https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86341
Bug ID: 86341
Summary: Aggressive loop unrolling in gcc 7.x produces
out-of-bounds index
Product: gcc
Version: 7.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: tomek.bury at gmail dot com
Target Milestone: ---
Created attachment 44331
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=44331&action=edit
test case, compile with 7.x gcc -Wall -O3
Compiling the below code produces a warning in gcc 7.x series:
test.c:
---
struct test
{
unsigned num_items;
int items[3];
};
struct test t = {2, {1, 2}};
int oopsie(void)
{
int foo = 0;
for(unsigned i = 0; i < t.num_items; i++)
{
const int *item_i = &t.items[i];
foo += *item_i;
for(unsigned j = i + 1; j < t.num_items; j++)
{
const int *item_j = &t.items[j];
foo += *item_j;
}
}
return foo;
}
---
$ gcc -Wall -o test.o -c test.c -O3
test.c: In function ‘oopsie’:
test.c:18:41: warning: array subscript is above array bounds [-Warray-bounds]
const int *item_j = &t.items[j];
~~~~~~~^~~
Copying the t.num_items to a local variable and using a local copy in any of
the loops makes the problem disappear. Also disabling aggressive loop unrolling
make the warning disappear. The gcc 6.x and 8.x seem to be all right.
