[PATCH 0/2] continuable/steppable/non-steppable watchpoints

[PATCH 0/2] continuable/steppable/non-steppable watchpoints

[Pedro Alves]

This is a follow up to
<https://sourceware.org/ml/gdb/2018-08/msg00054.html>.

Pedro Alves (2):
  Eliminate target_have_continuable_watchpoint
  Add comment describing continuable/steppable/non-steppable watchpoints

 gdb/gdbarch.h          |  2 ++
 gdb/gdbarch.sh         |  1 +
 gdb/nto-procfs.c       |  3 ---
 gdb/s390-linux-nat.c   |  1 -
 gdb/target-delegates.c | 27 ---------------------------
 gdb/target.h           | 41 ++++++++++++++++++++++++++++++++++-------
 gdb/x86-nat.h          |  2 --
 7 files changed, 37 insertions(+), 40 deletions(-)

-- 
2.14.4

[PATCH 1/2] Eliminate target_have_continuable_watchpoint

[Pedro Alves]

target_have_continuable_watchpoint isn't used anywhere so remove it.
The property isn't necessary because checking for "continuable" is the
same as checking for "!steppable && !non-steppable".

gdb/ChangeLog:
2018-08-31  Pedro Alves  <palves@redhat.com>

	* nto-procfs.c (nto_procfs_target::have_continuable_watchpoint):
	Delete.
	* s390-linux-nat.c
	(s390_linux_nat_target::have_continuable_watchpoint): Delete.
	* target.h (target_ops::have_continuable_watchpoint): Delete.
	(target_have_continuable_watchpoint): Delete.
	* x86-nat.h (x86_nat_target::have_continuable_watchpoint): Delete.
	* target-delegates.c: Regenerate.
---
 gdb/nto-procfs.c       |  3 ---
 gdb/s390-linux-nat.c   |  1 -
 gdb/target-delegates.c | 27 ---------------------------
 gdb/target.h           |  7 -------
 gdb/x86-nat.h          |  2 --
 5 files changed, 40 deletions(-)

diff --git a/gdb/nto-procfs.c b/gdb/nto-procfs.c
index 117edb161c3..54357af85b2 100644
--- a/gdb/nto-procfs.c
+++ b/gdb/nto-procfs.c
@@ -119,9 +119,6 @@ struct nto_procfs_target : public inf_child_target
 
   void interrupt () override;
 
-  bool have_continuable_watchpoint ()
-  { return true; }
-
   const char *extra_thread_info (struct thread_info *) override;
 
   char *pid_to_exec_file (int pid) override;
diff --git a/gdb/s390-linux-nat.c b/gdb/s390-linux-nat.c
index 2c605628105..7a04e854eb7 100644
--- a/gdb/s390-linux-nat.c
+++ b/gdb/s390-linux-nat.c
@@ -122,7 +122,6 @@ public:
   int remove_hw_breakpoint (struct gdbarch *, struct bp_target_info *)
     override;
   int region_ok_for_hw_watchpoint (CORE_ADDR, int) override;
-  bool have_continuable_watchpoint () override { return true; }
   bool stopped_by_watchpoint () override;
   int insert_watchpoint (CORE_ADDR, int, enum target_hw_bp_type,
 			 struct expression *) override;
diff --git a/gdb/target-delegates.c b/gdb/target-delegates.c
index 319a9bb46b4..03136dfacf2 100644
--- a/gdb/target-delegates.c
+++ b/gdb/target-delegates.c
@@ -36,7 +36,6 @@ struct dummy_target : public target_ops
   int remove_mask_watchpoint (CORE_ADDR arg0, CORE_ADDR arg1, enum target_hw_bp_type arg2) override;
   bool stopped_by_watchpoint () override;
   int have_steppable_watchpoint () override;
-  bool have_continuable_watchpoint () override;
   bool stopped_data_address (CORE_ADDR *arg0) override;
   bool watchpoint_addr_within_range (CORE_ADDR arg0, CORE_ADDR arg1, int arg2) override;
   int region_ok_for_hw_watchpoint (CORE_ADDR arg0, int arg1) override;
@@ -204,7 +203,6 @@ struct debug_target : public target_ops
   int remove_mask_watchpoint (CORE_ADDR arg0, CORE_ADDR arg1, enum target_hw_bp_type arg2) override;
   bool stopped_by_watchpoint () override;
   int have_steppable_watchpoint () override;
-  bool have_continuable_watchpoint () override;
   bool stopped_data_address (CORE_ADDR *arg0) override;
   bool watchpoint_addr_within_range (CORE_ADDR arg0, CORE_ADDR arg1, int arg2) override;
   int region_ok_for_hw_watchpoint (CORE_ADDR arg0, int arg1) override;
@@ -1016,31 +1014,6 @@ debug_target::have_steppable_watchpoint ()
   return result;
 }
 
-bool
-target_ops::have_continuable_watchpoint ()
-{
-  return this->beneath ()->have_continuable_watchpoint ();
-}
-
-bool
-dummy_target::have_continuable_watchpoint ()
-{
-  return false;
-}
-
-bool
-debug_target::have_continuable_watchpoint ()
-{
-  bool result;
-  fprintf_unfiltered (gdb_stdlog, "-> %s->have_continuable_watchpoint (...)\n", this->beneath ()->shortname ());
-  result = this->beneath ()->have_continuable_watchpoint ();
-  fprintf_unfiltered (gdb_stdlog, "<- %s->have_continuable_watchpoint (", this->beneath ()->shortname ());
-  fputs_unfiltered (") = ", gdb_stdlog);
-  target_debug_print_bool (result);
-  fputs_unfiltered ("\n", gdb_stdlog);
-  return result;
-}
-
 bool
 target_ops::stopped_data_address (CORE_ADDR *arg0)
 {
diff --git a/gdb/target.h b/gdb/target.h
index 39aa8c3c730..b29d34b8a65 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -552,8 +552,6 @@ struct target_ops
       TARGET_DEFAULT_RETURN (false);
     virtual int have_steppable_watchpoint ()
       TARGET_DEFAULT_RETURN (false);
-    virtual bool have_continuable_watchpoint ()
-      TARGET_DEFAULT_RETURN (false);
     virtual bool stopped_data_address (CORE_ADDR *)
       TARGET_DEFAULT_RETURN (false);
     virtual bool watchpoint_addr_within_range (CORE_ADDR, CORE_ADDR, int)
@@ -1933,11 +1931,6 @@ extern struct thread_info *target_thread_handle_to_thread_info
 #define target_have_steppable_watchpoint \
   (current_top_target ()->have_steppable_watchpoint ())
 
-/* Non-zero if we have continuable watchpoints  */
-
-#define target_have_continuable_watchpoint \
-  (current_top_target ()->have_continuable_watchpoint ())
-
 /* Provide defaults for hardware watchpoint functions.  */
 
 /* If the *_hw_beakpoint functions have not been defined
diff --git a/gdb/x86-nat.h b/gdb/x86-nat.h
index cc27fa43b4d..f6d8a26baf9 100644
--- a/gdb/x86-nat.h
+++ b/gdb/x86-nat.h
@@ -76,8 +76,6 @@ struct x86_nat_target : public BaseTarget
      the one that caused the trap.  Therefore we don't need to step
      over it.  But we do need to reset the status register to avoid
      another trap.  */
-  bool have_continuable_watchpoint () override
-  { return true; }
 
   int can_use_hw_breakpoint (enum bptype type, int cnt, int othertype) override
   { return x86_can_use_hw_breakpoint (type, cnt, othertype); }
-- 
2.14.4

Re: [PATCH 1/2] Eliminate target_have_continuable_watchpoint

[Tom Tromey]

>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

Pedro> target_have_continuable_watchpoint isn't used anywhere so remove it.
Pedro> The property isn't necessary because checking for "continuable" is the
Pedro> same as checking for "!steppable && !non-steppable".

Pedro>  /* If the *_hw_beakpoint functions have not been defined
Pedro> diff --git a/gdb/x86-nat.h b/gdb/x86-nat.h
Pedro> index cc27fa43b4d..f6d8a26baf9 100644
Pedro> --- a/gdb/x86-nat.h
Pedro> +++ b/gdb/x86-nat.h
Pedro> @@ -76,8 +76,6 @@ struct x86_nat_target : public BaseTarget
Pedro>       the one that caused the trap.  Therefore we don't need to step
Pedro>       over it.  But we do need to reset the status register to avoid
Pedro>       another trap.  */
Pedro> -  bool have_continuable_watchpoint () override
Pedro> -  { return true; }

I think the comment just before this function should also be removed.

Tom

Re: [PATCH 1/2] Eliminate target_have_continuable_watchpoint

[Pedro Alves]

On 08/31/2018 04:32 PM, Tom Tromey wrote:
>>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
> 
> Pedro> target_have_continuable_watchpoint isn't used anywhere so remove it.
> Pedro> The property isn't necessary because checking for "continuable" is the
> Pedro> same as checking for "!steppable && !non-steppable".
> 
> Pedro>  /* If the *_hw_beakpoint functions have not been defined
> Pedro> diff --git a/gdb/x86-nat.h b/gdb/x86-nat.h
> Pedro> index cc27fa43b4d..f6d8a26baf9 100644
> Pedro> --- a/gdb/x86-nat.h
> Pedro> +++ b/gdb/x86-nat.h
> Pedro> @@ -76,8 +76,6 @@ struct x86_nat_target : public BaseTarget
> Pedro>       the one that caused the trap.  Therefore we don't need to step
> Pedro>       over it.  But we do need to reset the status register to avoid
> Pedro>       another trap.  */
> Pedro> -  bool have_continuable_watchpoint () override
> Pedro> -  { return true; }
> 
> I think the comment just before this function should also be removed.

I thought it was still useful as is.  It's describing x86 watchpoints
generically enough before a set of functions implementing watchpoints
support that I thought that it's still positioned in a good place as
is.  WDYT?

Thanks,
Pedro Alves

Re: [PATCH 1/2] Eliminate target_have_continuable_watchpoint

[Tom Tromey]

>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

>> I think the comment just before this function should also be removed.

Pedro> I thought it was still useful as is.  It's describing x86 watchpoints
Pedro> generically enough before a set of functions implementing watchpoints
Pedro> support that I thought that it's still positioned in a good place as
Pedro> is.  WDYT?

I looked at that but I didn't understand how the comment related to the
nearby code; like I didn't see anything else in that file about the
status register.  However I think it's a minor point and if it makes
sense to you it is fine.

Tom

Re: [PATCH 1/2] Eliminate target_have_continuable_watchpoint

[Pedro Alves]

On 08/31/2018 04:43 PM, Tom Tromey wrote:
>>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
> 
>>> I think the comment just before this function should also be removed.
> 
> Pedro> I thought it was still useful as is.  It's describing x86 watchpoints
> Pedro> generically enough before a set of functions implementing watchpoints
> Pedro> support that I thought that it's still positioned in a good place as
> Pedro> is.  WDYT?
> 
> I looked at that but I didn't understand how the comment related to the
> nearby code; like I didn't see anything else in that file about the
> status register.  However I think it's a minor point and if it makes
> sense to you it is fine.

Ah, I was looking more at the bit explaining that the watchpoints
are continuable, so don't need to override target_has_steppable_watchpoint.

I won't miss it myself, so if it isn't helping others, might as well
remove it.

I've done that before pushing the patch in.

Thanks,
Pedro Alves

[PATCH 2/2] Add comment describing continuable/steppable/non-steppable watchpoints

[Pedro Alves]

These weren't described anywhere in the sources.

gdb/ChangeLog:
2018-08-31  Pedro Alves  <palves@redhat.com>

	* gdbarch.sh (have_nonsteppable_watchpoint): Add comment.
	* target.h (Hardware watchpoint interfaces): Describe
	continuable/steppable/non-steppable watchpoints.
---
 gdb/gdbarch.h  |  2 ++
 gdb/gdbarch.sh |  1 +
 gdb/target.h   | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index b563f8d165a..83408812b2a 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -818,6 +818,8 @@ extern void set_gdbarch_adjust_dwarf2_line (struct gdbarch *gdbarch, gdbarch_adj
 extern int gdbarch_cannot_step_breakpoint (struct gdbarch *gdbarch);
 extern void set_gdbarch_cannot_step_breakpoint (struct gdbarch *gdbarch, int cannot_step_breakpoint);
 
+/* See comment in target.h about continue, steppable and non-steppable watchpoints. */
+
 extern int gdbarch_have_nonsteppable_watchpoint (struct gdbarch *gdbarch);
 extern void set_gdbarch_have_nonsteppable_watchpoint (struct gdbarch *gdbarch, int have_nonsteppable_watchpoint);
 
diff --git a/gdb/gdbarch.sh b/gdb/gdbarch.sh
index 24a553b73c3..941c11b430e 100755
--- a/gdb/gdbarch.sh
+++ b/gdb/gdbarch.sh
@@ -707,6 +707,7 @@ f;CORE_ADDR;adjust_dwarf2_addr;CORE_ADDR pc;pc;;default_adjust_dwarf2_addr;;0
 # stop PC.
 f;CORE_ADDR;adjust_dwarf2_line;CORE_ADDR addr, int rel;addr, rel;;default_adjust_dwarf2_line;;0
 v;int;cannot_step_breakpoint;;;0;0;;0
+# See comment in target.h about continue, steppable and non-steppable watchpoints.
 v;int;have_nonsteppable_watchpoint;;;0;0;;0
 F;int;address_class_type_flags;int byte_size, int dwarf2_addr_class;byte_size, dwarf2_addr_class
 M;const char *;address_class_type_flags_to_name;int type_flags;type_flags
diff --git a/gdb/target.h b/gdb/target.h
index b29d34b8a65..229b5d0551a 100644
--- a/gdb/target.h
+++ b/gdb/target.h
@@ -1905,6 +1905,40 @@ extern struct thread_info *target_thread_handle_to_thread_info
 
 /* Hardware watchpoint interfaces.  */
 
+/* GDB's current model is that there are three "kinds" of watchpoints,
+   with respect to when they trigger and how you can move past them.
+
+   Those are: continuable, steppable, and non-steppable.
+
+   Continuable watchpoints are like x86's -- those trigger after the
+   memory access's side effects are fully committed to memory.  I.e.,
+   they trap with the PC pointing at the next instruction already.
+   Continuing past such a watchpoint is doable by just normally
+   continuing, hence the name.
+
+   Both steppable and non-steppable watchpoints trap before the memory
+   access.  I.e, the PC points at the instruction that is accessing
+   the memory.  So GDB needs to single-step once past the current
+   instruction in order to make the access effective and check whether
+   the instruction's side effects change the watched expression.
+
+   Now, in order to step past that instruction, depending on
+   architecture and target, you can have two situations:
+
+   - steppable watchpoints: you can single-step with the watchpoint
+     still armed, and the watchpoint won't trigger again.
+
+   - non-steppable watchpoints: if you try to single-step with the
+     watchpoint still armed, you'd trap the watchpoint again and the
+     thread wouldn't make any progress.  So GDB needs to temporarily
+     remove the watchpoint in order to step past it.
+
+   If your target/architecture does not signal that it has either
+   steppable or non-steppable watchpoints via either
+   target_have_steppable_watchpoint or
+   gdbarch_have_nonsteppable_watchpoint, GDB assumes continuable
+   watchpoints.  */
+
 /* Returns non-zero if we were stopped by a hardware watchpoint (memory read or
    write).  Only the INFERIOR_PTID task is being queried.  */
 
-- 
2.14.4

Re: [PATCH 2/2] Add comment describing continuable/steppable/non-steppable watchpoints

[Tom Tromey]

>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:

Pedro> These weren't described anywhere in the sources.

This is excellent, thank you very much for doing this.

Pedro> +# See comment in target.h about continue, steppable and non-steppable watchpoints.

Probably "continuable" here as well.

Tom

Re: [PATCH 2/2] Add comment describing continuable/steppable/non-steppable watchpoints

[Pedro Alves]

On 08/31/2018 04:40 PM, Tom Tromey wrote:
>>>>>> "Pedro" == Pedro Alves <palves@redhat.com> writes:
> 
> Pedro> These weren't described anywhere in the sources.
> 
> This is excellent, thank you very much for doing this.
> 
> Pedro> +# See comment in target.h about continue, steppable and non-steppable watchpoints.
> 
> Probably "continuable" here as well.

Whoops, yes.  Pushed with that fixed.

Thanks,
Pedro Alves