Wrong code at -O2/3 on x86_64-linux_gnu (a latent regression since Clang-15)

[shao-hua-li]

This looks like a long-standing issue since Clang-15. Clang at -O2/3 produces the wrong code. opt-bisect-limit suggests that the issue might be in LoopVectorizePass.

Compiler explorer: https://godbolt.org/z/dssMcdz55

% cat a.c
int printf(const char *, ...);
long a, c;
long *b = &a;
char d;
char *const e = &d;
int f, g, h;
int main() {
  long i[] = {1, 4, 4, 1, 4, 4};
  for (; g >= -6; g--) {
    f = 0;
    for (; f <= 5; f++)
      c = i[f] ^= *e > *b;
    *e = 1;
  }
  for (; h < 1; h++)
    printf("%d\n", (int)c);
}
%
% clang-tk -O0 a.c && ./a.out
4
% clang-tk -O2 a.c && ./a.out
1
% clang-tk -O3 a.c && ./a.out
1
%
% clang-tk -v
clang version 17.0.0 (https://github.com/llvm/llvm-project.git a5bba98a58b7406f81629d3942e03b1eff1e2b33)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /zdata/shaoli/compilers/ccbuilder-compilers
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/9
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/11
Candidate multilib: .;@m64
Selected multilib: .;@m64
%

[shao-hua-li]

Bisect to 4e5e042, which was commited by @igogo-x86 .

[igogo-x86]

@shao-hua-li, thank you! I'll look into it.

[igogo-x86]

Pre-commit - https://reviews.llvm.org/D157630
Fix - https://reviews.llvm.org/D157631

[igogo-x86]

/cherry-pick 2df9ed1 ac65fb8

[llvmbot]

/branch llvm/llvm-project-release-prs/issue64047