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

["Maciej W. Rozycki" <macro@embecosm.com>]

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;