[llvm-bugs] [Bug 35455] New: wrong code generated at all optimization levels for union assignment

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35455

            Bug ID: 35455
           Summary: wrong code generated at all optimization levels for
                    union assignment
           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-svn318792-1~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 {
  short c;
  int k;
} m;

int *f = (int*)&m;

void l() {
  m.k = 1;
  *f = 2;
  short *n = (short*)&m;
  *n = 0;
}

int main() {
  l();
  printf("%d  %d\n", m.c, m.k);
}

$ clang -O0 small.c; ./a.out
0  0

$ clang -O3 small.c; ./a.out
2  2


As we can find that, disable and enable optimization, the output is different.

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