GDB: Bring back the handling of DW_CC_program

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.

1 files changed, 27 insertions, 1 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index c910be8..b362dfd 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -9994,6 +9994,22 @@ inherit_abstract_dies (struct die_info *die, struct dwarf2_cu *cu)
     compute_delayed_physnames (origin_cu);
 }
 
+/* Return TRUE if the given DIE is the program's "main".  DWARF 4 has
+   defined a dedicated DW_AT_main_subprogram attribute to indicate the
+   starting function of the program, however with older versions the
+   DW_CC_program value of the DW_AT_calling_convention attribute was
+   used instead as the only means available.  We handle both variants.  */
+
+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 +10100,7 @@ read_func_scope (struct die_info *die, struct dwarf2_cu *cu)
   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 ());
 
@@ -16136,11 +16152,21 @@ cooked_indexer::scan_attributes (dwarf2_per_cu_data *scanning_per_cu,
 	    *linkage_name = attr.as_string ();
 	  break;
 
+	/* DWARF 4 has defined a dedicated DW_AT_main_subprogram
+	   attribute to indicate the starting function of the program...  */
 	case DW_AT_main_subprogram:
 	  if (attr.as_boolean ())
 	    *flags |= IS_MAIN;
 	  break;
 
+	/* ... however with older versions the DW_CC_program value of
+	   the DW_AT_calling_convention attribute was used instead as
+	   the only means available.  We handle both variants then.  */
+	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;