[Bug tree-optimization/96693] New: GCC produces incorrect code with -O2 for loops

[vsevolod.livinskij at frtk dot ru]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96693

            Bug ID: 96693
           Summary: GCC produces incorrect code with -O2 for loops
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vsevolod.livinskij at frtk dot ru
  Target Milestone: ---

Error:
>$ g++ -O2 driver.cpp func.cpp && ./a.out
0
>$ g++ -O0 driver.cpp func.cpp && ./a.out
42

Reproducer:
//func.cpp
extern char var_20;
extern short var_22;
void test(unsigned a,
          bool b,
          long long p12[23]) {
  for (int c = 0; c < 2;) {
    if (b) {
      for (int d = 0; d < 4082; d += 2)
        for (int e = 0; e < a; e = 2)
          var_20 = 0;
      var_22 = 0;
    }
    c = p12[c];
  }
}

//driver.cpp 
#include <stdio.h>

unsigned int var_4 = 140810747U;
bool var_7 = (bool)0;
unsigned char var_20 = (unsigned char)82;
unsigned short var_22 = 42;
long long int arr_9 [23] ;

void test(unsigned int var_4, bool var_7, long long int arr_9 [23]);

int main() {
    for (size_t i_0 = 0; i_0 < 23; ++i_0) 
        arr_9 [i_0] = -722784397873599555LL;
    test(var_4, var_7, arr_9);
    printf("%d\n", var_22);
}

GCC version:
11.0.0 20200816 (23747614cc8fc137c1f2ca64c8e224125a72fae5) + fix proposed in
bug 95396