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
