Re: [PATCH+DOC] Print registers not saved in the frame as "", instead of "".

Pedro Alves
2013-10-02T16:17:41Z

FYI, I've now applied this patch, as below:

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

Currently, in some scenarios, GDB prints <optimized out> when printing
outer frame registers. An <optimized out> register is a confusing
concept. What this really means is that the register is
call-clobbered, or IOW, not saved by the callee. This patch makes GDB
say that instead.

Before patch:

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

After patch:

(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>
when printing the variable. IOW, <not saved> is reserved for
inspecting registers at the machine level. The patch uses
lval_register+optimized_out to encode the not saved registers, and
makes it so that optimized out variables always end up in
!lval_register values.

** See <https://sourceware.org/ml/gdb-patches/2012-08/msg00787.html>.
Current/recent enough GCC doesn't mark variables/arguments as being in
call-clobbered registers in the ranges corresponding to function
calls, while older GCCs did. Newer GCCs will just not say where the
variable is, so GDB will end up realizing the variable is optimized
out.

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/
2013-10-02 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)
<DWARF_VALUE_REGISTER>: 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.
* NEWS: Mention <not saved>.

gdb/testsuite/
2013-10-02 Pedro Alves <palves@redhat.com>

* gdb.dwarf2/dw2-reg-undefined.exp <pattern_rax_rbx_rcx_print,
pattern_rax_rbx_rcx_info>: Set to "<not saved>".
* gdb.mi/mi-reg-undefined.exp (opt_out_pattern): Delete.
(not_saved_pattern): New.
Replace use of the former with the latter.

gdb/doc/
2013-10-02 Pedro Alves <palves@redhat.com>

* gdb.texinfo (Registers): Expand description of saved registers
in frames. Explain <not saved>.
---

gdb/NEWS | 14 +++++++++++
gdb/cp-valprint.c | 4 ++-
gdb/doc/gdb.texinfo | 31 +++++++++++++++++++++---
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 ++++
16 files changed, 111 insertions(+), 30 deletions(-)

diff --git a/gdb/NEWS b/gdb/NEWS
index af06a21..c0fe72a 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -15,6 +15,20 @@

* The "catch syscall" command now works on arm*-linux* targets.

+* GDB now consistently shows "<not saved>" when printing values of
+ registers the debug info indicates have not been saved in the frame
+ and there's nowhere to retrieve them from
+ (callee-saved/call-clobbered registers):
+
+ (gdb) p $rax
+ $1 = <not saved>
+
+ (gdb) info registers rax
+ rax <not saved>
+
+ Before, the former would print "<optimized out>", and the latter
+ "*value not available*".
+
* Python scripting

** Frame filters and frame decorators have been added.
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/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 60d2877..ffee6e1 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -10031,10 +10031,33 @@ were exited and their saved registers restored. In order to see the
true contents of hardware registers, you must select the innermost
frame (with @samp{frame 0}).

-However, @value{GDBN} must deduce where registers are saved, from the machine
-code generated by your compiler. If some registers are not saved, or if
-@value{GDBN} is unable to locate the saved registers, the selected stack
-frame makes no difference.
+@cindex caller-saved registers
+@cindex call-clobbered registers
+@cindex volatile registers
+@cindex <not saved> values
+Usually ABIs reserve some registers as not needed to be saved by the
+callee (a.k.a.: ``caller-saved'', ``call-clobbered'' or ``volatile''
+registers). It may therefore not be possible for @value{GDBN} to know
+the value a register had before the call (in other words, in the outer
+frame), if the register value has since been changed by the callee.
+@value{GDBN} tries to deduce where the inner frame saved
+(``callee-saved'') registers, from the debug info, unwind info, or the
+machine code generated by your compiler. If some register is not
+saved, and @value{GDBN} knows the register is ``caller-saved'' (via
+its own knowledge of the ABI, or because the debug/unwind info
+explicitly says the register's value is undefined), @value{GDBN}
+displays @w{@samp{<not saved>}} as the register's value. With targets
+that @value{GDBN} has no knowledge of the register saving convention,
+if a register was not saved by the callee, then its value and location
+in the outer frame are assumed to be the same of the inner frame.
+This is usually harmless, because if the register is call-clobbered,
+the caller either does not care what is in the register after the
+call, or has code to restore the value that it does care about. Note,
+however, that if you change such a register in the outer frame, you
+may also be affecting the inner frame. Also, the more ``outer'' the
+frame is you're looking at, the more likely a call-clobbered
+register's value is to be wrong, in the sense that it doesn't actually
+represent the value the register had just before the call.

@node Floating Point Hardware
@section Floating Point Hardware
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 e29dcde..46102a5 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 e8c4744..5cde1ae 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 bc1239d..d96d285 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