https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79877
Bug ID: 79877
Summary: -Wstrict-overflow false positive or misunderstanding?
Product: gcc
Version: 7.0.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: s-beyer at gmx dot net
Target Milestone: ---
Hi,
I have the following code:
----
$ cat foo.cc
#include <iostream>
static int strict_overflow_warning(int bitmask) {
int max = -1;
for (int i = 31; i > max; i--) {
if (bitmask & (1 << i)) {
max = i;
}
}
return max;
}
static int no_strict_overflow_warning(int bitmask) {
int max = -1;
for (int i = 31; i >= 0; i--) {
if (bitmask & (1 << i)) {
max = i;
break;
}
}
return max;
}
int main(int argc, char *argv[]) {
std::cout << strict_overflow_warning(argc) << std::endl;
std::cout << no_strict_overflow_warning(argc) << std::endl;
std::cout << strict_overflow_warning(0) << std::endl;
std::cout << no_strict_overflow_warning(0) << std::endl;
std::cout << strict_overflow_warning(-argc) << std::endl;
std::cout << no_strict_overflow_warning(-argc) << std::endl;
return 0;
}
----
I get the following output (depending on g++ version and -O flag):
----
$ g++-7 -O3 -Wstrict-overflow foo.cc && ./a.out 2 3 4 5 6 7 8
foo.cc: In function ‘int main(int, char**)’:
foo.cc:5:21: warning: assuming signed overflow does not occur when assuming
that (X - c) <= X is always true [-Wstrict-overflow]
for (int i = 31; i > max; i--) {
~~^~~~~
foo.cc:5:21: warning: assuming signed overflow does not occur when assuming
that (X - c) <= X is always true [-Wstrict-overflow]
for (int i = 31; i > max; i--) {
~~^~~~~
3
3
-1
-1
31
31
$ g++-7 -O2 -Wstrict-overflow foo.cc && ./a.out 2 3 4 5 6 7 8
3
3
-1
-1
31
31
$ g++-6 -O3 -Wstrict-overflow foo.cc && ./a.out 2 3 4 5 6 7 8
3
3
-1
-1
31
31
----
The two functions should be semantically equivalent (as the output
indicates). Why does the version using i > max (instead of using i >= 0
and the break) show me the strict-overflow warning?
Thank you,
Stephan
