[PATCH] gdb: Add switch to disable DWARF stack unwinders

[Andrew Burgess <andrew.burgess@embecosm.com>]

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.

gdb/ChangeLog:

	* dwarf2-frame-tailcall.c (tailcall_frame_sniffer): Exit early if
	dwarf unwinders are disabled.
	* dwarf2-frame.c (dwarf2_frame_sniffer): Likewise.
	(dwarf2_frame_unwinders_enabled_p): Define.
	(dwarf2_append_unwinders): Enable dwarf unwinders when they are
	first registered.
	* dwarf2-frame.h (dwarf2_frame_unwinders_enabled_p): Declare.
	* dwarf2read.c: Add dwarf2-frame.h include.
	(show_dwarf_unwinders_enabled_p): New function.
	(_initialize_dwarf2_read): Register switch to control unwinder
	use.
	* 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                    | 15 +++++++++++++++
 gdb/NEWS                         |  6 ++++++
 gdb/doc/ChangeLog                |  5 +++++
 gdb/doc/gdb.texinfo              | 24 ++++++++++++++++++++++++
 gdb/dwarf2-frame-tailcall.c      |  3 +++
 gdb/dwarf2-frame.c               |  9 +++++++++
 gdb/dwarf2-frame.h               |  6 ++++++
 gdb/dwarf2read.c                 | 26 ++++++++++++++++++++++++++
 gdb/testsuite/ChangeLog          |  5 +++++
 gdb/testsuite/gdb.base/maint.exp |  4 ++++
 10 files changed, 103 insertions(+)

diff --git a/gdb/NEWS b/gdb/NEWS
index acb9c34fb22..8b66cd8a9ad 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -7,6 +7,12 @@
   can be passed using the '[ADDRESS]:PORT' notation, or the regular
   'ADDRESS:PORT' method.
 
+* New commands
+
+maint set dwarf unwinders (on|off)
+maint show dwarf unwinders
+  Control whether DWARF unwinders can be used.
+
 *** Changes in GDB 8.2
 
 * The 'set disassembler-options' command now supports specifying options
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 2e9d76227c3..eb148cedef0 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -35802,6 +35802,30 @@
 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 functions 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
diff --git a/gdb/dwarf2-frame-tailcall.c b/gdb/dwarf2-frame-tailcall.c
index 1d3e1f445bb..f565a2eecc8 100644
--- a/gdb/dwarf2-frame-tailcall.c
+++ b/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)
diff --git a/gdb/dwarf2-frame.c b/gdb/dwarf2-frame.c
index 91e16cf024e..fdd41b360e6 100644
--- a/gdb/dwarf2-frame.c
+++ b/gdb/dwarf2-frame.c
@@ -169,6 +169,9 @@ static CORE_ADDR read_encoded_value (struct comp_unit *unit, gdb_byte encoding,
 				     CORE_ADDR func_base);
 \f
 
+/* See dwarf2-frame.h.  */
+int dwarf2_frame_unwinders_enabled_p = 0;
+
 /* 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 +1329,9 @@ 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.
@@ -1389,6 +1395,9 @@ dwarf2_append_unwinders (struct gdbarch *gdbarch)
 
   frame_unwind_append_unwinder (gdbarch, &dwarf2_frame_unwind);
   frame_unwind_append_unwinder (gdbarch, &dwarf2_signal_frame_unwind);
+
+  /* Mark dwarf frame unwinders as enabled.  */
+  dwarf2_frame_unwinders_enabled_p = 1;
 }
 \f
 
diff --git a/gdb/dwarf2-frame.h b/gdb/dwarf2-frame.h
index 471281a2c3f..56c90be8ceb 100644
--- a/gdb/dwarf2-frame.h
+++ b/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.  */
 
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index 372f45ee175..b49d4d9fbbc 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -90,6 +90,7 @@
 #include <forward_list>
 #include "rust-lang.h"
 #include "common/pathstuff.h"
+#include "dwarf2-frame.h"
 
 /* When == 1, print basic high level tracing messages.
    When > 1, be more verbose.
@@ -1423,6 +1424,19 @@ show_dwarf_max_cache_age (struct ui_file *file, int from_tty,
 			    "DWARF compilation units is %s.\n"),
 		    value);
 }
+
+/* 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);
+}
+
 \f
 /* local function prototypes */
 
@@ -25363,6 +25377,18 @@ conversational style, when possible."),
 			   &set_dwarf_cmdlist,
 			   &show_dwarf_cmdlist);
 
+  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);
+
   add_setshow_zuinteger_cmd ("dwarf-read", no_class, &dwarf_read_debug, _("\
 Set debugging of the DWARF reader."), _("\
 Show debugging of the DWARF reader."), _("\
diff --git a/gdb/testsuite/gdb.base/maint.exp b/gdb/testsuite/gdb.base/maint.exp
index 92086eec862..6125b6ec6f2 100644
--- a/gdb/testsuite/gdb.base/maint.exp
+++ b/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"} {
-- 
2.14.4