Fix language of compilation unit with unknown file extension

Here, in dwarfread.c:process_full_comp_unit:

      /* Set symtab language to language from DW_AT_language.  If the
	 compilation is from a C file generated by language preprocessors, do
	 not set the language if it was already deduced by start_subfile.  */
      if (!(cu->language == language_c
	    && COMPUNIT_FILETABS (cust)->language != language_c))
	COMPUNIT_FILETABS (cust)->language = cu->language;

in case start_subfile doesn't manage to deduce a language
COMPUNIT_FILETABS(cust)->language ends up as language_unknown, not
language_c.  So the condition above evals false and we never set the
language from the cu's language.

gdb/ChangeLog:
2015-08-20  Pedro Alves  <palves@redhat.com>

	* dwarf2read.c (process_full_comp_unit): To tell whether
	start_subfile managed to deduce a language, test for
	language_unknown instead of language_c.

gdb/testsuite/ChangeLog:
2015-08-20  Pedro Alves  <palves@redhat.com>

	* gdb.dwarf2/comp-unit-lang.exp: New file.
	* gdb.dwarf2/comp-unit-lang.c: New file.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2015-08-20  Pedro Alves  <palves@redhat.com>
+
+	* dwarf2read.c (process_full_comp_unit): To tell whether
+	start_subfile managed to deduce a language, test for
+	language_unknown instead of language_c.
+
 2015-08-20  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* ada-lex.l: Reset the start condition to INITIAL in the rule that

gdb/dwarf2read.c

@@ -8072,7 +8072,7 @@ process_full_comp_unit (struct dwarf2_per_cu_data *per_cu,
 	 compilation is from a C file generated by language preprocessors, do
 	 not set the language if it was already deduced by start_subfile.  */
       if (!(cu->language == language_c
-	    && COMPUNIT_FILETABS (cust)->language != language_c))
+	    && COMPUNIT_FILETABS (cust)->language != language_unknown))
 	COMPUNIT_FILETABS (cust)->language = cu->language;
 
       /* GCC-4.0 has started to support -fvar-tracking.  GCC-3.x still can

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2015-08-20  Pedro Alves  <palves@redhat.com>
+
+	* gdb.dwarf2/comp-unit-lang.exp: New file.
+	* gdb.dwarf2/comp-unit-lang.c: New file.
+
 2015-08-20  Pierre-Marie de Rodat  <derodat@adacore.com>
 
 	* gdb.ada/attr_ref_and_charlit.exp: New testcase.

gdb/testsuite/gdb.dwarf2/comp-unit-lang.c

@@ -0,0 +1,34 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2011-2015 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/>.  */
+
+asm (".section	\".text\"");
+asm (".balign 8");
+asm ("func_start: .globl func_start");
+
+static void
+func (void)
+{
+}
+
+asm ("func_end: .globl func_end");
+
+int
+main (void)
+{
+  func ();
+  return 0;
+}

gdb/testsuite/gdb.dwarf2/comp-unit-lang.exp

@@ -0,0 +1,80 @@
+# Copyright 2014-2015 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/>.
+
+# Test that GDB determines the frame's language based on the comp
+# unit's language, even if the file has an unknown file extension,
+# such as ".txt".
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use
+# gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile .c comp-unit-lang.S
+
+# Assemble the DWARF using CU_LANG as compilation unit's language.
+# Run to a function in that compilation unit and check that GDB
+# figures out that the language is GDB_LANG.
+
+proc do_test {cu_lang gdb_lang} {
+    global testfile srcfile srcfile2
+
+    # Make some DWARF for the test.
+    set asm_file [standard_output_file $srcfile2]
+    Dwarf::assemble $asm_file {
+	upvar cu_lang cu_lang
+
+	# Creating a CU with 4-byte addresses lets this test link on
+	# both 32- and 64-bit machines.
+	cu { addr_size 4 } {
+	    extern func_start func_end
+
+	    compile_unit {
+		{name file1.txt}
+		{language @$cu_lang}
+		{low_pc func_start addr}
+		{high_pc func_end addr}
+	    } {
+		subprogram {
+		    {external 1 flag}
+		    {name func}
+		    {low_pc func_start addr}
+		    {high_pc func_end addr}
+		} {
+		}
+	    }
+	}
+    }
+
+    if { [prepare_for_testing "failed to prepare" ${testfile} \
+	      [list $srcfile $asm_file] {nodebug}] } {
+	return -1
+    }
+
+    if ![runto func] {
+	return -1
+    }
+
+    gdb_test "show language" "\"auto; currently $gdb_lang\".*"
+}
+
+# Some paths in the debugger fall back to C.  Check C++ as well to
+# make sure the test doesn't happen to work because of such a
+# fallback.
+do_test DW_LANG_C "c"
+do_test DW_LANG_C_plus_plus "c\\+\\+"