[Bug tree-optimization/81162] New: UBSAN switch triggers incorrect optimization

[babokin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81162

            Bug ID: 81162
           Summary: UBSAN switch triggers incorrect optimization
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: babokin at gmail dot com
  Target Milestone: ---

gcc trunk rev249427, x86_64

In presence of -fsanitize=undefined optimisations (-O2) produce wrong code.

> cat f.cpp
#include <stdio.h>
short s;
int i1 = 1;
int i2 = 1;
unsigned char uc = 147;

int main() {
  s = (-uc + 2147483647) << 0;
  if (9031239389974324562ULL >= (-((i1 && i2) + uc) ^ -21096) ) {
    printf("OK\n");
  } else {
    printf("FAIL\n");
  }

  return 0;
}

> g++ -std=c++11 -fsanitize=undefined -O0 f.cpp -o out
> ./out
OK
> g++ -std=c++11 -fsanitize=undefined -O2 f.cpp -o out
> ./out
FAIL