https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80815
Bug ID: 80815
Summary: wrong code because of broken runtime alias check in
vectorizer
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: amker at gcc dot gnu.org
Target Milestone: ---
Hi,
I was suspecting that vect_prune_runtime_alias_check_list is broken, now I can
create a test case for it:
#include <stdio.h>
int arr[2048];
int
foo (int *a, int *b)
{
int i;
int *a1 = a;
int *a0 = a1 - 512;
for (i = 0; i < 500; i++)
{
*b = *a0 + *a1;
b++;
a0--;
a1--;
}
return 0;
}
int main (void)
{
int *a = &arr[1027];
int *b = &arr[1024];
int i;
for (i = 0; i < 2048; i++)
arr[i] = i;
foo (a, b);
for (i = 0; i < 2048; i++)
fprintf (stdout, "%d: %d\n", i, arr[i]);
return 0;
}
Compiled on x86_64 with below two commands:
Binary 1)
$ ../gcc -O2 -fno-inline ../x.c -o x.exe
$ ./x.exe >/tmp/x
Binary 2)
../gcc -Ofast -fno-inline ../x.c -o y.exe
$ ./y.exe >/tmp/y
We have:
$ diff /tmp/x /tmp/y
1027,1028c1027,1028
< 1026: 2053
< 1027: 2054
---
> 1026: 1538
> 1027: 1536
I think the root cause is in below code:
/* Generally the new segment length is the maximum of the
left segment size and the right segment size plus the distance.
??? We can also build tree MAX_EXPR here but it's not clear this
is profitable. */
else if (tree_fits_uhwi_p (dr_a1->seg_len)
&& tree_fits_uhwi_p (dr_a2->seg_len))
{
unsigned HOST_WIDE_INT seg_len_a1 = tree_to_uhwi
(dr_a1->seg_len);
unsigned HOST_WIDE_INT seg_len_a2 = tree_to_uhwi
(dr_a2->seg_len);
dr_a1->seg_len = size_int (MAX (seg_len_a1, diff + seg_len_a2));
do_remove = true;
}
Here dr_a1 is *a0, dr_a2 is *a1 and dr_b1/dr_b2 is *b, and their ranges is like
-------------------------------------------------------------->
a0_en a0 a1_end b a1 b_end
It should merge range_a0/range_a1 and result in range of a1, but because:
1) seg_len_a1 wraps to a large unsigned integer;
2) seg_len_a2 wraps to a large unsigned integer;
3) diff + seg_len_a2 wraps again to a small unsigned integer;
The program falsely merges ranges to range_a0. As a result, loop is vectorized
because range_a0 and range_b are considered not alias to each other. This is
wrong because *a1 and *b overlaps at the first several iterations and have
circle in dependence.
