[Bug tree-optimization/78731] New: Possible bug with switch when optimization is turned on.

["wilson at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78731

            Bug ID: 78731
           Summary: Possible bug with switch when optimization is turned
                    on.
           Product: gcc
           Version: 5.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilson at gcc dot gnu.org
  Target Milestone: ---

Creating a bug from the original message, which is here
https://gcc.gnu.org/ml/gcc-bugs/2016-12/msg00678.html

Hi

The following code seems to be correctly executed when compiled with
GCC 4.4.7 and LLVM 6.1. It does not correctly compile with gcc version
5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.4).

The following is what I have reduced the problem to:

#include <stdio.h>
#include <stdlib.h>

#define GENERAL 1
#define BRACKETS 2
#define QUOTES 3

//This method contains the issue.
void foo(char *qb, char* into) {
  //The starting state is important for the bug.
  int state = QUOTES;
  int save_state = BRACKETS;

  while (qb) { //Always true, it seems this can't just be '1'
    printf("State is %d\n", state);
    switch (state) {
    case BRACKETS:
      printf("Yay this was correctly executed\n");
      exit(0);
      break;
    case GENERAL:
        printf("Oh no how did you get here?\n");
        printf("State is %d\n", state);
        exit(1);
      break;
    case QUOTES:
        state = save_state;
        printf("State went to %d btw BRACKETS is %d\n", state, BRACKETS);
        save_state = GENERAL; //Remove this line and it will work even
when optimised.
        printf("After save state, state went to %d btw BRACKETS is
%d\n", state, BRACKETS);
      break;
    default: ;
    }
    printf("State %d btw GENERAL %d\n", state, GENERAL);
  }
  printf("If you see this then something is really wrong.\n");
  exit(4);
}

int main() {
    //These don't seem to matter don't concern yourself with them.
    char *b = "123";
    char out[4];
    foo(b, out);
    return 1;
}

If I compile this with:
gcc -O0 -g -Wall -Werror sillyswitch.c -o sillyswitch

It will correctly print "Yay this was correctly executed"

However if I compile this with:
gcc -O -g -Wall -Werror sillyswitch.c -o sillyswitch

It will print "Oh no how did you get here?"

I suspect this is a bug. I am unable to create an account to report
this as a bug though.

-Luke