https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122298
Bug ID: 122298
Summary: -Warray-bounds -O2 false positive with bounds computed
using modulus
Product: gcc
Version: 15.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: ldr709 at gmail dot com
Target Milestone: ---
GCC 15 seems to have introduced a `-Warray-bounds` false positive. This is as
small as I've been able to minimize the test example.
inline void bar(unsigned int* x, unsigned int round)
{
unsigned int offset = (round % 3 + 1) * 2;
x += offset;
for (unsigned int i = 0; i < 6 - offset; ++i) x[i] += 1;
}
void foo()
{
unsigned int x[6] = {0};
for (unsigned int round = 2; round <= 10; round += 3)
bar(x, round);
}
(Or see https://godbolt.org/z/7MEb14oEv)
Compiler warnings and version:
# gcc test.c -c -o test.o -Warray-bounds -O2
In function ‘bar’,
inlined from ‘foo’ at test.c:13:9:
test.c:5:52: warning: array subscript 6 is outside array bounds of ‘unsigned
int[6]’ [-Warray-bounds=]
5 | for (unsigned int i = 0; i < 6 - offset; ++i) x[i] += 1;
| ~^~~
test.c: In function ‘foo’:
test.c:10:18: note: at offset 24 into object ‘x’ of size 24
10 | unsigned int x[6] = {0};
| ^
In function ‘bar’,
inlined from ‘foo’ at test.c:13:9:
test.c:5:56: warning: array subscript 6 is outside array bounds of ‘unsigned
int[6]’ [-Warray-bounds=]
5 | for (unsigned int i = 0; i < 6 - offset; ++i) x[i] += 1;
| ~~~~~^~~~
test.c: In function ‘foo’:
test.c:10:18: note: at offset 24 into object ‘x’ of size 24
10 | unsigned int x[6] = {0};
| ^
# gcc --version
gcc (GCC) 15.2.1 20250813
Copyright (C) 2025 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
