PR symtab/16264 - support DW_AT_main_subprogram

This patch adds support for DW_AT_main_subprogram.
This is PR symtab/16264.

DW_AT_main_subprogram is used to mark a program's entry point.  GCC
can emit this, and I hope to change the Rust compiler to emit it as
well.

GDB already supports an older, pre-DWARF 4 convention adopted by
FORTRAN compilers, namely to emit DW_AT_calling_convention for the
"main" function.  However, I think this support in GDB had a small
bug, in that it seems to rely on the DW_AT_name being read before
DW_AT_calling_convention.  This patch fixes this as well.

Built and regtested on x86-64 Fedora 24 and the buildbot.  New test
case included.

2016-12-02  Tom Tromey  <tom@tromey.com>

	PR symtab/16264:
	* dwarf2read.c (struct partial_die_info) <main_subprogram>: New
	member.
	(add_partial_symbol): Call set_objfile_main_name.
	(read_partial_die): Handle DW_AT_main_subprogram.
	<DW_AT_calling_convention>: don't call set_objfile_main_name, but
	set main_subprogram flag.

2016-12-02  Tom Tromey  <tom@tromey.com>

	* gdb.dwarf2/main-subprogram.c: New file.
	* gdb.dwarf2/main-subprogram.exp: New file.

5 files changed, 138 insertions, 12 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index fa02377..33d8243 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,13 @@
+2016-12-02  Tom Tromey  <tom@tromey.com>
+
+	PR symtab/16264:
+	* dwarf2read.c (struct partial_die_info) <main_subprogram>: New
+	member.
+	(add_partial_symbol): Call set_objfile_main_name.
+	(read_partial_die): Handle DW_AT_main_subprogram.
+	<DW_AT_calling_convention>: don't call set_objfile_main_name, but
+	set main_subprogram flag.
+
 2016-12-02  Simon Marchi  <simon.marchi@ericsson.com>
 
 	* tracefile-tfile.c (tfile_write_status): Adjust to renames.
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 558159a..ed10e03 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -1105,6 +1105,9 @@ struct partial_die_info
     unsigned int has_pc_info : 1;
     unsigned int may_be_inlined : 1;
 
+    /* This DIE has been marked DW_AT_main_subprogram.  */
+    unsigned int main_subprogram : 1;
+
     /* Flag set if the SCOPE field of this structure has been
        computed.  */
     unsigned int scope_set : 1;
@@ -6930,6 +6933,9 @@ add_partial_symbol (struct partial_die_info *pdi, struct dwarf2_cu *cu)
 			       &objfile->static_psymbols,
 			       addr, cu->language, objfile);
 	}
+
+      if (pdi->main_subprogram && actual_name != NULL)
+	set_objfile_main_name (objfile, actual_name, cu->language);
       break;
     case DW_TAG_constant:
       {
@@ -15949,19 +15955,18 @@ read_partial_die (const struct die_reader_specs *reader,
 	     to describe functions' calling conventions.
 
 	     However, because it's a necessary piece of information in
-	     Fortran, and because DW_CC_program is the only piece of debugging
-	     information whose definition refers to a 'main program' at all,
-	     several compilers have begun marking Fortran main programs with
-	     DW_CC_program --- even when those functions use the standard
-	     calling conventions.
-
-	     So until DWARF specifies a way to provide this information and
-	     compilers pick up the new representation, we'll support this
-	     practice.  */
+	     Fortran, and before DWARF 4 DW_CC_program was the only
+	     piece of debugging information whose definition refers to
+	     a 'main program' at all, several compilers marked Fortran
+	     main programs with DW_CC_program --- even when those
+	     functions use the standard calling conventions.
+
+	     Although DWARF now specifies a way to provide this
+	     information, we support this practice for backward
+	     compatibility.  */
 	  if (DW_UNSND (&attr) == DW_CC_program
-	      && cu->language == language_fortran
-	      && part_die->name != NULL)
-	    set_objfile_main_name (objfile, part_die->name, language_fortran);
+	      && cu->language == language_fortran)
+	    part_die->main_subprogram = 1;
 	  break;
 	case DW_AT_inline:
 	  if (DW_UNSND (&attr) == DW_INL_inlined
@@ -15978,6 +15983,10 @@ read_partial_die (const struct die_reader_specs *reader,
 	    }
 	  break;
 
+	case DW_AT_main_subprogram:
+	  part_die->main_subprogram = DW_UNSND (&attr);
+	  break;
+
 	default:
 	  break;
 	}
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 0ec07ae..0e06d9e 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2016-12-02  Tom Tromey  <tom@tromey.com>
+
+	* gdb.dwarf2/main-subprogram.c: New file.
+	* gdb.dwarf2/main-subprogram.exp: New file.
+
 2016-12-01  Luis Machado  <lgustavo@codesourcery.com>
 
 	* gdb.base/maint.exp: Remove timeout handling for gdb_test_multiple.
diff --git a/gdb/testsuite/gdb.dwarf2/main-subprogram.c b/gdb/testsuite/gdb.dwarf2/main-subprogram.c
new file mode 100644
index 0000000..97ca22e
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/main-subprogram.c
@@ -0,0 +1,29 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   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/>.  */
+
+int
+mymain ()
+{
+  asm ("mymain_label: .globl mymain_label");
+  return 0;
+}
+
+int
+main()
+{
+  return mymain ();
+}
diff --git a/gdb/testsuite/gdb.dwarf2/main-subprogram.exp b/gdb/testsuite/gdb.dwarf2/main-subprogram.exp
new file mode 100644
index 0000000..08c076b
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/main-subprogram.exp
@@ -0,0 +1,73 @@
+# 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/>.
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-4 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile main-subprogram.c enum-type-dw.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+
+    set range [function_range mymain ${srcdir}/${subdir}/${srcfile}]
+    set mymain_start [lindex $range 0]
+    set mymain_length [lindex $range 1]
+
+    cu {} {
+	DW_TAG_compile_unit {
+                {DW_AT_language @DW_LANG_C}
+                {DW_AT_name     main-subprogram.c}
+                {DW_AT_comp_dir /tmp}
+        } {
+	    declare_labels type
+
+            type: DW_TAG_base_type {
+                {DW_AT_byte_size 4 DW_FORM_sdata}
+                {DW_AT_encoding  @DW_ATE_signed}
+                {DW_AT_name      int}
+            }
+
+	    DW_TAG_subprogram {
+		{name mymain}
+		{low_pc $mymain_start addr}
+                {high_pc "$mymain_start + $mymain_length" addr}
+		{type :$type}
+		{DW_AT_main_subprogram 1 flag}
+	    } {
+	    }
+	}
+    }
+}
+
+if {[prepare_for_testing ${testfile}.exp ${testfile} \
+	 [list $srcfile $asm_file] {nodebug}]} {
+    return -1
+}
+
+# Test that the "start" command stops in the "mymain" function.
+# This should happen because we used DW_AT_main_subprogram to tell gdb
+# that this was the real "main".
+
+if {[gdb_start_cmd] < 0} {
+    untested "could not start ${testfile}"
+    return -1
+}
+
+gdb_test "" "Temporary breakpoint .* mymain.*" "stopped at mymain"