GDB: Favor full symbol main name for backtrace stop

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>

3 files changed, 85 insertions, 10 deletions

diff --git a/gdb/frame.c b/gdb/frame.c
index de3c7de..4f420f1 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -2542,29 +2542,34 @@ inside_main_func (frame_info_ptr this_frame)
   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.  */
diff --git a/gdb/testsuite/gdb.fortran/backtrace.exp b/gdb/testsuite/gdb.fortran/backtrace.exp
new file mode 100644
index 0000000..8553c31
--- /dev/null
+++ b/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 \[^\r\n\]+" ]
diff --git a/gdb/testsuite/gdb.fortran/backtrace.f90 b/gdb/testsuite/gdb.fortran/backtrace.f90
new file mode 100644
index 0000000..a5f03f1
--- /dev/null
+++ b/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