Re: [3/3] [PATCH] value_optimized_out and value_fetch_lazy

[Pedro Alves <palves at redhat dot com>]

On 07/01/2013 07:06 PM, Andrew Burgess wrote:
> On 19/06/2013 4:50 PM, Pedro Alves wrote:
>> On 06/10/2013 11:24 AM, Andrew Burgess wrote:
>>
>>> I ran into an issue with gdb that appears to be caused by an incorrect
>>> use of value_optimized_out.
>>>
>>
>> I'm finding the patch a bit hard to read though.  Could you
>> split it up?  
> 
> This tiny patch notices that when we mark a value as optimized out
> we can also mark the value as no longer lazy.
> This patch is not required, but felt like a good thing to me, not
> sure if everyone will agree though.
> 
> Should I apply?
> 
> Thanks
> Andrew
> 
> 
> 
> 
> gdb/ChangeLog
> 
> 2013-07-01  Andrew Burgess  <aburgess@broadcom.com>
> 
> 	* value.c (set_value_optimized_out): A value that is optimized out
> 	is no longer lazy.

Hmm.

I tried going a step further.  If the value is optimized
out, then we don't really need its contents buffer.

That is:

 gdb/value.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/gdb/value.c b/gdb/value.c
index 970fb66..50f8889 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -690,6 +690,8 @@ allocate_value_lazy (struct type *type)
 void
 allocate_value_contents (struct value *val)
 {
+  gdb_assert (!val->optimized_out);
+
   if (!val->contents)
     val->contents = (gdb_byte *) xzalloc (TYPE_LENGTH (val->enclosing_type));
 }
@@ -1064,6 +1066,13 @@ void
 set_value_optimized_out (struct value *value, int val)
 {
   value->optimized_out = val;
+
+  if (val)
+    {
+      set_value_lazy (value, 0);
+      xfree (value->contents);
+      value->contents = NULL;
+    }
 }

and then ran the testsuite.  Sure enough, that catches a gdb crash.
In gdb.dwarf2/dw2-op-out-param.exp:

#0  0x000000000056bed0 in extract_signed_integer (addr=0x0, len=8, byte_order=BFD_ENDIAN_LITTLE) at ../../src/gdb/findvar.c:78
#1  0x000000000057e761 in unpack_long (type=0x1793f80, valaddr=0x0) at ../../src/gdb/value.c:2453
#2  0x000000000059baae in print_scalar_formatted (valaddr=0x0, type=0x1793f80, options=0x7fffdb2376c0, size=0, stream=0x1929a00) at ../../src/gdb/printcmd.c:404
#3  0x000000000059763b in val_print_scalar_formatted (type=0x1793f80, valaddr=0x0, embedded_offset=0, val=0x19dfcd0, options=0x7fffdb2376c0, size=0, stream=0x1929a00)
    at ../../src/gdb/valprint.c:970
#4  0x00000000006d1f2f in c_val_print (type=0x1793f80, valaddr=0x0, embedded_offset=0, address=9225056, stream=0x1929a00, recurse=3, original_value=0x19dfcd0, options=
    0x7fffdb237860) at ../../src/gdb/c-valprint.c:407
#5  0x0000000000596f4a in val_print (type=0x1793f80, valaddr=0x0, embedded_offset=0, address=9225056, stream=0x1929a00, recurse=3, val=0x19dfcd0, options=0x7fffdb237940,
    language=0x8d34a0) at ../../src/gdb/valprint.c:778
#6  0x00000000006d3491 in cp_print_value_fields (type=0x1796110, real_type=0x1796110, valaddr=0x0, offset=0, address=9225056, stream=0x1929a00, recurse=2, val=0x19dfcd0,
    options=0x7fffdb237d40, dont_print_vb=0x0, dont_print_statmem=0) at ../../src/gdb/cp-valprint.c:363
#7  0x00000000006d37ef in cp_print_value_fields_rtti (type=0x1796110, valaddr=0x0, offset=0, address=9225056, stream=0x1929a00, recurse=2, val=0x19dfcd0, options=
    0x7fffdb237d40, dont_print_vb=0x0, dont_print_statmem=0) at ../../src/gdb/cp-valprint.c:458
#8  0x00000000006d1e45 in c_val_print (type=0x1796110, valaddr=0x0, embedded_offset=0, address=9225056, stream=0x1929a00, recurse=2, original_value=0x19dfcd0, options=
    0x7fffdb237d40) at ../../src/gdb/c-valprint.c:393
#9  0x0000000000596f4a in val_print (type=0x1796110, valaddr=0x0, embedded_offset=0, address=9225056, stream=0x1929a00, recurse=2, val=0x19dfcd0, options=0x7fffdb237e60,
    language=0x8d34a0) at ../../src/gdb/valprint.c:778
#10 0x0000000000597157 in common_val_print (val=0x19dfcd0, stream=0x1929a00, recurse=2, options=0x7fffdb237e60, language=0x8d34a0) at ../../src/gdb/valprint.c:840
#11 0x00000000005d9ea6 in print_frame_arg (arg=0x7fffdb237f30) at ../../src/gdb/stack.c:284
#12 0x00000000005daa49 in print_frame_args (func=0x1796af0, frame=0x1b5b800, num=-1, stream=0x168fd10) at ../../src/gdb/stack.c:645
#13 0x00000000005db9df in print_frame (frame=0x1b5b800, print_level=1, print_what=LOCATION, print_args=1, sal=...) at ../../src/gdb/stack.c:1172
#14 0x00000000005daf95 in print_frame_info (frame=0x1b5b800, print_level=1, print_what=LOCATION, print_args=1) at ../../src/gdb/stack.c:824
#15 0x00000000005dcdc5 in backtrace_command_1 (count_exp=0x0, show_locals=0, no_filters=0, from_tty=1) at ../../src/gdb/stack.c:1763
#16 0x00000000005dd1b6 in backtrace_command (arg=0x0, from_tty=1) at ../../src/gdb/stack.c:1860
#17 0x00000000004dc53f in do_cfunc (c=0x151bec0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:113
#18 0x00000000004df5d4 in cmd_func (cmd=0x151bec0, args=0x0, from_tty=1) at ../../src/gdb/cli/cli-decode.c:1888
#19 0x00000000006e43d1 in execute_command (p=0x14623a2 "", from_tty=1) at ../../src/gdb/top.c:478

Odd that this bit of val_print_scalar_formatted:

  /* A scalar object that does not have all bits available can't be
     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);

didn't catch this.  I then added this further.

--- c/gdb/value.c
+++ w/gdb/value.c
@@ -1078,6 +1078,8 @@ set_value_optimized_out (struct value *value, int val)
 int
 value_entirely_optimized_out (const struct value *value)
 {
+  gdb_assert (value->optimized_out || !value->lazy);
+
   if (!value->optimized_out)
     return 0;
   if (value->lval != lval_computed
@@ -1089,6 +1091,8 @@ value_entirely_optimized_out (const struct value *value)
 int
 value_bits_valid (const struct value *value, int offset, int length)
 {
+  gdb_assert (value->optimized_out || !value->lazy);
+
   if (!value->optimized_out)
     return 1;
   if (value->lval != lval_computed

And I see:

Breakpoint 2, 0x000000000040058f in breakpt ()
(gdb) PASS: gdb.dwarf2/dw2-op-out-param.exp: continue to breakpoint: Stop in breakpt for struct_param_two_reg_pieces
bt
#0  0x000000000040058f in breakpt ()
#1  0x00000000004005c2 in struct_param_two_reg_pieces (operand0=../../src/gdb/value.c:1081: internal-error: value_entirely_optimized_out: Assertion `value->optimized_out || !value->lazy' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n) FAIL: gdb.dwarf2/dw2-op-out-param.exp: Backtrace for test struct_param_two_reg_pieces (GDB internal error)
Resyncing due to internal error.

... and then I ran out of time.  Gotta run an errand.  :-)

-- 
Pedro Alves