooks for gdb for debugging GCC
+# Copyright (C) 2013 Free Software Foundation, Inc.
+
+# Contributed by David Malcolm <dmalcolm@redhat.com>
+
+# This file is part of GCC.
+
+# GCC is free software; you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free
+# Software Foundation; either version 3, or (at your option) any later
+# version.
+
+# GCC is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+# for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+"""
+Enabling the debugging hooks
+----------------------------
+gcc/configure (from configure.ac) generates a .gdbinit within the "gcc"
+subdirectory of the build directory, and when run by gdb, this imports
+gcc/gdbhooks.py from the source directory, injecting useful Python code
+into gdb.
+
+You may see a message from gdb of the form:
+  "path-to-build/gcc/.gdbinit" auto-loading has been declined by your `auto-load safe-path'
+as a protection against untrustworthy python scripts.  See
+  http://sourceware.org/gdb/onlinedocs/gdb/Auto_002dloading-safe-path.html
+
+The fix is to mark the paths of the build/gcc directory as trustworthy.
+An easy way to do so is by adding the following to your ~/.gdbinit script:
+  add-auto-load-safe-path /absolute/path/to/build/gcc
+for the build directories for your various checkouts of gcc.
+
+If it's working, you should see the message:
+  Successfully loaded GDB hooks for GCC
+as gdb starts up.
+
+During development, I've been manually invoking the code in this way, as a
+precanned way of printing a variety of different kinds of value:
+
+  gdb \
+    -ex "break expand_gimple_stmt" \
+    -ex "run" \
+    -ex "bt" \
+    --args \
+      ./cc1 foo.c -O3
+
+Examples of output using the pretty-printers
+--------------------------------------------
+Pointer values are generally shown in the form:
+  <type address extra_info>
+
+For example, an opt_pass* might appear as:
+  (gdb) p pass
+  $2 = <opt_pass* 0x188b600 "expand"(170)>
+
+The name of the pass is given ("expand"), together with the
+static_pass_number.
+
+Note that you can dereference the pointer in the normal way:
+  (gdb) p *pass
+  $4 = {type = RTL_PASS, name = 0x120a312 "expand",
+  [etc, ...snipped...]
+
+and you can suppress pretty-printers using /r (for "raw"):
+  (gdb) p /r pass
+  $3 = (opt_pass *) 0x188b600
+
+Basic blocks are shown with their index in parentheses, apart from the
+CFG's entry and exit blocks, which are given as "ENTRY" and "EXIT":
+  (gdb) p bb
+  $9 = <basic_block 0x7ffff041f1a0 (2)>
+  (gdb) p cfun->cfg->x_entry_block_ptr
+  $10 = <basic_block 0x7ffff041f0d0 (ENTRY)>
+  (gdb) p cfun->cfg->x_exit_block_ptr
+  $11 = <basic_block 0x7ffff041f138 (EXIT)>
+
+CFG edges are shown with the src and dest blocks given in parentheses:
+  (gdb) p e
+  $1 = <edge 0x7ffff043f118 (ENTRY -> 6)>
+
+Tree nodes are printed using Python code that emulates print_node_brief,
+running in gdb, rather than in the inferior:
+  (gdb) p cfun->decl
+  $1 = <function_decl 0x7ffff0420b00 foo>
+For usability, the type is printed first (e.g. "function_decl"), rather
+than just "tree".
+
+RTL expressions use a kludge: they are pretty-printed by injecting
+calls into print-rtl.c into the inferior:
+  Value returned is $1 = (note 9 8 10 [bb 3] NOTE_INSN_BASIC_BLOCK)
+  (gdb) p $1
+  $2 = (note 9 8 10 [bb 3] NOTE_INSN_BASIC_BLOCK)
+  (gdb) p /r $1
+  $3 = (rtx_def *) 0x7ffff043e140
+This won't work for coredumps, and probably in other circumstances, but
+it's a quick way of getting lots of debuggability quickly.
+
+Callgraph nodes are printed with the name of the function decl, if
+available:
+  (gdb) frame 5
+  #5  0x00000000006c288a in expand_function (node=<cgraph_node* 0x7ffff0312720 "foo">) at ../../src/gcc/cgraphunit.c:1594
+  1594	  execute_pass_list (g->get_passes ()->all_passes);
+  (gdb) p node
+  $1 = <cgraph_node* 0x7ffff0312720 "foo">
+"""
+import re
+
+import gdb
+import gdb.printing
+import gdb.types
+
+# Convert "enum tree_code" (tree.def and tree.h) to a dict:
+tree_code_dict = gdb.types.make_enum_dict(gdb.lookup_type('enum tree_code'))
+
+# ...and look up specific values for use later:
+IDENTIFIER_NODE = tree_code_dict['IDENTIFIER_NODE']
+TYPE_DECL = tree_code_dict['TYPE_DECL']
+
+# Similarly for "enum tree_code_class" (tree.h):
+tree_code_class_dict = gdb.types.make_enum_dict(gdb.lookup_type('enum tree_code_class'))
+tcc_type = tree_code_class_dict['tcc_type']
+tcc_declaration = tree_code_class_dict['tcc_declaration']
+
+class Tree:
+    """
+    Wrapper around a gdb.Value for a tree, with various methods
+    corresponding to macros in gcc/tree.h
+    """
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def is_nonnull(self):
+        return long(self.gdbval)
+
+    def TREE_CODE(self):
+        """
+        Get gdb.Value corresponding to TREE_CODE (self)
+        as per:
+          #define TREE_CODE(NODE) ((enum tree_code) (NODE)->base.code)
+        """
+        return self.gdbval['base']['code']
+
+    def DECL_NAME(self):
+        """
+        Get Tree instance corresponding to DECL_NAME (self)
+        """
+        return Tree(self.gdbval['decl_minimal']['name'])
+
+    def TYPE_NAME(self):
+        """
+        Get Tree instance corresponding to result of TYPE_NAME (self)
+        """
+        return Tree(self.gdbval['type_common']['name'])
+
+    def IDENTIFIER_POINTER(self):
+        """
+        Get str correspoinding to result of IDENTIFIER_NODE (self)
+        """
+        return self.gdbval['identifier']['id']['str'].string()
+
+class TreePrinter:
+    "Prints a tree"
+
+    def __init__ (self, gdbval):
+        self.gdbval = gdbval
+        self.node = Tree(gdbval)
+
+    def to_string (self):
+        # like gcc/print-tree.c:print_node_brief
+        # #define TREE_CODE(NODE) ((enum tree_code) (NODE)->base.code)
+        # tree_code_name[(int) TREE_CODE (node)])
+        if long(self.gdbval) == 0:
+            return '<tree 0x0>'
+
+        val_TREE_CODE = self.node.TREE_CODE()
+
+        # extern const enum tree_code_class tree_code_type[];
+        # #define TREE_CODE_CLASS(CODE)	tree_code_type[(int) (CODE)]
+
+        val_tree_code_type = gdb.parse_and_eval('tree_code_type')
+        val_tclass = val_tree_code_type[val_TREE_CODE]
+
+        val_tree_code_name = gdb.parse_and_eval('tree_code_name')
+        val_code_name = val_tree_code_name[long(val_TREE_CODE)]
+        #print val_code_name.string()
+
+        result = '<%s 0x%x' % (val_code_name.string(), long(self.gdbval))
+        if long(val_tclass) == tcc_declaration:
+            tree_DECL_NAME = self.node.DECL_NAME()
+            if tree_DECL_NAME.is_nonnull():
+                 result += ' %s' % tree_DECL_NAME.IDENTIFIER_POINTER()
+            else:
+                pass # TODO: labels etc
+        elif long(val_tclass) == tcc_type:
+            tree_TYPE_NAME = Tree(self.gdbval['type_common']['name'])
+            if tree_TYPE_NAME.is_nonnull():
+                if tree_TYPE_NAME.TREE_CODE() == IDENTIFIER_NODE:
+                    result += ' %s' % tree_TYPE_NAME.IDENTIFIER_POINTER()
+                elif tree_TYPE_NAME.TREE_CODE() == TYPE_DECL:
+                    if tree_TYPE_NAME.DECL_NAME().is_nonnull():
+                        result += ' %s' % tree_TYPE_NAME.DECL_NAME().IDENTIFIER_POINTER()
+        if self.node.TREE_CODE() == IDENTIFIER_NODE:
+            result += ' %s' % self.node.IDENTIFIER_POINTER()
+        # etc
+        result += '>'
+        return result
+
+######################################################################
+# Callgraph pretty-printers
+######################################################################
+
+class CGraphNodePrinter:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def to_string (self):
+        result = '<cgraph_node* 0x%x' % long(self.gdbval)
+        if long(self.gdbval):
+            # symtab_node_name calls lang_hooks.decl_printable_name
+            # default implementation (lhd_decl_printable_name) is:
+            #    return IDENTIFIER_POINTER (DECL_NAME (decl));
+            symbol = self.gdbval['symbol']
+            tree_decl = Tree(symbol['decl'])
+            result += ' "%s"' % tree_decl.DECL_NAME().IDENTIFIER_POINTER()
+        result += '>'
+        return result
+
+######################################################################
+
+class GimplePrinter:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def to_string (self):
+        if long(self.gdbval) == 0:
+            return '<gimple 0x0>'
+        val_gimple_code = self.gdbval['gsbase']['code']
+        val_gimple_code_name = gdb.parse_and_eval('gimple_code_name')
+        val_code_name = val_gimple_code_name[long(val_gimple_code)]
+        result = '<%s 0x%x' % (val_code_name.string(),
+                               long(self.gdbval))
+        result += '>'
+        return result
+
+######################################################################
+# CFG pretty-printers
+######################################################################
+
+def bb_index_to_str(index):
+    if index == 0:
+        return 'ENTRY'
+    elif index == 1:
+        return 'EXIT'
+    else:
+        return '%i' % index
+
+class BasicBlockPrinter:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def to_string (self):
+        result = '<basic_block 0x%x' % long(self.gdbval)
+        if long(self.gdbval):
+            result += ' (%s)' % bb_index_to_str(long(self.gdbval['index']))
+        result += '>'
+        return result
+
+class CfgEdgePrinter:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def to_string (self):
+        result = '<edge 0x%x' % long(self.gdbval)
+        if long(self.gdbval):
+            src = bb_index_to_str(long(self.gdbval['src']['index']))
+            dest = bb_index_to_str(long(self.gdbval['dest']['index']))
+            result += ' (%s -> %s)' % (src, dest)
+        result += '>'
+        return result
+
+######################################################################
+
+class Rtx:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def GET_CODE(self):
+        return self.gdbval['code']
+
+def GET_RTX_LENGTH(code):
+    val_rtx_length = gdb.parse_and_eval('rtx_length')
+    return long(val_rtx_length[code])
+
+def GET_RTX_NAME(code):
+    val_rtx_name = gdb.parse_and_eval('rtx_name')
+    return val_rtx_name[code].string()
+
+def GET_RTX_FORMAT(code):
+    val_rtx_format = gdb.parse_and_eval('rtx_format')
+    return val_rtx_format[code].string()
+
+class RtxPrinter:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+        self.rtx = Rtx(gdbval)
+
+    def to_string (self):
+        """
+        For now, a cheap kludge: invoke the inferior's print
+        function to get a string to use the user, and return an empty
+        string for gdb
+        """
+        # We use print_inline_rtx to avoid a trailing newline
+        gdb.execute('call print_inline_rtx (stderr, (const_rtx) %s, 0)'
+                    % long(self.gdbval))
+        return ''
+
+        # or by hand; based on gcc/print-rtl.c:print_rtx
+        result = ('<rtx_def 0x%x'
+                  % (long(self.gdbval)))
+        code = self.rtx.GET_CODE()
+        result += ' (%s' % GET_RTX_NAME(code)
+        format_ = GET_RTX_FORMAT(code)
+        for i in range(GET_RTX_LENGTH(code)):
+            print format_[i]
+        result += ')>'
+        return result
+
+######################################################################
+
+class PassPrinter:
+    def __init__(self, gdbval):
+        self.gdbval = gdbval
+
+    def to_string (self):
+        result = '<opt_pass* 0x%x' % long(self.gdbval)
+        if long(self.gdbval):
+            result += (' "%s"(%i)'
+                       % (self.gdbval['name'].string(),
+                          long(self.gdbval['static_pass_number'])))
+        result += '>'
+        return result
+
+######################################################################
+
+# TODO:
+#   * vec
+#   * hashtab
+#   * location_t
+
+class GdbSubprinter(gdb.printing.SubPrettyPrinter):
+    def __init__(self, name, str_type_, class_):
+        super(GdbSubprinter, self).__init__(name)
+        self.str_type_ = str_type_
+        self.class_ = class_
+
+class GdbPrettyPrinters(gdb.printing.PrettyPrinter):
+    def __init__(self, name):
+        super(GdbPrettyPrinters, self).__init__(name, [])
+
+    def add_printer(self, name, exp, class_):
+        self.subprinters.append(GdbSubprinter(name, exp, class_))
+
+    def __call__(self, gdbval):
+        type_ = gdbval.type.unqualified()
+        str_type_ = str(type_)
+        for printer in self.subprinters:
+            if printer.enabled and str_type_ == printer.str_type_:
+                return printer.class_(gdbval)
+
+        # Couldn't find a pretty printer (or it was disabled):
+        return None
+
+
+def build_pretty_printer():
+    pp = GdbPrettyPrinters('gcc')
+    pp.add_printer('tree', 'tree', TreePrinter)
+    pp.add_printer('cgraph_node', 'cgraph_node *', CGraphNodePrinter)
+    pp.add_printer('gimple', 'gimple', GimplePrinter)
+    pp.add_printer('basic_block', 'basic_block', BasicBlockPrinter)
+    pp.add_printer('edge', 'edge', CfgEdgePrinter)
+    pp.add_printer('rtx_def', 'rtx_def *', RtxPrinter)
+    pp.add_printer('opt_pass', 'opt_pass *', PassPrinter)
+    return pp
+
+gdb.printing.register_pretty_printer(
+    gdb.current_objfile(),
+    build_pretty_printer())
+
+print('Successfully loaded GDB hooks for GCC')