gdb: Add switch to disable DWARF stack unwinders

Add a maintenance command to disable the DWARF stack unwinders.
Normal users would not need this feature, but it is useful to allow
extended testing of fallback stack unwinding strategies, for example,
prologue scanners.

This is a partial implementation of the idea discussed in pr gdb/8434,
which talks about a generic ability to disable any frame unwinder.

Being able to arbitrarily disable any frame unwinder would be a more
complex patch, and I was unsure how useful such a feature would really
be, however, I can see (and have) a real need to disable DWARF
unwinders.  That's why this patch only targets that specific set of
unwinders.

If in the future we find ourselves adding more switches to disable
different unwinders, then we should probably move to a more generic
solution, and remove this patch.

gdb/ChangeLog:

	* dwarf2-frame-tailcall.c (tailcall_frame_sniffer): Exit early if
	DWARF unwinders are disabled.
	* dwarf2-frame.c: Add dwarf2read.h include.
	(dwarf2_frame_sniffer): Exit early if DWARF unwinders are
	disabled.
	(dwarf2_frame_unwinders_enabled_p): Define.
	(show_dwarf_unwinders_enabled_p): New function.
	(_initialize_dwarf2_frame): Register switch to control DWARF
	unwinder use.
	* dwarf2-frame.h (dwarf2_frame_unwinders_enabled_p): Declare.
	* dwarf2read.c (set_dwarf_cmdlist): Remove static keyword.
	(show_dwarf_cmdlist): Remove static keyword.
	* dwarf2read.h (set_dwarf_cmdlist): Declare.
	(show_dwarf_cmdlist): Declare.
	* NEWS: Document new feature.

gdb/doc/ChangeLog:

	* gdb.texinfo (Maintenance Commands): Add description of
	maintenance command to control dwarf unwinders.

gdb/testsuite/ChangeLog:

	* gdb.base/maint.exp: Add check that dwarf unwinders control flag
	is visible.

gdb/ChangeLog

@@ -1,3 +1,21 @@
+2018-07-26  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* dwarf2-frame-tailcall.c (tailcall_frame_sniffer): Exit early if
+	DWARF unwinders are disabled.
+	* dwarf2-frame.c: Add dwarf2read.h include.
+	(dwarf2_frame_sniffer): Exit early if DWARF unwinders are
+	disabled.
+	(dwarf2_frame_unwinders_enabled_p): Define.
+	(show_dwarf_unwinders_enabled_p): New function.
+	(_initialize_dwarf2_frame): Register switch to control DWARF
+	unwinder use.
+	* dwarf2-frame.h (dwarf2_frame_unwinders_enabled_p): Declare.
+	* dwarf2read.c (set_dwarf_cmdlist): Remove static keyword.
+	(show_dwarf_cmdlist): Remove static keyword.
+	* dwarf2read.h (set_dwarf_cmdlist): Declare.
+	(show_dwarf_cmdlist): Declare.
+	* NEWS: Document new feature.
+
 2018-07-26  Tom de Vries  <tdevries@suse.de>
 
 	PR breakpoints/23366

gdb/NEWS

@@ -27,6 +27,10 @@ tfaas COMMAND
   output).
   Shortcut for 'thread apply all -s frame apply all -s COMMAND'.
 
+maint set dwarf unwinders (on|off)
+maint show dwarf unwinders
+  Control whether DWARF unwinders can be used.
+
 * Changed commands
 
 thread apply [all | COUNT | -COUNT] [FLAG]... COMMAND

gdb/doc/ChangeLog

@@ -1,3 +1,8 @@
+2018-07-26  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.texinfo (Maintenance Commands): Add description of
+	maintenance command to control dwarf unwinders.
+
 2018-07-12  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
 
 	* gdb.texinfo (Debugging Programs with Multiple Threads):

gdb/doc/gdb.texinfo

@@ -35986,6 +35986,30 @@ compilation units will be stored in memory longer, and more total
 memory will be used.  Setting it to zero disables caching, which will
 slow down @value{GDBN} startup, but reduce memory consumption.
 
+@kindex maint set dwarf unwinders
+@kindex maint show dwarf unwinders
+@item maint set dwarf unwinders
+@itemx maint show dwarf unwinders
+Control use of the DWARF frame unwinders.
+
+@cindex DWARF frame unwinders
+Many targets that support DWARF debugging use @value{GDBN}'s DWARF
+frame unwinders to build the backtrace.  Many of these targets will
+also have a second mechanism for building the backtrace for use in
+cases where DWARF information is not available, this second mechanism
+is often an analysis of a function's prologue.
+
+In order to extend testing coverage of the second level stack
+unwinding mechanisms it is helpful to be able to disable the DWARF
+stack unwinders, this can be done with this switch.
+
+In normal use of @value{GDBN} disabling the DWARF unwinders is not
+advisable, there are cases that are better handled through DWARF than
+prologue analysis, and the debug experience is likely to be better
+with the DWARF frame unwinders enabled.
+
+If DWARF frame unwinders are not supported for a particular target
+architecture, then enabling this flag does not cause them to be used.
 @kindex maint set profile
 @kindex maint show profile
 @cindex profiling GDB

gdb/dwarf2-frame-tailcall.c

@@ -318,6 +318,9 @@ tailcall_frame_sniffer (const struct frame_unwind *self,
   int next_levels;
   struct tailcall_cache *cache;
 
+  if (!dwarf2_frame_unwinders_enabled_p)
+    return 0;
+
   /* Inner tail call element does not make sense for a sentinel frame.  */
   next_frame = get_next_frame (this_frame);
   if (next_frame == NULL)

gdb/dwarf2-frame.c

@@ -35,6 +35,7 @@
 
 #include "complaints.h"
 #include "dwarf2-frame.h"
+#include "dwarf2read.h"
 #include "ax.h"
 #include "dwarf2loc.h"
 #include "dwarf2-frame-tailcall.h"
@@ -169,6 +170,9 @@ static CORE_ADDR read_encoded_value (struct comp_unit *unit, gdb_byte encoding,
 				     CORE_ADDR func_base);
 
 
+/* See dwarf2-frame.h.  */
+int dwarf2_frame_unwinders_enabled_p = 1;
+
 /* Store the length the expression for the CFA in the `cfa_reg' field,
    which is unused in that case.  */
 #define cfa_exp_len cfa_reg
@@ -1326,6 +1330,9 @@ static int
 dwarf2_frame_sniffer (const struct frame_unwind *self,
 		      struct frame_info *this_frame, void **this_cache)
 {
+  if (!dwarf2_frame_unwinders_enabled_p)
+    return 0;
+
   /* Grab an address that is guarenteed to reside somewhere within the
      function.  get_frame_pc(), with a no-return next function, can
      end up returning something past the end of this function's body.
@@ -2394,12 +2401,36 @@ dwarf2_build_frame_info (struct objfile *objfile)
   set_objfile_data (objfile, dwarf2_frame_objfile_data, fde_table2);
 }
 
+/* Handle 'maintenance show dwarf unwinders'.  */
+
+static void
+show_dwarf_unwinders_enabled_p (struct ui_file *file, int from_tty,
+				struct cmd_list_element *c,
+				const char *value)
+{
+  fprintf_filtered (file,
+		    _("The DWARF stack unwinders are currently %s.\n"),
+		    value);
+}
+
 void
 _initialize_dwarf2_frame (void)
 {
   dwarf2_frame_data = gdbarch_data_register_pre_init (dwarf2_frame_init);
   dwarf2_frame_objfile_data = register_objfile_data ();
 
+  add_setshow_boolean_cmd ("unwinders", class_obscure,
+			   &dwarf2_frame_unwinders_enabled_p , _("\
+Set whether the DWARF stack frame unwinders are used."), _("\
+Show whether the DWARF stack frame unwinders are used."), _("\
+When enabled the DWARF stack frame unwinders can be used for architectures\n\
+that support the DWARF unwinders.  Enabling the DWARF unwinders for an\n\
+architecture that doesn't support them will have no effect."),
+			   NULL,
+			   show_dwarf_unwinders_enabled_p,
+			   &set_dwarf_cmdlist,
+			   &show_dwarf_cmdlist);
+
 #if GDB_SELF_TEST
   selftests::register_test_foreach_arch ("execute_cfa_program",
 					 selftests::execute_cfa_program_test);

gdb/dwarf2-frame.h

@@ -210,6 +210,12 @@ struct dwarf2_frame_state
   bool armcc_cfa_offsets_reversed = false;
 };
 
+/* When this is true the DWARF frame unwinders can be used if they are
+   registered with the gdbarch.  Not all architectures can or do use the
+   DWARF unwinders.  Setting this to true on a target that does not
+   otherwise support the DWARF unwinders has no effect.  */
+extern int dwarf2_frame_unwinders_enabled_p;
+
 /* Set the architecture-specific register state initialization
    function for GDBARCH to INIT_REG.  */
 

gdb/dwarf2read.c

@@ -25345,8 +25345,8 @@ partial_die_eq (const void *item_lhs, const void *item_rhs)
   return part_die_lhs->sect_off == part_die_rhs->sect_off;
 }
 
-static struct cmd_list_element *set_dwarf_cmdlist;
-static struct cmd_list_element *show_dwarf_cmdlist;
+struct cmd_list_element *set_dwarf_cmdlist;
+struct cmd_list_element *show_dwarf_cmdlist;
 
 static void
 set_dwarf_cmd (const char *args, int from_tty)

gdb/dwarf2read.h

@@ -23,6 +23,10 @@
 #include "filename-seen-cache.h"
 #include "gdb_obstack.h"
 
+/* Hold 'maintenance (set|show) dwarf' commands.  */
+extern struct cmd_list_element *set_dwarf_cmdlist;
+extern struct cmd_list_element *show_dwarf_cmdlist;
+
 typedef struct dwarf2_per_cu_data *dwarf2_per_cu_ptr;
 DEF_VEC_P (dwarf2_per_cu_ptr);
 

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2018-07-26  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.base/maint.exp: Add check that dwarf unwinders control flag
+	is visible.
+
 2018-07-26  Tom de Vries  <tdevries@suse.de>
 
 	PR breakpoints/23366

gdb/testsuite/gdb.base/maint.exp

@@ -526,6 +526,10 @@ gdb_test_no_output "maint info line-table xxx.c" \
 
 set timeout $oldtimeout
 
+# Just check that the DWARF unwinders control flag is visible.
+gdb_test "maint show dwarf unwinders" \
+    "The DWARF stack unwinders are currently (on|off)\\."
+
 #============test help on maint commands
 
 test_prefix_command_help {"maint info" "maintenance info"} {