[llvm-bugs] [Bug 47578] New: Incorrectly lift a modulo computation outside an if statement

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=47578

            Bug ID: 47578
           Summary: Incorrectly lift a modulo computation outside an if
                    statement
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: k.even-mendoza at imperial.ac.uk
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

#include <stdio.h>

static long r = 0;
static long *m[1] = {&r};

static int i = 1;
static int *pi = &i;

int main() {
   unsigned x = 0;
   if (i) { // Evaluates to true
     ++x; // Guaranteed to be reached, and afterwards x == 1
     int test = pi == (int*) 1;
   }

   printf("x = %d\n", x);
   // At this point, x == 1
   if (!x) { // Evaluates to false
     i = 7UL % (m[0] == (long*) 1); // Unreachable.  (It would have UB if it
*were* reachable, becuase of a mod by zero.)
   }
}

Get "floating point exception" with -02, Ubuntun, and clang-6/Clang-10.
The bug is no appearing with -O0 flag.

Restor the bug:
clang-10 -O2 -w test1.c 
./a.out 
Floating point exception (core dumped)
======
clang-10 -O0 -w test1.c 
./a.out 
x = 1

I tried with two versions, both got the bugs:
1) from llvm trunk: clang version 10.0.0 (trunk), Target:
x86_64-unknown-linux-gnu, Thread model: posix, InstalledDir:
/home/user42/llvm-csmith-0/llvm-install/bin (from 03/09/2020)
2) Ubuntu's: clang version 10.0.0-4ubuntu1~18.04.2, Target:
x86_64-pc-linux-gnu, Thread model: posix, InstalledDir: /usr/bin

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