[SCEV] wrong code at -O2 on x86_64-linux_gnu

[zhendongsu]

This is a regression from 13.0.*.

Compiler Explorer: https://godbolt.org/z/9r1o5v5eq

% clangtk -v
clang version 17.0.0 (https://github.com/llvm/llvm-project.git 1b9b4f3bfa8bc7ed7dddd30dd30a07676891bedb)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /local/suz-local/opfuzz/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/10
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64
% 
% clangtk -O2 small.c
% ./a.out
Floating point exception
% 
% cat small.c
short a, b, *c = &b;
unsigned d = 4294967295U;
int e() {
  while (1) {
    while (d <= 1 && b <= d)
      *c = 1;
    int f = d;
    d = a / f;
    if (f < 4294967295U)
      continue;
    return 1;
  }
}
int main() {
  e();
  return 0;
}

[EKCKABATOP54]

Isn't that ub? https://en.cppreference.com/w/cpp/language/ub ; Infinite loop without side-effects

[zhendongsu]

Isn't that ub? https://en.cppreference.com/w/cpp/language/ub ; Infinite loop without side-effects

It's not an infinite loop; the body of the outer loop will be executed once and return 1.

[efriedma-quic]

I think 99d2582 broke this; it attaches incorrect nsw/nuw flags on SCEV expressions in loop conditions. CC @wsmoses @LebedevRI @preames

[luxufan]

I have tried to fix this and posted a patch https://reviews.llvm.org/D144861, but I am not sure if it is the right fix.