Hi, there!
I am new for using GCC mail list, please forgive me if something is wrong.
I have some issues about how GCC deal with the different optimizations in a
UB program.
For example,
small.cc
*#include <iostream>unsigned long long a;void b(unsigned long long *c, int
h) { *c = h; }int d = 0;int main() { long e = -5392885090005969454L, f =
-7099246355752164823LL, g = f - d + e; b(&a, g); std::cout << a <<
std::endl;}*
Here are the instructions I used and outputs
*$g++-8 --version*
g++-8 (Ubuntu 8.4.0-1ubuntu1~18.04) 8.4.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
*$g++-8 -O0 small.cc ; ./a.out*
18446744073174349819
*$g++-8 -O1 small.cc ; ./a.out *
0
*$g++-8 -O0 -fsanitize=undefined small.cc ; ./a.out*
small.cc:6:63: runtime error: signed integer overflow: -5392885090005969454
+ -7099246355752164823 cannot be represented in type 'long int'
18446744073174349819
*$g++-8 -O1 -fsanitize=undefined small.cc ; ./a.out*
small.cc:6:63: runtime error: signed integer overflow: -5392885090005969454
+ -7099246355752164823 cannot be represented in type 'long int'
18446744073174349819
The O2~Os produce the same results with O1. It's so weird I add
"-fsanitize=undefined" with "-O1" and produce a different result compared
to only with "-O1".
So, my concerns are
* It's a bug in GCC with the above results? Or is it a bug if GCC produces
different results in UB programs with different optimization options?
* How GCC deal with other UB programs, do GCC have any default strategy to
deal with that?
Thank you all, and any suggestions are welcome!
Best,
Haoxin
