range stepping: gdb

range stepping: gdb

This patch teaches GDB to take advantage of target-assisted range
stepping.  It adds a new 'r ADDR1,ADDR2' action to vCont (vCont;r),
meaning, "step once, and keep stepping as long as the thread is in the
[ADDR1,ADDR2) range".

Rationale:

When user issues the "step" command on the following line of source,

   a = b + c + d * e - a;

GDB single-steps every single instruction until the program reaches a
new different line.  E.g., on x86_64, that line compiles to:

   0x08048434 <+65>:    mov    0x1c(%esp),%eax
   0x08048438 <+69>:    mov    0x30(%esp),%edx
   0x0804843c <+73>:    add    %eax,%edx
   0x0804843e <+75>:    mov    0x18(%esp),%eax
   0x08048442 <+79>:    imul   0x2c(%esp),%eax
   0x08048447 <+84>:    add    %edx,%eax
   0x08048449 <+86>:    sub    0x34(%esp),%eax
   0x0804844d <+90>:    mov    %eax,0x34(%esp)
   0x08048451 <+94>:    mov    0x1c(%esp),%eax

and the following is the RSP traffic between GDB and GDBserver:

 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:3c840408;thread:p2e13.2e13;core:1;
 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:3e840408;thread:p2e13.2e13;core:2;
 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:42840408;thread:p2e13.2e13;core:2;
 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:47840408;thread:p2e13.2e13;core:0;
 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:49840408;thread:p2e13.2e13;core:0;
 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:4d840408;thread:p2e13.2e13;core:0;
 --> vCont;s:p2e13.2e13;c
 <-- T0505:68efffbf;04:30efffbf;08:51840408;thread:p2e13.2e13;core:0;

IOW, a lot of roundtrips between GDB and GDBserver.

If we add a new command to the RSP, meaning "keep stepping and don't
report a stop until the program goes out of the [0x08048434,
0x08048451) address range", then the RSP traffic can be reduced down
to:

 --> vCont;r8048434,8048451:p2db0.2db0;c
 <-- T0505:68efffbf;04:30efffbf;08:51840408;thread:p2db0.2db0;core:1;

As number of packets is reduced dramatically, the performance of
stepping source lines is much improved.

In case something is wrong with range stepping on the stub side, the
debug info or even gdb, this adds a "set/show range-stepping" command
to be able to turn range stepping off.

gdb/
2013-05-23  Yao Qi  <yao@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	* gdbthread.h (struct thread_control_state) <may_range_step>: New
	field.
	* infcmd.c (step_once, until_next_command): Enable range stepping.
	* infrun.c (displaced_step_prepare): Disable range stepping.
	(resume): Disable range stepping if stepping over a breakpoint or
	we have software watchpoints.  If range stepping is enabled,
	assert the thread is in the stepping range.
	(clear_proceed_status_thread): Clear may_range_step.
	(handle_inferior_event): Disable range stepping as soon as we know
	the thread that hit the event.  Re-enable it whenever we're going
	to step with a step range.
	* remote.c (struct vCont_action_support) <r>: New field.
	(use_range_stepping): New global.
	(remote_vcont_probe): Handle 'r' action.
	(append_resumption): Append an 'r' action if the thread may range
	step.
	(show_range_stepping): New function.
	(set_range_stepping): New function.
	(_initialize_remote): Call add_setshow_boolean_cmd to register the
	'set range-stepping' and 'show range-stepping' commands.
	* NEWS: Mention range stepping, the new vCont;r action, and the
	new "set/show range-stepping" commands.

gdb/doc/
2013-05-23  Yao Qi  <yao@codesourcery.com>
	    Pedro Alves  <palves@redhat.com>

	* gdb.texinfo (Packets): Document 'vCont;r'.
	(Continuing and Stepping): Document target-assisted range
	stepping, and the 'set range-stepping' and 'show range-stepping'
	commands.