https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69612
Bug ID: 69612
Summary: Optimizer does not consider overflow
Product: gcc
Version: 5.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: roarl at pvv dot org
Target Milestone: ---
I found a short code snippet that produces different results after optimizing
with -O2. I believe my code should work as it only uses a simple test to check
if incrementation has overflowed. It looks like the optimizer, having
established that a variable is >= 0, does not believe the variable can become <
0 after incrementing. However, I am relying on just that to catch the overflow.
In the code below, the __attribute__((noinline)) is not necessary to
demonstrate the "effect", however without it the "test" routine would have to
reside in another compilation unit.
% cat bug.c
int __attribute__((noinline)) test(int a) {
if (a < 0)
return 1;
a++;
if (a < 0) // gcc -O2 thinks this can't happen, since a>=0 from above
return 2;
return a;
}
#include <stdio.h>
int main()
{
printf("%d\n", test(0x7FFFFFFF));
}
% gcc bug.c -o bug
% ./bug
2
% gcc -O2 bug.c -o bug
% ./bug
-2147483648
% gcc --version
gcc (SUSE Linux) 5.2.1 20151008 [gcc-5-branch revision 228597]
...
