[Bug middle-end/54218] New: Debug info for function parameters is incorrect when compiled with -O0

[Bug middle-end/54218] New: Debug info for function parameters is incorrect when compiled with -O0

[senthil_kumar.selvaraj at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

             Bug #: 54218
           Summary: Debug info for function parameters is incorrect when
                    compiled with -O0
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: senthil_kumar.selvaraj@atmel.com


Created attachment 27980
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27980
Failing dejagnu test case

When compiling a function, like below, with no optimizations (-O0), on a x86_64
target (also occurs for atleast one other target (avr))

void func(int p)
{
    p = 0; 
    p = 32;
}

wrong debug information is emitted for the function parameter (p), that cause
the debugger to keep showing the value of the actual argument, even after p is
overwritten. A failing dejagnu test is attached.

Based on a preliminary analysis, the actual problem seems to be that stack
space for a function parameter (that is not used??) is allocated twice when
gimple_expand_cfg runs - once when expand_used_vars runs, and later when
assign_parm_setup_stack runs. expand_used_vars allocates stack space despite
the check for a default definition being present in the partition, because the
function parameter node (which I guess should be the default definition) is not
present when iterating over num_ssa_names.

[Bug middle-end/54218] Debug info for function parameters is incorrect when compiled with -O0

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |wrong-debug
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-08-10
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-08-10 08:53:46 UTC ---
That's because the actual parameter value is not used:

func (int p)
{
;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  p_1 = 0;
  p_2 = 32;
  return;

Partition 0 (p_1 - 1 2 )


Does -fvar-tracking "fix" it?

[Bug middle-end/54218] Debug info for function parameters is incorrect when compiled with -O0

[senthil_kumar.selvaraj at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

--- Comment #2 from Senthil Kumar Selvaraj <senthil_kumar.selvaraj at atmel dot com> 2012-08-10 10:23:55 UTC ---
Comment on attachment 27980
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27980
Failing dejagnu test case

>/* { dg-do run } */
>/* { dg-options "-g" } */
>/* { dg-skip-if "" { *-*-* }  { "*" } { "-O0" } } */
>
>void func(int p)
>{
>    p = 0; /* { dg-final { gdb-test 8 "p" "0" } } */
>    p = 32;/* { dg-final { gdb-test 8 "p" "42" } } */
>}
>
>int
>main (void)
>{
>    int local = 42;
>    func(local);
>}

[Bug middle-end/54218] Debug info for function parameters is incorrect when compiled with -O0

[senthil_kumar.selvaraj at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

Senthil Kumar Selvaraj <senthil_kumar.selvaraj at atmel dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #27980|0                           |1
        is obsolete|                            |

--- Comment #3 from Senthil Kumar Selvaraj <senthil_kumar.selvaraj at atmel dot com> 2012-08-10 10:25:03 UTC ---
Created attachment 27981
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27981
Failing dejagnu test case (right line number)

[Bug middle-end/54218] Debug info for function parameters is incorrect when compiled with -O0

[senthil_kumar.selvaraj at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

--- Comment #4 from Senthil Kumar Selvaraj <senthil_kumar.selvaraj at atmel dot com> 2012-08-10 10:39:32 UTC ---
(In reply to comment #1)
> That's because the actual parameter value is not used:
> 
> func (int p)
> {
> ;;   basic block 2, loop depth 0
> ;;    pred:       ENTRY
>   p_1 = 0;
>   p_2 = 32;
>   return;
> 
> Partition 0 (p_1 - 1 2 )
> 
> 
> Does -fvar-tracking "fix" it?

Yes, it does. Doesn't change the code generated though - the initial copy is
still at a different frame offset.

(In reply to comment #1)
> That's because the actual parameter value is not used:
> 
> func (int p)
> {
> ;;   basic block 2, loop depth 0
> ;;    pred:       ENTRY
>   p_1 = 0;
>   p_2 = 32;
>   return;
> 
> Partition 0 (p_1 - 1 2 )
> 
> 
> Does -fvar-tracking "fix" it?

(In reply to comment #1)
> That's because the actual parameter value is not used:
> 
> func (int p)
> {
> ;;   basic block 2, loop depth 0
> ;;    pred:       ENTRY
>   p_1 = 0;
>   p_2 = 32;
>   return;
> 
> Partition 0 (p_1 - 1 2 )
> 
> 
> Does -fvar-tracking "fix" it?

[Bug middle-end/54218] Debug info for function parameters is incorrect when compiled with -O0

[george.thomas at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

--- Comment #5 from George Thomas <george.thomas at atmel dot com> 2013-01-07 12:04:22 UTC ---
Created attachment 29095
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=29095
Draft Patch for the fix of 54218

The issue is happening because stack space is allocated twice

1. assign_params_setup_stack and
2. expand_used_vars

The proposed fix is to allocate the space only once in assign_params_stack
by explicitly checking in expand_used_vars if the tree node is of type
PARM_DECL.

if its PARM_DECL, it would mean that it would already have been expanded and
hence do not require further expansion.

This fixes the issue and allows debugging to work properly.

I would like to know if it would be an acceptable change.

[Bug middle-end/54218] Debug info for function parameters is incorrect when compiled with -O0

[george.thomas at atmel dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54218

George Thomas <george.thomas at atmel dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |george.thomas at atmel dot
                   |                            |com

--- Comment #6 from George Thomas <george.thomas at atmel dot com> 2013-01-07 12:06:01 UTC ---
Another alternative thought of for the fix was to,

Make the Parameters to be the default def of the next use. In this case,
the condition mentioned above will not expand the variable.

if (!bitmap_bit_p (SA.partition_has_default_def, i))
  {
    expand_one_var (var, true, true);
    gcc_assert (SA.partition_to_pseudo[i]);
  }