[PATCH v2] Add support for the --readnever command-line option (DWARF only)

[Sergio Durigan Junior <sergiodj at redhat dot com>]

[ Reviving the thread.  ]

Hey there,

So, since I'm working on upstreaming most of the patches we carry on
Fedora GDB, this one caught my attention (thanks to Pedro for bringing
this to me).

I applied it to a local tree, did some tests and adjustments (see
below), and now I'm resubmitting it for another set of reviews.

I hope we can get it pushed this time :-).

Please see comments below.

On Tuesday, October 04 2016, Pedro Alves wrote:

> On 07/12/2016 03:27 PM, Yao Qi wrote:
>> Hi Joel,
>> 
>> On Wed, Jul 6, 2016 at 9:54 PM, Joel Brobecker <brobecker@adacore.com> wrote:
>>> Hello,
>>>
>>> One of our customers asked us about this option, which they could
>>> see as being available in the version of GDB shipped by RedHat but
>>> not in the version that AdaCore supports.
>>>
>>> The purpose of this option is to turn the load of debugging information
>>> off. The implementation proposed here is mostly a copy of the patch
>>> distributed with RedHat Fedora, and looking at the patch itself and
>>> the history, I can see some reasons why it was never submitted:
>> 
>> Andrew Cagney posted the patch
>> https://www.sourceware.org/ml/gdb/2004-11/msg00216.html
>> 
>>>   - The patch appears to have been introduced as a workaround, at
>>>     least initially;
>>>   - The patch is far from perfect, as it simply shunts the load of
>>>     DWARF debugging information, without really worrying about the
>>>     other debug format.
>>>   - Who really does non-symbolic debugging anyways?
>> 
>> The reason, IMO, it was posted is that people want GDB avoid reading
>> debug info and efficiently dump stack backtrace.  I think Red Hat people
>> must know why Fedora is shipping this patch.
>> 
>> I don't object to this approach.
>> 
>
> This predates my gdb involvement, I don't really know the history.
> Maybe Jan knows.
>
> In any case, I don't object to the approach.
>
> Is this skipping _unwind_ info as well though?  I think the
> documentation should be clear on that, because if it does
> skip dwarf info for unwinding as well, then you
> may get a faster, but incorrect backtrace.

So, I looked a bit into this, and it seems that unwind information is
also skipped, which is unfortunate.  I am not an expert in this area of
GDB so by all means please comment if you have more details, but here's
the simple test I did.

I modified gdb.dwarf2/dw2-dup-frame.exp to use --readnever, and ran it.
The tests failed, and from what I checked this is because GDB was unable
to tell that the frame stack is corrupted (because there are two
identical frames in it).  By doing some debugging, I noticed that
gdb/dwarf2-frame.c:dwarf2_frame_sniffer is always returning 0 because it
can't find any FDE's, which are found by using DWARF information that
GDB hasn't read.

On Tuesday, October 04 2016, Pedro Alves wrote:

> On 07/06/2016 09:54 PM, Joel Brobecker wrote:
>
>>  #include <fcntl.h>
>>  #include <sys/types.h>
>> @@ -2062,6 +2063,9 @@ int
>>  dwarf2_has_info (struct objfile *objfile,
>>                   const struct dwarf2_debug_sections *names)
>>  {
>> +  if (readnever_symbol_files)
>> +    return 0;
>
> Guess that means '--readnever --readnow' is the same as
> --readnever in practice?

I've modified our main.c to detect whether --readnow and --readnever are
given at the same time, and print an error in this case.

>
>> +if { [gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable {debug}] != "" } {
>> +    untested "Couldn't compile ${srcfile}"
>> +    return -1
>> +}
>
> Maybe use build_executable.

Done.

>> +set saved_gdbflags $GDBFLAGS
>> +set GDBFLAGS "$GDBFLAGS --readnever"
>> +clean_restart ${binfile}
>> +set GDBFLAGS $saved_gdbflags
>
> Nowadays we have save_vars:
>
> save_vars { GDBFLAGS } {
>   append GDBFLAGS " --readnever"
>   clean_restart ${binfile}
> }

Done.

Here's the updated patch.  I've made a few cosmetic modifications
(s/RedHat/Red Hat/, for example) to the commit message, BTW.

-- 
Sergio
GPG key ID: 237A 54B1 0287 28BF 00EF  31F4 D0EB 7628 65FC 5E36
Please send encrypted e-mail if possible
http://sergiodj.net/

>From 858798fa4ce56e05c24e86098072518950135b4f Mon Sep 17 00:00:00 2001
From: Joel Brobecker <brobecker@adacore.com>
Date: Wed, 6 Jul 2016 13:54:23 -0700
Subject: [PATCH] Add support for the --readnever command-line option (DWARF
 only)

Hello,

One of our customers asked us about this option, which they could see
as being available in the version of GDB shipped by Red Hat but not in
the version that AdaCore supports.

The purpose of this option is to turn the load of debugging
information off. The implementation proposed here is mostly a copy of
the patch distributed with Fedora, and looking at the patch itself and
the history, I can see some reasons why it was never submitted:

  - The patch appears to have been introduced as a workaround, at
    least initially;
  - The patch is far from perfect, as it simply shunts the load of
    DWARF debugging information, without really worrying about the
    other debug format.
  - Who really does non-symbolic debugging anyways?

One use of this is when a user simply wants to do the following
sequence: attach, dump core, detach. Loading the debugging information
in this case is an unnecessary cause of delay.

I started looking at a more general way of implementing this feature.
For instance, I was hoping for a patch maybe at the objfile reader
level, or even better at the generic part of the objfile reader.
But it turns out this is not trivial at all.  The loading of debugging
information appears to be done as part of either the sym_fns->sym_read
(most cases), or during the sym_fns->sym_read_psymbols phase ("lazy
loading", ELF). Reading the code there, it wasn't obvious to me
what the consequence would be to add some code to ignore debugging
information, and since I would not be able to test those changes,
I opted towards touching only the targets that use DWARF.

This is why I ended up choosing the same approach as Red Hat.  It's
far from perfect, but has the benefit of working for what I hope is
the vast majority of people, and being fairly unintrusive. I thought,
since it's useful to AdaCore, and it's been useful to Red Hat, maybe
others might find it useful, so here it is.

The changes I made to the patch from Fedora are:

      - dwarf2_has_info: Return immediately if readnever_symbol_files
                         is set - faster return, and easier to read, IMO;
      - main.c: Update the --help output to mention that this feature
                only supports DWARF;
      - gdb.texinfo: Add a paragraph to explain that this option only
        supports DWARF.

If you guys don't think it's a good idea, then I'll understand
(hence the RFC).  At least we'll have it in the archives, in case
someone else wants it.

gdb/ChangeLog:

	Andrew Cagney  <cagney@redhat.com>
	Joel Brobecker  <brobecker@adacore.com>
	Sergio Durigan Junior  <sergiodj@redhat.com>

	* symfile.c (readnever_symbol_files): New global.
	* top.h (readnever_symbol_files): New extern global.
	* main.c (captured_main): Add support for --readnever.
	(print_gdb_help): Document --readnever.
	* dwarf2read.c: #include "top.h".
	(dwarf2_has_info): Return 0 if READNEVER_SYMBOL_FILES is set.

gdb/doc/ChangeLog:

	Andrew Cagney  <cagney@redhat.com>
	Joel Brobecker  <brobecker@adacore.com>

	* gdb.texinfo (File Options): Document --readnever.

gdb/testsuite/ChangeLog:

        Joel Brobecker  <brobecker@adacore.com>
	Sergio Durigan Junior  <sergiodj@redhat.com>

        * gdb.base/readnever.c, gdb.base/readnever.exp: New files.

Tested on x86_64-linux.
---
 gdb/doc/gdb.texinfo                  |  8 ++++++
 gdb/dwarf2read.c                     |  4 +++
 gdb/main.c                           | 32 +++++++++++++++++++++---
 gdb/symfile.c                        |  1 +
 gdb/testsuite/gdb.base/readnever.c   | 39 ++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.base/readnever.exp | 47 ++++++++++++++++++++++++++++++++++++
 gdb/top.h                            |  1 +
 7 files changed, 129 insertions(+), 3 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/readnever.c
 create mode 100644 gdb/testsuite/gdb.base/readnever.exp

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index ab05a3718d..7d3d651185 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -1037,6 +1037,14 @@ Read each symbol file's entire symbol table immediately, rather than
 the default, which is to read it incrementally as it is needed.
 This makes startup slower, but makes future operations faster.
 
+@item --readnever
+@cindex @code{--readnever}
+Do not read each symbol file's symbolic debug information.  This makes
+startup faster but at the expense of not being able to perform
+symbolic debugging.
+
+This option is currently limited to debug information in DWARF format.
+For all other format, this option has no effect.
 @end table
 
 @node Mode Options
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 334d8c2e05..686fa10148 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -82,6 +82,7 @@
 #include <unordered_set>
 #include <unordered_map>
 #include "selftest.h"
+#include "top.h"
 
 /* When == 1, print basic high level tracing messages.
    When > 1, be more verbose.
@@ -2319,6 +2320,9 @@ int
 dwarf2_has_info (struct objfile *objfile,
                  const struct dwarf2_debug_sections *names)
 {
+  if (readnever_symbol_files)
+    return 0;
+
   dwarf2_per_objfile = ((struct dwarf2_per_objfile *)
 			objfile_data (objfile, dwarf2_objfile_data_key));
   if (!dwarf2_per_objfile)
diff --git a/gdb/main.c b/gdb/main.c
index 61168faf50..3ca64f48ef 100644
--- a/gdb/main.c
+++ b/gdb/main.c
@@ -579,14 +579,17 @@ captured_main_1 (struct captured_main_args *context)
       OPT_NOWINDOWS,
       OPT_WINDOWS,
       OPT_IX,
-      OPT_IEX
+      OPT_IEX,
+      OPT_READNOW,
+      OPT_READNEVER
     };
     static struct option long_options[] =
     {
       {"tui", no_argument, 0, OPT_TUI},
       {"dbx", no_argument, &dbx_commands, 1},
-      {"readnow", no_argument, &readnow_symbol_files, 1},
-      {"r", no_argument, &readnow_symbol_files, 1},
+      {"readnow", no_argument, NULL, OPT_READNOW},
+      {"readnever", no_argument, NULL, OPT_READNEVER},
+      {"r", no_argument, NULL, OPT_READNOW},
       {"quiet", no_argument, &quiet, 1},
       {"q", no_argument, &quiet, 1},
       {"silent", no_argument, &quiet, 1},
@@ -809,6 +812,26 @@ captured_main_1 (struct captured_main_args *context)
 	    }
 	    break;
 
+	  case OPT_READNOW:
+	    {
+	      if (readnever_symbol_files)
+		error (_("%s: '--readnow' and '--readnever' cannot be "
+			 "specified simultaneously"),
+		       gdb_program_name);
+	      readnow_symbol_files = 1;
+	    }
+	    break;
+
+	  case OPT_READNEVER:
+	    {
+	      if (readnow_symbol_files)
+		error (_("%s: '--readnow' and '--readnever' cannot be "
+			 "specified simultaneously"),
+		       gdb_program_name);
+	      readnever_symbol_files = 1;
+	    }
+	    break;
+
 	  case '?':
 	    error (_("Use `%s --help' for a complete list of options."),
 		   gdb_program_name);
@@ -1183,6 +1206,9 @@ Selection of debuggee and its files:\n\n\
   --se=FILE          Use FILE as symbol file and executable file.\n\
   --symbols=SYMFILE  Read symbols from SYMFILE.\n\
   --readnow          Fully read symbol files on first access.\n\
+  --readnever        Do not read symbol files.\n\
+                     There is currently a known limitation that this only\n\
+                     has an effect on symbol files provided in DWARF format.\n\
   --write            Set writing into executable and core files.\n\n\
 "), stream);
   fputs_unfiltered (_("\
diff --git a/gdb/symfile.c b/gdb/symfile.c
index feb50f8b79..14d3d8a6da 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -81,6 +81,7 @@ static void clear_symtab_users_cleanup (void *ignore);
 
 /* Global variables owned by this file.  */
 int readnow_symbol_files;	/* Read full symbols immediately.  */
+int readnever_symbol_files;	/* Never read full symbols.  */
 
 /* Functions this file defines.  */
 
diff --git a/gdb/testsuite/gdb.base/readnever.c b/gdb/testsuite/gdb.base/readnever.c
new file mode 100644
index 0000000000..803a5542f9
--- /dev/null
+++ b/gdb/testsuite/gdb.base/readnever.c
@@ -0,0 +1,39 @@
+/* Copyright 2016 Free Software Foundation, Inc.
+
+   This program 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 of the License, or
+   (at your option) any later version.
+
+   This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+void
+fun_three (void)
+{
+  /* Do nothing.  */
+}
+
+void
+fun_two (void)
+{
+  fun_three ();
+}
+
+void
+fun_one (void)
+{
+  fun_two ();
+}
+
+int
+main (void)
+{
+  fun_one ();
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/readnever.exp b/gdb/testsuite/gdb.base/readnever.exp
new file mode 100644
index 0000000000..24220f85c6
--- /dev/null
+++ b/gdb/testsuite/gdb.base/readnever.exp
@@ -0,0 +1,47 @@
+# Copyright 2016 Free Software Foundation, Inc.
+
+# This program 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 of the License, or
+# (at your option) any later version.
+#
+# This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This file is part of the gdb testsuite.  It is intended to test that
+# gdb can correctly print arrays with indexes for each element of the
+# array.
+
+standard_testfile .c
+
+if { [build_executable "failed to build" $testfile $srcfile { debug }] == -1 } {
+    untested "Couldn't compile ${srcfile}"
+    return -1
+}
+
+save_vars { GDBFLAGS } {
+    append GDBFLAGS " --readnever"
+    clean_restart ${binfile}
+}
+
+if ![runto_main] then {
+    perror "couldn't run to breakpoint"
+    continue
+}
+
+gdb_test "break fun_three" \
+         "Breakpoint $decimal at $hex"
+
+gdb_test "continue" \
+         "Breakpoint $decimal, $hex in fun_three \\(\\)"
+
+gdb_test "backtrace" \
+         [multi_line "#0  $hex in fun_three \\(\\)" \
+                     "#1  $hex in fun_two \\(\\)" \
+                     "#2  $hex in fun_one \\(\\)" \
+                     "#3  $hex in main \\(\\)" ]
diff --git a/gdb/top.h b/gdb/top.h
index 26fe87842f..a1df64f383 100644
--- a/gdb/top.h
+++ b/gdb/top.h
@@ -267,6 +267,7 @@ extern int gdb_in_secondary_prompt_p (struct ui *ui);
 
 /* From random places.  */
 extern int readnow_symbol_files;
+extern int readnever_symbol_files;
 
 /* Perform _initialize initialization.  */
 extern void gdb_init (char *);
-- 
2.13.3