[llvm-bugs] [Bug 35615] New: [Clang-optimization] wrong code for elements in a union pointed by two types of global pointers

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35615

            Bug ID: 35615
           Summary: [Clang-optimization] wrong code for elements in a
                    union pointed by two types of global pointers
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: yangyibiao at nju.edu.cn
                CC: llvm-bugs at lists.llvm.org

$ clang -v
clang version 6.0.0-svn320001-1~exp1 (trunk)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.1
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/6.3.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.3.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.2.0
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.2.0
Candidate multilib: .;@m64
Selected multilib: .;@m64

$ cat small.c
int printf(const char *, ...);

union a {
  int b;
  long c;
} e[] = {{0}, {8}};

int d = 0;
int *f = &e[1].b;
long *g = &e[1].c;

void h() {
  for (; d < 2; d++) {
    *g = 0;
    if (*f)
      return;
  }
  return;
}

int main() {
  h();
  printf("%d\n", d);
}

$ clang -O0 small.c; ./a.out
2
$ clang -Ofast small.c; ./a.out
0
$ clang -fsanitize=alignment small.c; ./a.out
2

I am not very sure whether there exist any undefined behavior in this code.
When I am using sanitize option, clang can not detect any undefined behavior in
this code.

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