A bug in vrp_meet?

[Qing Zhao]

Hi,

I have been debugging a runtime error caused by value range propagation. and 
finally located to the following gcc routine:

vrp_meet_1 in gcc/tree-vrp.c

====
/* Meet operation for value ranges.  Given two value ranges VR0 and
   VR1, store in VR0 a range that contains both VR0 and VR1.  This
   may not be the smallest possible such range.  */

static void 
vrp_meet_1 (value_range *vr0, const value_range *vr1)
{
  value_range saved;

  if (vr0->type == VR_UNDEFINED)
    {    
      set_value_range (vr0, vr1->type, vr1->min, vr1->max, vr1->equiv);
      return;
    }    

  if (vr1->type == VR_UNDEFINED)
    {    
      /* VR0 already has the resulting range.  */
      return;
    }    
====

In the above, when one of vr0 or vr1 is VR_UNDEFINED,  the meet result of these 
two will be  the other VALUE. 

This seems not correct to me. 

For example, the following is the located incorrect value range propagation:  
(portion from the dump file *.181t.dom3)

====
Visiting PHI node: i_83 = PHI <_152(20), 0(22)>
    Argument #0 (20 -> 10 executable)
        _152: UNDEFINED
    Argument #1 (22 -> 10 executable)
        0: [0, 0]
Meeting
  UNDEFINED
and
  [0, 0]
to
  [0, 0]
Intersecting
  [0, 0]
and
  [0, 65535]
to
  [0, 0]
====


In the above, “i_83” is defined as PHI <_152(20), 0(22)>,   the 1st argument is 
UNDEFINED at this time(but its value range definitely is NOT [0,0]),
 and the 2nd argument is 0.

“vrp_meet” generate a VR_RANGE with [0,0] for “i_83” based on the current 
algorithm.  Obviously, this result VR_RANGE with [0,0] does NOT 
contain the value ranges for _152. 

 the result of “vrp_meet” is Not correct.  and this incorrect value range 
result finally caused the runtime error. 

I ‘d like to modify the vrp_meet_1 as following:

====
static void 
vrp_meet_1 (value_range *vr0, const value_range *vr1)
{
  value_range saved;

  if (vr0->type == VR_UNDEFINED)
    {    
      /* VR0 already has the resulting range. */
      return;
    }    

  if (vr1->type == VR_UNDEFINED)
    {    
      set_value_range_to_undefined (vr0)
     return;
    }    
====

let me know your opinion.

thanks a lot for the help.

Qing