Unexpected behavior of g++-8 g++-9 -O2 optimization

Asked May 27 '19 at 00:49

Active May 30 '19 at 10:54

Viewed 90 times

#include <stdio.h>
#include <iostream>
__attribute__((optimize("O2"))) static void* verify(char *str)
{
  if (str[0] == 0)
  {
    puts("MM");
    exit(-2);
  }
}

int main(int argc,char **argv)
{
  verify(argv[0]);
  puts("GG");
  return 1;
}

Here is my code. When I compile it using g++8/g++9/clang++-6/clang++-8 with -O2 optimization, the program prints "MM" instead of "GG". I know return statement is missing in function verify, but I never use the return value. Is there any explanation?

By the way, my test environment is ubuntu 1804 and g++-7 generates correct program.

edited May 30 '19 at 10:54

Markus W Mahlberg

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asked May 27 '19 at 00:49

Mr Pang

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If you define your function as returning a value, but don't actually return anything from it, and call the function, undefined behavior means all bets are off. Fix your function and see if it still happens. – Shawn May 27 '19 at 03:28
@Shawn adding a _return_ GG is written using gcc 8.1.0 – bruno May 27 '19 at 08:32
Is it really an undefined behavior when I omit return statement? I think using return value is undefined behavior, but missing return statement not to be. – Mr Pang May 27 '19 at 09:28
This may be a case where a C tag vs a C++ tag is important... – Marc Glisse May 27 '19 at 09:40
1

https://stackoverflow.com/q/1610030/1918193 – Marc Glisse May 27 '19 at 09:43
Oh, Great link. C++ has different behavior – Mr Pang May 28 '19 at 00:58

Unexpected behavior of g++-8 g++-9 -O2 optimization

0 Answers0