[PATCH] Print registers not saved in the frame as "<not saved>", instead of "<optimized out>". (was: Re: [PATCH] Consistent display of "<optimized out>")

[Pedro Alves <palves at redhat dot com>]

Hi guys,

Getting back to this, trying to make progress.

On 08/19/2013 11:24 AM, Andrew Burgess wrote:
> On 16/08/2013 7:41 PM, Pedro Alves wrote:
>> On 08/12/2013 09:01 PM, Mark Kettenis wrote:
>>>> Date: Mon, 12 Aug 2013 14:55:04 +0100
>>>> From: Pedro Alves <palves@redhat.com>
>>>>
>>>> On 08/12/2013 02:31 PM, Andrew Burgess wrote:
>>>>> On 06/08/2013 7:39 PM, Pedro Alves wrote:
>>>>>> On 08/06/2013 04:41 PM, Mark Kettenis wrote:
>>>>>>>> Date: Tue, 6 Aug 2013 14:49:03 +0100
>>>>>>>> From: "Andrew Burgess" <aburgess@broadcom.com>
>>>>>>
>>>>>>>> 3. My understanding was that values lost due to the ABI of a call site
>>>>>>>> were recorded as optimized out.  For evidence I would present
>>>>>>>> dwarf2_frame_prev_register, and how DWARF2_FRAME_REG_UNDEFINED is handled.
>>>>>>>>
>>>>>>>> For these reasons I believe my patch should still be considered, what do
>>>>>>>> you think?
>>>>>>>
>>>>>>> I think that registers are either available or unavailble.  A register
>>>>>>> being unavailble implies that a variable that is supposed to live in
>>>>>>> such a register may have been optimized out.  Whether GDB's pseudo
>>>>>>> variables that respresent registers are considered unavailable or
>>>>>>> optimized out in that case is arguable.
>>>>>>
>>>>>> I think improving consistency as in Andrew's patch is good.
>>>>>
>>>>> Given almost a week has passed with no further feedback I plan to
>>>>> commit this patch tomorrow unless there's any further discussion to be had.
>>>>
>>>> TBC, note my opinion doesn't get to overrule Mark's.  Consensus
>>>> works much better, and Mark does have deep knowledge of all
>>>> ABI/pseudo registers/etc. gdb things.
>>>> That said, Mark, if you still disagree, please counter argue,
>>>> otherwise, we'll just have to assume you do agree with the
>>>> rationales and clarifications.
>>>
>>> Can't say I agree.  It simply doesn't make sense for registers to be
>>> "optimized out".  I guess there are two reasons why GDB can't display
>>> the contents of a register in a frame:
>>>
>>> 1. The register contents aren't made available by the debugging
>>>    interface, i.e. ptrace(2) or the remote stub doesn't tell us.
>>>
>>> 2. The register wasn't saved before calling another function.
>>>
>>> I guess after Andrew's chnages 1) would be shown as <unavailable> and
>>> 2) would become <optimized out>.  But in the latter case something
>>> like <not saved> would make more sense.
>>>
>>> That said, Pedro, you're pretty much the expert for this area of GDB.
>>> So If you think Andrew should go ahead with this, feel free to ignore
>>> me.
>>
>> This is a tough call.  I do agree that "optimized out" for registers
>> is a bit confusing.  However, we already do print "<optimized out>" in
>> other places, such as when printing expressions, and consistency
>> is good.  If we did add a distinction, I agree with Andrew that it should
>> be done in a more systematic way.  However, I'm not really sure we need
>> much machinery.  Wouldn't something like:
>>
>> void
>> val_print_optimized_out (const struct value *val, struct ui_file *stream)
>> {
>>   if (value_lval_const (val) == lval_register)
>>     fprintf_filtered (stream, _("<not saved>"));
>>   else
>>     fprintf_filtered (stream, _("<optimized out>"));
>> }
>>
>> work?  What could be the register value cases that would print
>> "not saved" that we'd still want to print "optimized out" ?
> 
> The only case I can immediately think of where this would cause a
> problem would be for computed locations, (lval_computed).  The easy
> answer would be (in that case) the blame the compiler - why say the
> location is in a register if that register is volatile - but sadly I see
> this way too often.

Hmm, OK, but then lval_computed values with that change won't
ever show "<not saved>", due to the lval_register check.  IOW,
we'd have to do something else in addition to lval_computed values
to make them print something other than the current <optimized out>.

However, I've come to think there's a really simple rule to
follow here -- We should only ever print <not saved> for values
that represent machine/pseudo registers.  IOW, $pc, $rax, etc.
If the debug info happens to describe a variable as being located
in some optimized out register, we should still print
<optimized out>.  The previous version of the patch failed that:

 (gdb) PASS: gdb.dwarf2/dw2-op-out-param.exp: continue to breakpoint: Stop in breakpt for test int_param_single_reg_loc
  bt
  #0  0x000000000040058f in breakpt ()
 -#1  0x00000000004005a2 in int_param_single_reg_loc (operand0=<optimized out>, operand1=0xdeadbe00deadbe01, operand2=<optimized out>)
 +#1  0x00000000004005a2 in int_param_single_reg_loc (operand0=<not saved>, operand1=0xdeadbe00deadbe01, operand2=<not saved>)
  #2  0x0000000000400577 in main ()

It didn't really make a lot of sense.  This new version doesn't have
that change anymore.

That simple rule suggests that whatever the internal representation,
we should be easily able to have a single central point where to tag
such values.  In fact, I think that already exists in value_of_register.

> However, exchanging what I see as the current larger inconsistency, for
> this much smaller one seems like a good deal to me, especially if it
> gets this patch unblocked...

Alright, what do you (all) think of of this (supposedly finished) patch
on top of yours (Andrew's) then?

Force-pushed here (along with Andrew's patch) for convenience:
 https://github.com/palves/gdb/commits/register_not_saved
 git@github.com:palves/gdb.git register_not_saved

--------------
Subject: [PATCH] Print registers not saved in the frame as "<not saved>"
 instead of "<optimized out>".

Currently we print not saved registers as <optimized out>.  That's
confusing, because what this really means is that the register was not
saved in the frame before calling another function.

Before:

 (gdb) p/x $rax $1 = <optimized out>
 (gdb) info registers rax
 rax            <optimized out>

After:

 (gdb) p/x $rax
 $1 = <not saved>
 (gdb) info registers rax
 rax            <not saved>

However, if for some reason the debug info describes a variable as being
in such a register, we still want to print <optimized out>.  IOW, <not
saved> is reserved for inspecting registers at the machine level.
The patch uses lval_register+optimized_out to encode the latter.

frame_unwind_got_optimized creates not_lval optimized out registers,
so by default, in most cases, we'll see <optimized out>.

value_of_register is the function eval.c uses for evaluating
OP_REGISTER (again, $pc, etc.), and related bits.  It isn't used for
anything else.  This function makes sure to return lval_register
values.  The patch makes "info registers" and the MI equivalent use it
too.  I think it just makes a lot of sense, as this makes it so that
when printing machine registers ($pc, etc.), we go through a central
function.

We're likely to need a different encoding at some point, if/when we
support partially saved registers.  Even then, I think
value_of_register will still be the spot to tag the intention to print
machine register values differently.

value_from_register however may also return optimized out
lval_register values, so at a couple places where we're computing a
variable's location from a dwarf expression, we convert the resulting
value away from lval_register to a regular optimized out value.

Tested on x86_64 Fedora 17

gdb/
2013-09-05  Pedro Alves  <palves@redhat.com>

	* cp-valprint.c (cp_print_value_fields): Adjust calls to
	val_print_optimized_out.
	* jv-valprint.c (java_print_value_fields): Likewise.
	* p-valprint.c (pascal_object_print_value_fields): Likewise.
	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full) <xxxxxx>: If the register
	was not saved, return a new optimized out value.
	* findvar.c (address_from_register): Likewise.
	* frame.c (put_frame_register): Tweak error string to say the
	register was not saved, rather than optimized out.
	* infcmd.c (default_print_one_register_info): Adjust call to
	val_print_optimized_out.  Use value_of_register instead of
	get_frame_register_value.
	* mi/mi-main.c (output_register): Use value_of_register instead of
	get_frame_register_value.
	* valprint.c (valprint_check_validity): Likewise.
	(val_print_optimized_out): New value parameter.  If the value is
	lval_register, print <not saved> instead.
	(value_check_printable, val_print_scalar_formatted): Adjust calls
	to val_print_optimized_out.
	* valprint.h (val_print_optimized_out): New value parameter.
	* value.c (struct value) <optimized_out>: Extend comment.
	(error_value_optimized_out): New function.
	(require_not_optimized_out): Use it.  Use a different string for
	lval_register values.
	* value.h (error_value_optimized_out): New declaration.

gdb/testsuite/
2013-09-05  Pedro Alves  <palves@redhat.com>

	* gdb.mi/mi-reg-undefined.exp (opt_out_pattern): Delete.
	(not_saved_pattern): New.
	Replace uses for the former with the latter.
---
 gdb/cp-valprint.c                              |  4 ++--
 gdb/dwarf2loc.c                                | 16 +++++++++++++---
 gdb/findvar.c                                  |  9 +++++++++
 gdb/frame.c                                    |  2 +-
 gdb/infcmd.c                                   |  4 ++--
 gdb/jv-valprint.c                              |  4 ++--
 gdb/mi/mi-main.c                               |  2 +-
 gdb/p-valprint.c                               |  4 ++--
 gdb/testsuite/gdb.dwarf2/dw2-reg-undefined.exp |  4 ++--
 gdb/testsuite/gdb.mi/mi-reg-undefined.exp      |  4 ++--
 gdb/valprint.c                                 | 13 ++++++++-----
 gdb/valprint.h                                 |  3 ++-
 gdb/value.c                                    | 22 +++++++++++++++++++---
 gdb/value.h                                    |  5 +++++
 14 files changed, 70 insertions(+), 26 deletions(-)

diff --git a/gdb/cp-valprint.c b/gdb/cp-valprint.c
index e83d979..1d7147c 100644
--- a/gdb/cp-valprint.c
+++ b/gdb/cp-valprint.c
@@ -298,7 +298,7 @@ cp_print_value_fields (struct type *type, struct type *real_type,
 					  TYPE_FIELD_BITPOS (type, i),
 					  TYPE_FIELD_BITSIZE (type, i)))
 		{
-		  val_print_optimized_out (stream);
+		  val_print_optimized_out (val, stream);
 		}
 	      else
 		{
@@ -334,7 +334,7 @@ cp_print_value_fields (struct type *type, struct type *real_type,
 				      _("<error reading variable: %s>"),
 				      ex.message);
 		  else if (v == NULL)
-		    val_print_optimized_out (stream);
+		    val_print_optimized_out (NULL, stream);
 		  else
 		    cp_print_static_field (TYPE_FIELD_TYPE (type, i),
 					   v, stream, recurse + 1,
diff --git a/gdb/dwarf2loc.c b/gdb/dwarf2loc.c
index 55d43f1..1313e17 100644
--- a/gdb/dwarf2loc.c
+++ b/gdb/dwarf2loc.c
@@ -2290,11 +2290,21 @@ dwarf2_evaluate_loc_desc_full (struct type *type, struct frame_info *frame,
 	    if (byte_offset != 0)
 	      error (_("cannot use offset on synthetic pointer to register"));
 	    do_cleanups (value_chain);
-	    if (gdb_regnum != -1)
-	      retval = value_from_register (type, gdb_regnum, frame);
-	    else
+	   if (gdb_regnum == -1)
 	      error (_("Unable to access DWARF register number %d"),
 		     dwarf_regnum);
+	   retval = value_from_register (type, gdb_regnum, frame);
+	   if (value_optimized_out (retval))
+	     {
+	       /* This means the register has undefined value / was
+		  not saved.  As we're computing the location of some
+		  variable etc. in the program, not a value for
+		  inspecting a register ($pc, $sp, etc.), return a
+		  generic optimized out value instead, so that we show
+		  <optimized out> instead of <not saved>.  */
+	       do_cleanups (value_chain);
+	       retval = allocate_optimized_out_value (type);
+	     }
 	  }
 	  break;
 
diff --git a/gdb/findvar.c b/gdb/findvar.c
index 25242be..c3550b4 100644
--- a/gdb/findvar.c
+++ b/gdb/findvar.c
@@ -757,6 +757,15 @@ address_from_register (struct type *type, int regnum, struct frame_info *frame)
   value = value_from_register (type, regnum, frame);
   gdb_assert (value);
 
+  if (value_optimized_out (value))
+    {
+      /* This function is used while computing a location expression.
+	 Complain about the value being optimized out, rather than
+	 letting value_as_address complain about some random register
+	 the expression depends on not being saved.  */
+      error_value_optimized_out ();
+    }
+
   result = value_as_address (value);
   release_value (value);
   value_free (value);
diff --git a/gdb/frame.c b/gdb/frame.c
index d52c26a..be1a1b1 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -1140,7 +1140,7 @@ put_frame_register (struct frame_info *frame, int regnum,
   frame_register (frame, regnum, &optim, &unavail,
 		  &lval, &addr, &realnum, NULL);
   if (optim)
-    error (_("Attempt to assign to a value that was optimized out."));
+    error (_("Attempt to assign to a register that was not saved."));
   switch (lval)
     {
     case lval_memory:
diff --git a/gdb/infcmd.c b/gdb/infcmd.c
index f6a5290..2bba49e 100644
--- a/gdb/infcmd.c
+++ b/gdb/infcmd.c
@@ -2035,7 +2035,7 @@ default_print_one_register_info (struct ui_file *file,
     }
   else if (value_optimized_out (val))
     {
-      val_print_optimized_out (file);
+      val_print_optimized_out (val, file);
       fprintf_filtered (file, "\n");
       return;
     }
@@ -2142,7 +2142,7 @@ default_print_registers_info (struct gdbarch *gdbarch,
 
       default_print_one_register_info (file,
 				       gdbarch_register_name (gdbarch, i),
-				       get_frame_register_value (frame, i));
+				       value_of_register (i, frame));
     }
 }
 
diff --git a/gdb/jv-valprint.c b/gdb/jv-valprint.c
index 3b90e54..cb89a85 100644
--- a/gdb/jv-valprint.c
+++ b/gdb/jv-valprint.c
@@ -395,7 +395,7 @@ java_print_value_fields (struct type *type, const gdb_byte *valaddr,
 	      else if (!value_bits_valid (val, TYPE_FIELD_BITPOS (type, i),
 					  TYPE_FIELD_BITSIZE (type, i)))
 		{
-		  val_print_optimized_out (stream);
+		  val_print_optimized_out (val, stream);
 		}
 	      else
 		{
@@ -420,7 +420,7 @@ java_print_value_fields (struct type *type, const gdb_byte *valaddr,
 		  struct value *v = value_static_field (type, i);
 
 		  if (v == NULL)
-		    val_print_optimized_out (stream);
+		    val_print_optimized_out (NULL, stream);
 		  else
 		    {
 		      struct value_print_options opts;
diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
index e6e98b6..503b7bf 100644
--- a/gdb/mi/mi-main.c
+++ b/gdb/mi/mi-main.c
@@ -1161,7 +1161,7 @@ output_register (struct frame_info *frame, int regnum, int format,
 {
   struct gdbarch *gdbarch = get_frame_arch (frame);
   struct ui_out *uiout = current_uiout;
-  struct value *val = get_frame_register_value (frame, regnum);
+  struct value *val = value_of_register (regnum, frame);
   struct cleanup *tuple_cleanup;
   struct value_print_options opts;
   struct ui_file *stb;
diff --git a/gdb/p-valprint.c b/gdb/p-valprint.c
index 05d4c6f..e6d4b91 100644
--- a/gdb/p-valprint.c
+++ b/gdb/p-valprint.c
@@ -629,7 +629,7 @@ pascal_object_print_value_fields (struct type *type, const gdb_byte *valaddr,
 	      else if (!value_bits_valid (val, TYPE_FIELD_BITPOS (type, i),
 					  TYPE_FIELD_BITSIZE (type, i)))
 		{
-		  val_print_optimized_out (stream);
+		  val_print_optimized_out (val, stream);
 		}
 	      else
 		{
@@ -657,7 +657,7 @@ pascal_object_print_value_fields (struct type *type, const gdb_byte *valaddr,
 		  v = value_field_bitfield (type, i, valaddr, offset, val);
 
 		  if (v == NULL)
-		    val_print_optimized_out (stream);
+		    val_print_optimized_out (NULL, stream);
 		  else
 		    pascal_object_print_static_field (v, stream, recurse + 1,
 						      options);
diff --git a/gdb/testsuite/gdb.dwarf2/dw2-reg-undefined.exp b/gdb/testsuite/gdb.dwarf2/dw2-reg-undefined.exp
index 4686648..44bf8e9 100644
--- a/gdb/testsuite/gdb.dwarf2/dw2-reg-undefined.exp
+++ b/gdb/testsuite/gdb.dwarf2/dw2-reg-undefined.exp
@@ -45,8 +45,8 @@ for {set f 0} {$f < 3} {incr f} {
 	set pattern_r8_r9_print "$hex"
 	set pattern_r8_r9_info "$hex\\s+$decimal"
     } else {
-	set pattern_rax_rbx_rcx_print "<optimized out>"
-	set pattern_rax_rbx_rcx_info "<optimized out>"
+	set pattern_rax_rbx_rcx_print "<not saved>"
+	set pattern_rax_rbx_rcx_info "<not saved>"
 	set pattern_r8_r9_print "$hex"
 	set pattern_r8_r9_info "$hex\\s+$decimal"
     }
diff --git a/gdb/testsuite/gdb.mi/mi-reg-undefined.exp b/gdb/testsuite/gdb.mi/mi-reg-undefined.exp
index 8bcbb21..cb3a716 100644
--- a/gdb/testsuite/gdb.mi/mi-reg-undefined.exp
+++ b/gdb/testsuite/gdb.mi/mi-reg-undefined.exp
@@ -52,13 +52,13 @@ mi_gdb_test "111-stack-list-frames" \
     "111\\^done,stack=\\\[frame=\{level=\"0\",addr=\"$hex\",func=\"stop_frame\",.*\},frame=\{level=\"1\",addr=\"$hex\",func=\"first_frame\",.*\},frame=\{level=\"2\",addr=\"$hex\",func=\"main\",.*\}\\\]" \
     "stack frame listing"
 
-set opt_out_pattern "<optimized out>"
+set not_saved_pattern "<not saved>"
 
 for {set f 0} {$f < 3} {incr f} {
     if {${f} == 0} {
 	set pattern_0_1_2 ${hex}
     } else {
-	set pattern_0_1_2 ${opt_out_pattern}
+	set pattern_0_1_2 ${not_saved_pattern}
     }
 
     mi_gdb_test "22${f}-data-list-register-values --thread 1 --frame ${f} x  0 1 2 8 9" \
diff --git a/gdb/valprint.c b/gdb/valprint.c
index 0f6d65e..61492cc 100644
--- a/gdb/valprint.c
+++ b/gdb/valprint.c
@@ -314,7 +314,7 @@ valprint_check_validity (struct ui_file *stream,
       if (!value_bits_valid (val, TARGET_CHAR_BIT * embedded_offset,
 			     TARGET_CHAR_BIT * TYPE_LENGTH (type)))
 	{
-	  val_print_optimized_out (stream);
+	  val_print_optimized_out (val, stream);
 	  return 0;
 	}
 
@@ -336,9 +336,12 @@ valprint_check_validity (struct ui_file *stream,
 }
 
 void
-val_print_optimized_out (struct ui_file *stream)
+val_print_optimized_out (const struct value *val, struct ui_file *stream)
 {
-  fprintf_filtered (stream, _("<optimized out>"));
+  if (val != NULL && value_lval_const (val) == lval_register)
+    fprintf_filtered (stream, _("<not saved>"));
+  else
+    fprintf_filtered (stream, _("<optimized out>"));
 }
 
 void
@@ -805,7 +808,7 @@ value_check_printable (struct value *val, struct ui_file *stream,
       if (options->summary && !val_print_scalar_type_p (value_type (val)))
 	fprintf_filtered (stream, "...");
       else
-	val_print_optimized_out (stream);
+	val_print_optimized_out (val, stream);
       return 0;
     }
 
@@ -966,7 +969,7 @@ val_print_scalar_formatted (struct type *type,
      printed, because all bits contribute to its representation.  */
   if (!value_bits_valid (val, TARGET_CHAR_BIT * embedded_offset,
 			      TARGET_CHAR_BIT * TYPE_LENGTH (type)))
-    val_print_optimized_out (stream);
+    val_print_optimized_out (val, stream);
   else if (!value_bytes_available (val, embedded_offset, TYPE_LENGTH (type)))
     val_print_unavailable (stream);
   else
diff --git a/gdb/valprint.h b/gdb/valprint.h
index e7073b6..1d86ed7 100644
--- a/gdb/valprint.h
+++ b/gdb/valprint.h
@@ -160,7 +160,8 @@ extern int read_string (CORE_ADDR addr, int len, int width,
 			enum bfd_endian byte_order, gdb_byte **buffer,
 			int *bytes_read);
 
-extern void val_print_optimized_out (struct ui_file *stream);
+extern void val_print_optimized_out (const struct value *val,
+				     struct ui_file *stream);
 
 extern void val_print_unavailable (struct ui_file *stream);
 
diff --git a/gdb/value.c b/gdb/value.c
index 42a8d2f..871b57c 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -197,8 +197,13 @@ struct value
      reset, be sure to consider this use as well!  */
   unsigned int lazy : 1;
 
-  /* If nonzero, this is the value of a variable which does not
-     actually exist in the program.  */
+  /* If nonzero, this is the value of a variable that does not
+     actually exist in the program.  If nonzero, and LVAL is
+     lval_register, this is a register ($pc, $sp, etc., never a
+     program variable) that has not been saved in the frame.  All
+     optimized-out values are treated pretty much the same, except
+     registers have a different string representation and related
+     error strings.  */
   unsigned int optimized_out : 1;
 
   /* If value is a variable, is it initialized or not.  */
@@ -902,11 +907,22 @@ value_actual_type (struct value *value, int resolve_simple_types,
   return result;
 }
 
+void
+error_value_optimized_out (void)
+{
+  error (_("value has been optimized out"));
+}
+
 static void
 require_not_optimized_out (const struct value *value)
 {
   if (value->optimized_out)
-    error (_("value has been optimized out"));
+    {
+      if (value->lval == lval_register)
+	error (_("register has not been saved in frame"));
+      else
+	error_value_optimized_out ();
+    }
 }
 
 static void
diff --git a/gdb/value.h b/gdb/value.h
index 98dbadf..db964e3 100644
--- a/gdb/value.h
+++ b/gdb/value.h
@@ -273,6 +273,11 @@ extern void set_value_lazy (struct value *value, int val);
 extern int value_stack (struct value *);
 extern void set_value_stack (struct value *value, int val);
 
+/* Throw an error complaining that the value has been optimized
+   out.  */
+
+extern void error_value_optimized_out (void);
+
 /* value_contents() and value_contents_raw() both return the address
    of the gdb buffer used to hold a copy of the contents of the lval.
    value_contents() is used when the contents of the buffer are needed
-- 
1.7.11.7