[gdb] Don't set initial language if set manually

Initially, gdb sets the language to auto/c:
...
$ gdb -q
(gdb) show language
The current source language is "auto; currently c".
...

And after loading a c++ executable, that changes to auto/c++:
...
(gdb) file a.out
Reading symbols from a.out...
(gdb) show language
The current source language is "auto; currently c++".
...

Now consider setting the language manually to c:
...
$ gdb -q
(gdb) show language
The current source language is "auto; currently c".
(gdb) set language c
(gdb) show language
The current source language is "c".
...
The resulting language is manual/c.

Surprisingly, a subsequent load of the c++ executable:
...
(gdb) file a.out
Reading symbols from a.out...
(gdb) show language
The current source language is "c++".
...
gets us language manual/c++.

Loading the file should get us either:
- auto/c++, or
- manual/c.
That is, either the manual setting should be reset by loading, or the manual
setting should persist.

Fix this in the manual/c fashion. [ Though we could make some gdb setting to
choose one or the other. ]

Build and reg-tested on x86_64-linux.

[ Note: In PR23710 comment 1 a cc1 binary is attached for which gdb is slow when
loading and settting a breakpoint on do_rpo_vn:
...
$ time.sh gdb cc1 -batch -ex "b do_rpo_vn"
Breakpoint 1 at 0xd40e30: do_rpo_vn. (2 locations)
maxmem: 1463496
real: 8.88
user: 8.59
system: 0.35
...

This fix enables a speedup by manually setting the language before
loading, reducing executing time with ~17%, due to not having to load the full
symtab containing main:
...
$ time.sh gdb -iex "set language c++" cc1 -batch -ex "b do_rpo_vn"
Breakpoint 1 at 0xd40e30: do_rpo_vn. (2 locations)
maxmem: 1067308
real: 7.36
user: 7.14
system: 0.28
... ]

gdb/ChangeLog:

2020-02-26  Tom de Vries  <tdevries@suse.de>

	PR gdb/25603
	* symfile.c (set_initial_language): Exit-early if
	language_mode == language_mode_manual.

gdb/testsuite/ChangeLog:

2020-02-26  Tom de Vries  <tdevries@suse.de>

	PR gdb/25603
	* gdb.base/persistent-lang.cc: New test.
	* gdb.base/persistent-lang.exp: New file.

5 files changed, 75 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index aa4b01b..fb4cc03 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-26  Tom de Vries  <tdevries@suse.de>
+
+	PR gdb/25603
+	* symfile.c (set_initial_language): Exit-early if
+	language_mode == language_mode_manual.
+
 2020-02-25  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* dwarf2/loc.h (dwarf2_read_addr_index): Move...
diff --git a/gdb/symfile.c b/gdb/symfile.c
index f7bada7..f1edf2d 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -1681,6 +1681,8 @@ symbol_file_command (const char *args, int from_tty)
 void
 set_initial_language (void)
 {
+  if (language_mode == language_mode_manual)
+    return;
   enum language lang = main_language ();
 
   if (lang == language_unknown)
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 720febb..a352c74 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+2020-02-26  Tom de Vries  <tdevries@suse.de>
+
+	PR gdb/25603
+	* gdb.base/persistent-lang.cc: New test.
+	* gdb.base/persistent-lang.exp: New file.
+
 2020-02-25  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* gdb.fortran/derived-type-striding.exp: Add a new test.
diff --git a/gdb/testsuite/gdb.base/persistent-lang.cc b/gdb/testsuite/gdb.base/persistent-lang.cc
new file mode 100644
index 0000000..f4825c8
--- /dev/null
+++ b/gdb/testsuite/gdb.base/persistent-lang.cc
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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
+main ()
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/persistent-lang.exp b/gdb/testsuite/gdb.base/persistent-lang.exp
new file mode 100644
index 0000000..bcc4ef7
--- /dev/null
+++ b/gdb/testsuite/gdb.base/persistent-lang.exp
@@ -0,0 +1,39 @@
+# Copyright 2020 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/>.
+
+standard_testfile .cc
+
+if {[build_executable "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+clean_restart
+
+set auto_c \
+    "The current source language is \"auto; currently c\"\."
+set manual_c \
+    "The current source language is \"c\"\."
+
+gdb_test "show language" $auto_c \
+    "initial language is auto/c"
+
+gdb_test_no_output "set language c"
+gdb_test "show language" $manual_c \
+    "language updated to manual/c after set"
+
+set binfile [standard_output_file ${testfile}]
+gdb_load ${binfile}
+gdb_test "show language" $manual_c \
+    "language still manual/c after load"