gdb: Eliminate the 'stop_pc' global

In my multi-target work, I need to add a few more
scoped_restore_current_thread and switch_to_thread calls in some
places, and in some lower-level places I was fighting against the fact
that switch_to_thread reads/refreshes the stop_pc global.

Instead of piling on workarounds, let's just finally eliminate the
stop_pc global.  We already have the per-thread
thread_info->suspend.stop_pc field, so it's mainly a matter of using
that more/instead.

gdb/ChangeLog:
2018-06-28  Pedro Alves  <[email protected]>

	* gdbthread.h (struct thread_suspend_state) <stop_pc>: Extend
	comments.
	(switch_to_thread_no_regs): Adjust comment.
	* infcmd.c (stop_pc): Delete.
	(post_create_inferior, info_program_command): Replace references
	to stop_pc with references to thread_info->suspend.stop_pc.
	* inferior.h (stop_pc): Delete declaration.
	* infrun.c (proceed, handle_syscall_event, fill_in_stop_func)
	(handle_inferior_event_1, handle_signal_stop)
	(process_event_stop_test, keep_going_stepped_thread)
	(handle_step_into_function, handle_step_into_function_backward)
	(print_stop_location): Replace references to stop_pc with
	references to thread_info->suspend.stop_pc.
	(struct infcall_suspend_state) <stop_pc>: Delete field.
	(save_infcall_suspend_state, restore_infcall_suspend_state):
	Remove references to inf_stat->stop_pc.
	* linux-fork.c (fork_load_infrun_state): Likewise.
	* record-btrace.c (record_btrace_set_replay): Likewise.
	* record-full.c (record_full_goto_entry): Likewise.
	* remote.c (print_one_stopped_thread): Likewise.
	* target.c (target_resume): Extend comment.
	* thread.c (set_executing_thread): New.
	(set_executing): Use it.
	(switch_to_thread_no_regs, switch_to_no_thread, switch_to_thread):
	Remove references to stop_pc.

gdb/ChangeLog

@@ -1,3 +1,31 @@
+2018-06-28  Pedro Alves  <[email protected]>
+
+	* gdbthread.h (struct thread_suspend_state) <stop_pc>: Extend
+	comments.
+	(switch_to_thread_no_regs): Adjust comment.
+	* infcmd.c (stop_pc): Delete.
+	(post_create_inferior, info_program_command): Replace references
+	to stop_pc with references to thread_info->suspend.stop_pc.
+	* inferior.h (stop_pc): Delete declaration.
+	* infrun.c (proceed, handle_syscall_event, fill_in_stop_func)
+	(handle_inferior_event_1, handle_signal_stop)
+	(process_event_stop_test, keep_going_stepped_thread)
+	(handle_step_into_function, handle_step_into_function_backward)
+	(print_stop_location): Replace references to stop_pc with
+	references to thread_info->suspend.stop_pc.
+	(struct infcall_suspend_state) <stop_pc>: Delete field.
+	(save_infcall_suspend_state, restore_infcall_suspend_state):
+	Remove references to inf_stat->stop_pc.
+	* linux-fork.c (fork_load_infrun_state): Likewise.
+	* record-btrace.c (record_btrace_set_replay): Likewise.
+	* record-full.c (record_full_goto_entry): Likewise.
+	* remote.c (print_one_stopped_thread): Likewise.
+	* target.c (target_resume): Extend comment.
+	* thread.c (set_executing_thread): New.
+	(set_executing): Use it.
+	(switch_to_thread_no_regs, switch_to_no_thread, switch_to_thread):
+	Remove references to stop_pc.
+
 2018-06-28  Pedro Alves  <[email protected]>
 
 	* infrun.c (handle_inferior_event_1) <TARGET_WAITKIND_EXECD>:

gdb/gdbthread.h

@@ -168,10 +168,19 @@ struct thread_suspend_state
 
   /* Record the pc of the thread the last time it stopped.  (This is
      not the current thread's PC as that may have changed since the
-     last stop, e.g., "return" command, or "p $pc = 0xf000").  This is
-     used in coordination with stop_reason and waitstatus_pending_p:
-     if the thread's PC is changed since it last stopped, a pending
-     breakpoint waitstatus is discarded.  */
+     last stop, e.g., "return" command, or "p $pc = 0xf000").
+
+     - If the thread's PC has not changed since the thread last
+       stopped, then proceed skips a breakpoint at the current PC,
+       otherwise we let the thread run into the breakpoint.
+
+     - If the thread has an unprocessed event pending, as indicated by
+       waitstatus_pending_p, this is used in coordination with
+       stop_reason: if the thread's PC has changed since the thread
+       last stopped, a pending breakpoint waitstatus is discarded.
+
+     - If the thread is running, this is set to -1, to avoid leaving
+       it with a stale value, to make it easier to catch bugs.  */
   CORE_ADDR stop_pc;
 };
 
@@ -498,8 +507,7 @@ extern void switch_to_thread (struct thread_info *thr);
 /* Switch context to no thread selected.  */
 extern void switch_to_no_thread ();
 
-/* Switch from one thread to another.  Does not read registers and
-   sets STOP_PC to -1.  */
+/* Switch from one thread to another.  Does not read registers.  */
 extern void switch_to_thread_no_regs (struct thread_info *thread);
 
 /* Marks or clears thread(s) PTID as resumed.  If PTID is

gdb/infcmd.c

@@ -94,10 +94,6 @@ static char *inferior_io_terminal_scratch;
 
 ptid_t inferior_ptid;
 
-/* Address at which inferior stopped.  */
-
-CORE_ADDR stop_pc;
-
 /* Nonzero if stopped due to completion of a stack dummy routine.  */
 
 enum stop_stack_kind stop_stack_dummy;
@@ -448,10 +444,12 @@ post_create_inferior (struct target_ops *target, int from_tty)
   /* Now that we know the register layout, retrieve current PC.  But
      if the PC is unavailable (e.g., we're opening a core file with
      missing registers info), ignore it.  */
-  stop_pc = 0;
+  thread_info *thr = inferior_thread ();
+
+  thr->suspend.stop_pc = 0;
   TRY
     {
-      stop_pc = regcache_read_pc (get_current_regcache ());
+      thr->suspend.stop_pc = regcache_read_pc (get_current_regcache ());
     }
   CATCH (ex, RETURN_MASK_ERROR)
     {
@@ -2108,7 +2106,7 @@ info_program_command (const char *args, int from_tty)
 
   target_files_info ();
   printf_filtered (_("Program stopped at %s.\n"),
-		   paddress (target_gdbarch (), stop_pc));
+		   paddress (target_gdbarch (), tp->suspend.stop_pc));
   if (tp->control.stop_step)
     printf_filtered (_("It stopped after being stepped.\n"));
   else if (stat != 0)

gdb/inferior.h

@@ -208,10 +208,6 @@ extern void prepare_execution_command (struct target_ops *target,
    the target is started up with a shell.  */
 extern int startup_with_shell;
 
-/* Address at which inferior stopped.  */
-
-extern CORE_ADDR stop_pc;
-
 /* Nonzero if stopped due to completion of a stack dummy routine.  */
 
 extern enum stop_stack_kind stop_stack_dummy;