[PATCH v2 0/3] Fixes and improvements for the starting function

[PATCH v2 0/3] Fixes and improvements for the starting function

[Maciej W. Rozycki]

Hi,

 Here is a small patch series to fix and improve the handling of the 
starting (aka main) function of the program, specifically for Fortran.

 This v2 update to the series brings documentation pieces missing from the 
original submission.

 They have been regression-tested with the `x86_64-linux-gnu' native 
configuration and GCC versions 11.3.0 and 4.4.7.  See individual change 
descriptions for details.

 OK to apply?

  Maciej

[PATCH v2 1/3] GDB: Favor full symbol main name for backtrace stop

[Maciej W. Rozycki]

From: Richard Bunt <richard.bunt@linaro.org>

In the case where a Fortran program has a program name of "main" and 
there is also a minimal symbol called main, such as with programs built 
with GCC version 4.4.7 or below, the backtrace will erroneously stop at 
the minimal symbol rather than the user specified main, e.g.:

(gdb) bt
#0  bar () at .../gdb/testsuite/gdb.fortran/backtrace.f90:17
#1  0x0000000000402556 in foo () at .../gdb/testsuite/gdb.fortran/backtrace.f90:21
#2  0x0000000000402575 in main () at .../gdb/testsuite/gdb.fortran/backtrace.f90:31
#3  0x00000000004025aa in main ()
(gdb) 

This patch fixes this issue by increasing the precedence of the full 
symbol when the language of the current frame is Fortran.

Newer versions of GCC transform the program name to "MAIN__" in this 
case, avoiding the problem.

Co-Authored-By: Maciej W. Rozycki <macro@embecosm.com>
---
Changes from v1:

- Remove testsuite status mistakenly copied to the change description.
---
 gdb/frame.c                             |   25 ++++++++++++---------
 gdb/testsuite/gdb.fortran/backtrace.exp |   38 ++++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.fortran/backtrace.f90 |   32 ++++++++++++++++++++++++++
 3 files changed, 85 insertions(+), 10 deletions(-)

gdb-richard-favour-full-symbol-main-name-for-backtrace-stop.diff
Index: src/gdb/frame.c
===================================================================
--- src.orig/gdb/frame.c
+++ src/gdb/frame.c
@@ -2542,29 +2542,34 @@ inside_main_func (frame_info_ptr this_fr
   bound_minimal_symbol msymbol
     = lookup_minimal_symbol (name, NULL,
 			     current_program_space->symfile_object_file);
-  if (msymbol.minsym == nullptr)
+
+  if (msymbol.minsym != nullptr)
+    sym_addr = msymbol.value_address ();
+
+  /* Favor a full symbol in Fortran, for the case where the Fortran main
+     is also called "main".  */
+  if (msymbol.minsym == nullptr
+      || get_frame_language (this_frame) == language_fortran)
     {
       /* In some language (for example Fortran) there will be no minimal
 	 symbol with the name of the main function.  In this case we should
 	 search the full symbols to see if we can find a match.  */
       struct block_symbol bs = lookup_symbol (name, NULL, VAR_DOMAIN, 0);
-      if (bs.symbol == nullptr)
-	return false;
 
       /* We might have found some unrelated symbol.  For example, the
 	 Rust compiler can emit both a subprogram and a namespace with
 	 the same name in the same scope; and due to how gdb's symbol
 	 tables currently work, we can't request the one we'd
 	 prefer.  */
-      if (bs.symbol->aclass () != LOC_BLOCK)
+      if (bs.symbol != nullptr && bs.symbol->aclass () == LOC_BLOCK)
+	{
+	  const struct block *block = bs.symbol->value_block ();
+	  gdb_assert (block != nullptr);
+	  sym_addr = block->start ();
+	}
+      else if (msymbol.minsym == nullptr)
 	return false;
-
-      const struct block *block = bs.symbol->value_block ();
-      gdb_assert (block != nullptr);
-      sym_addr = block->start ();
     }
-  else
-    sym_addr = msymbol.value_address ();
 
   /* Convert any function descriptor addresses into the actual function
      code address.  */
Index: src/gdb/testsuite/gdb.fortran/backtrace.exp
===================================================================
--- /dev/null
+++ src/gdb/testsuite/gdb.fortran/backtrace.exp
@@ -0,0 +1,38 @@
+# Copyright 2023 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/>.
+
+require allow_fortran_tests
+
+load_lib fortran.exp
+
+standard_testfile .f90
+
+if {[prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} {debug f90}]} {
+    return -1
+}
+
+if {![fortran_runto_main]} {
+    perror "Could not run to main."
+    return
+}
+
+gdb_breakpoint [gdb_get_line_number "Break here"]
+gdb_continue_to_breakpoint "continue to bar"
+
+gdb_test "bt" \
+    [multi_line \
+	 "#0\[ \t\]*bar \\(\\) at \[^\r\n\]+" \
+	 "#1\[ \t\]*$hex in foo \\(\\) at \[^\r\n\]+" \
+	 "#2\[ \t\]*$hex in \(?:MAIN__\|main\) \\(\\) at \[^\r\n\]+" ]
Index: src/gdb/testsuite/gdb.fortran/backtrace.f90
===================================================================
--- /dev/null
+++ src/gdb/testsuite/gdb.fortran/backtrace.f90
@@ -0,0 +1,32 @@
+! Copyright 2023 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/>.
+
+subroutine bar ()
+  return                ! Break here
+end subroutine bar
+
+subroutine foo ()
+  call bar ()
+end subroutine foo
+
+program main
+  interface
+    subroutine bar ()
+    end subroutine bar
+    subroutine foo ()
+    end subroutine foo
+  end interface
+  call foo ()
+end program main

[PATCH v2 2/3] GDB: Bring back the handling of DW_CC_program

[Maciej W. Rozycki]

Fix a functional regression and restore the handling of DW_CC_program 
code of DW_AT_calling_convention attribute for determining the name of 
the starting function of the program where the DW_AT_main_subprogram 
attribute has not been provided, such as with Fortran code compiled with 
GCC versions 4.5.4 and below, or where DWARF version 3 or below has been 
requested.  Without it "main" is considered the starting function.  Cf. 
GCC PR fortran/43414.

Original code was removed with commit 6209cde4ddb8 ("Delete DWARF 
psymtab code"), and then an update to complement commit 81873cc81eff 
("[gdb/symtab] Support DW_AT_main_subprogram with -readnow.") has also 
been included here.
---
NB this was discovered with and is covered by the test case included with 
3/3; it has been verified to trigger with GCC 4.4.7 and will likely also 
with other configurations.

No change from v1.
---
 gdb/dwarf2/read.c |   19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

Index: src/gdb/dwarf2/read.c
===================================================================
--- src.orig/gdb/dwarf2/read.c
+++ src/gdb/dwarf2/read.c
@@ -9994,6 +9994,18 @@ inherit_abstract_dies (struct die_info *
     compute_delayed_physnames (origin_cu);
 }
 
+/* Return TRUE if the given DIE is the program's "main".  */
+
+static bool
+dwarf2_func_is_main_p (struct die_info *die, struct dwarf2_cu *cu)
+{
+  if (dwarf2_flag_true_p (die, DW_AT_main_subprogram, cu))
+    return true;
+  struct attribute *attr = dwarf2_attr (die, DW_AT_calling_convention, cu);
+  return (attr != nullptr
+	  && attr->constant_value (DW_CC_normal) == DW_CC_program);
+}
+
 static void
 read_func_scope (struct die_info *die, struct dwarf2_cu *cu)
 {
@@ -10084,7 +10096,7 @@ read_func_scope (struct die_info *die, s
   newobj = cu->get_builder ()->push_context (0, lowpc);
   newobj->name = new_symbol (die, read_type_die (die, cu), cu, templ_func);
 
-  if (dwarf2_flag_true_p (die, DW_AT_main_subprogram, cu))
+  if (dwarf2_func_is_main_p (die, cu))
     set_objfile_main_name (objfile, newobj->name->linkage_name (),
 			   cu->lang ());
 
@@ -16141,6 +16153,11 @@ cooked_indexer::scan_attributes (dwarf2_
 	    *flags |= IS_MAIN;
 	  break;
 
+	case DW_AT_calling_convention:
+	  if (attr.constant_value (DW_CC_normal) == DW_CC_program)
+	    *flags |= IS_MAIN;
+	  break;
+
 	case DW_AT_declaration:
 	  is_declaration = attr.as_boolean ();
 	  break;

[PATCH v2 3/3] GDB: Add `info main' command

[Maciej W. Rozycki]

From: Richard Bunt <richard.bunt@linaro.org>

Allow consumers of GDB to extract the name of the main method.  This is
most useful for Fortran programs which have a variable main method.

Used by both MAP and DDT e.g. it is used to detect the presence of debug 
information.

Co-Authored-By: Maciej W. Rozycki <macro@embecosm.com>
---
Changes from v1:

- Add documentation pieces.
---
 gdb/NEWS                                |    3 ++
 gdb/doc/gdb.texinfo                     |    6 ++++
 gdb/symtab.c                            |   11 ++++++++
 gdb/testsuite/gdb.fortran/info-main.exp |   42 ++++++++++++++++++++++++++++++++
 gdb/testsuite/gdb.fortran/info-main.f90 |   19 ++++++++++++++
 5 files changed, 81 insertions(+)

gdb-richard-info-main-command.diff
Index: src/gdb/NEWS
===================================================================
--- src.orig/gdb/NEWS
+++ src/gdb/NEWS
@@ -83,6 +83,9 @@ show always-read-ctf
   When off, CTF is only read if DWARF is not present.  When on, CTF is
   read regardless of whether DWARF is present.  Off by default.
 
+info main
+  Get main symbol to identify entry point into program.
+
 * New convenience function "$_shell", to execute a shell command and
   return the result.  This lets you run shell commands in expressions.
   Some examples:
Index: src/gdb/doc/gdb.texinfo
===================================================================
--- src.orig/gdb/doc/gdb.texinfo
+++ src/gdb/doc/gdb.texinfo
@@ -20162,6 +20162,12 @@ The optional flag @samp{-q}, which stand
 printing header information and messages explaining why no functions
 or variables have been printed.
 
+@kindex info main
+@item info main
+Print the name of the starting function of the program.  This serves
+primarily Fortran programs, which have a user-supplied name for the
+main subroutine.
+
 @kindex info classes
 @cindex Objective-C, classes and selectors
 @item info classes
Index: src/gdb/symtab.c
===================================================================
--- src.orig/gdb/symtab.c
+++ src/gdb/symtab.c
@@ -5282,6 +5282,14 @@ info_modules_command (const char *args,
 		      from_tty);
 }
 
+/* Implement the 'info main' command.  */
+
+static void
+info_main_command (const char *args, int from_tty)
+{
+  gdb_printf ("%s\n", main_name ());
+}
+
 static void
 rbreak_command (const char *regexp, int from_tty)
 {
@@ -6873,6 +6881,9 @@ Options:\n\
 		_("All module names, or those matching REGEXP."));
   set_cmd_completer_handle_brkchars (c, info_types_command_completer);
 
+  add_info ("main", info_main_command,
+	    _("Get main symbol to identify entry point into program."));
+
   add_basic_prefix_cmd ("module", class_info, _("\
 Print information about modules."),
 			&info_module_cmdlist, 0, &infolist);
Index: src/gdb/testsuite/gdb.fortran/info-main.exp
===================================================================
--- /dev/null
+++ src/gdb/testsuite/gdb.fortran/info-main.exp
@@ -0,0 +1,42 @@
+# Copyright 2023 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/>.
+
+require allow_fortran_tests
+
+load_lib fortran.exp
+
+standard_testfile .f90
+
+set old_gdbflags $GDBFLAGS
+set GDBFLAGS [string map {"-readnow" ""} "$GDBFLAGS"]
+
+if {[prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} {debug f90}]} {
+    return -1
+}
+
+gdb_test "info main" "simple" "info main prior to start"
+
+if ![fortran_runto_main] {
+    untested "could not run to main"
+    return -1
+}
+
+gdb_test "info main" "simple" "info main post start"
+
+set GDBFLAGS "$GDBFLAGS -readnow"
+clean_restart $testfile
+gdb_test "info main" "simple" "info main with readnow"
+
+set GDBFLAGS $old_gdbflags
Index: src/gdb/testsuite/gdb.fortran/info-main.f90
===================================================================
--- /dev/null
+++ src/gdb/testsuite/gdb.fortran/info-main.f90
@@ -0,0 +1,19 @@
+! Copyright 2023 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/>.
+
+program simple
+    implicit none
+    print *, "Nothing."
+end program simple

Re: [PATCH v2 3/3] GDB: Add `info main' command

[Eli Zaretskii]

> Date: Tue, 28 Mar 2023 19:49:02 +0100 (BST)
> From: "Maciej W. Rozycki" <macro@embecosm.com>
> cc: Richard Bunt <richard.bunt@linaro.org>
> 
> From: Richard Bunt <richard.bunt@linaro.org>
> 
> Allow consumers of GDB to extract the name of the main method.  This is
> most useful for Fortran programs which have a variable main method.
> 
> Used by both MAP and DDT e.g. it is used to detect the presence of debug 
> information.
> 
> Co-Authored-By: Maciej W. Rozycki <macro@embecosm.com>
> ---
> Changes from v1:
> 
> - Add documentation pieces.
> ---
>  gdb/NEWS                                |    3 ++
>  gdb/doc/gdb.texinfo                     |    6 ++++
>  gdb/symtab.c                            |   11 ++++++++
>  gdb/testsuite/gdb.fortran/info-main.exp |   42 ++++++++++++++++++++++++++++++++
>  gdb/testsuite/gdb.fortran/info-main.f90 |   19 ++++++++++++++
>  5 files changed, 81 insertions(+)

Thanks, the documentation parts are OK.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>

Re: [PATCH v2 2/3] GDB: Bring back the handling of DW_CC_program

[Tom Tromey]

>>>>> "Maciej" == Maciej W Rozycki <macro@embecosm.com> writes:

Maciej> Original code was removed with commit 6209cde4ddb8 ("Delete DWARF 
Maciej> psymtab code"), and then an update to complement commit 81873cc81eff 
Maciej> ("[gdb/symtab] Support DW_AT_main_subprogram with -readnow.") has also 
Maciej> been included here.

Sorry about that.

Maciej> NB this was discovered with and is covered by the test case included with 
Maciej> 3/3; it has been verified to trigger with GCC 4.4.7 and will likely also 
Maciej> with other configurations.

Maciej> +/* Return TRUE if the given DIE is the program's "main".  */
Maciej> +
Maciej> +static bool
Maciej> +dwarf2_func_is_main_p (struct die_info *die, struct dwarf2_cu *cu)
Maciej> +{
Maciej> +  if (dwarf2_flag_true_p (die, DW_AT_main_subprogram, cu))
Maciej> +    return true;
Maciej> +  struct attribute *attr = dwarf2_attr (die, DW_AT_calling_convention, cu);
Maciej> +  return (attr != nullptr
Maciej> +	  && attr->constant_value (DW_CC_normal) == DW_CC_program);

Should the DW_AT_calling_convention thing only apply to Fortran CUs?  I
have only ever heard of this approach being used by Fortran compilers.

Maciej> +	case DW_AT_calling_convention:
Maciej> +	  if (attr.constant_value (DW_CC_normal) == DW_CC_program)
Maciej> +	    *flags |= IS_MAIN;

Both the spots doing this check could probably use a comment explaining
why.  Maybe someday we'll get to remove this again.

Tom

Re: [PATCH v2 2/3] GDB: Bring back the handling of DW_CC_program

[Maciej W. Rozycki]

On Tue, 28 Mar 2023, Tom Tromey wrote:

> Maciej> +/* Return TRUE if the given DIE is the program's "main".  */
> Maciej> +
> Maciej> +static bool
> Maciej> +dwarf2_func_is_main_p (struct die_info *die, struct dwarf2_cu *cu)
> Maciej> +{
> Maciej> +  if (dwarf2_flag_true_p (die, DW_AT_main_subprogram, cu))
> Maciej> +    return true;
> Maciej> +  struct attribute *attr = dwarf2_attr (die, DW_AT_calling_convention, cu);
> Maciej> +  return (attr != nullptr
> Maciej> +	  && attr->constant_value (DW_CC_normal) == DW_CC_program);
> 
> Should the DW_AT_calling_convention thing only apply to Fortran CUs?  I
> have only ever heard of this approach being used by Fortran compilers.

 Well, DWARFv4 has this:

'If the semantics of the language of the compilation unit containing the 
subroutine entry distinguishes between ordinary subroutines and 
subroutines that can serve as the "main program," that is, subroutines 
that cannot be called directly according to the ordinary calling 
conventions, then the debugging information entry for such a subroutine 
may have a calling convention attribute whose value is the constant 
DW_CC_program.'

as a part of the standard, and then:

'The DW_CC_program value is intended to support Fortran main programs 
which in some implementations may not be callable or which must be invoked 
in a special way.  It is not intended as a way of finding the entry 
address for the program.'

as an additional explanation (in italics, therefore outside the standard 
proper), so while it does mention Fortran, there's nothing specific to 
that language about the attribute value.

 Similarly for DW_AT_main_subprogram:

'11. A DW_AT_main_subprogram attribute, which is a flag whose presence 
     indicates that the compilation unit contains a subprogram that has 
     been identified as the starting function of the program.  If more 
     than one compilation unit contains this flag, any one of them may
     contain the starting function.'

again clarified thus:

'    Fortran has a PROGRAM statement which is used to specify and provide 
     a user-specified name for the main subroutine of a program.  C uses 
     the name "main" to identify the main subprogram of a program.  Some 
     other languages provide similar or other means to identify the main 
     subprogram of a program.'

 It's not clear to me why the DW_AT_main_subprogram has been added, but 
offhand I guess it is just because DW_AT_calling_convention carried two 
pieces of information at once, while there may exist languages where the 
starting function of the program does follow the ordinary calling
conventions and may therefore be called directly, yet we still want to 
be able to identify it.

 GCC sources have this note:

      /* DWARF 2 doesn't provide a way to identify a program's source-level
	entry point.  DW_AT_calling_convention attributes are only meant
	to describe functions' calling conventions.  However, lacking a
	better way to signal the Fortran main program, we used this for
	a long time, following existing custom.  Now, DWARF 4 has
	DW_AT_main_subprogram, which we add below, but some tools still
	rely on the old way, which we thus keep.  */

but I don't find it convincing as written; I find the DWARF wording and 
the name itself of DW_CC_program both indicating the attribute value does 
identify the starting function of the program even if in conjunction with 
a different calling convention.

 Then GCC does emit DW_CC_program for Fortran only, but so it does 
DW_AT_main_subprogram.  I can't speak for other compilers.

 So I think let's keep the arrangement generic i.e. universally enabled 
and let the producers decide how to shape debug information.

> Maciej> +	case DW_AT_calling_convention:
> Maciej> +	  if (attr.constant_value (DW_CC_normal) == DW_CC_program)
> Maciej> +	    *flags |= IS_MAIN;
> 
> Both the spots doing this check could probably use a comment explaining
> why.  Maybe someday we'll get to remove this again.

 It's not clear to me why we'd ever want to selectively remove this piece,
but I guess we can mention how the DWARF standard evolved giving the two 
ways to annotate the starting function of the program.  I'll post v3 with 
such an update.

 Thank you for your input.

  Maciej