https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94809
Bug ID: 94809
Summary: Different results between gcc-9 and gcc-6
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: k.even-mendoza at imperial dot ac.uk
Target Milestone: ---
Seen on: 18.04.4 LTS
kar@kar-VirtualBox:~/ex1$ gcc-9 ex2.c -o ex
kar@kar-VirtualBox:~/ex1$ ./ex
0
kar@kar-VirtualBox:~/ex1$ gcc-6 ex2.c -o ex
kar@kar-VirtualBox:~/ex1$ ./ex
1
kar@kar-VirtualBox:~/ex1$ more ex2.c
#include <stdio.h>
int main() {
int a = 0;
unsigned long long one = 1;
((18446744073709551615UL / one) < a++, one);
printf("%d\n", a);
}
===
1. Output shall be 1 instead of 0.
2. GCC-7, GCC-8, GCC-9 and HEAD 562bfb1 produce 0.
gcc-9 (Ubuntu 9.2.1-17ubuntu1~18.04.1) 9.2.1 20191102
gcc-8 (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
gcc-7 (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0
gcc-6 (Ubuntu 6.5.0-2ubuntu1~18.04) 6.5.0 20181026
compiled: gcc-9 ex2.c -o ex
3. Explanation:
((18446744073709551615UL / one) < a++, one); the part before "," is skipped
including the increase of "a" by "1" (a++). As a result the printf outputs 0
instead of 1.
4. GCC-6 and LLVM (for example) produce 1. Note: You can try to compile the
code with GCC-6 or llvm to observe the behaviour where the output is 1.
