[llvm-bugs] [Bug 35507] New: [Clang-Optimization] wrong code for union assignment optimization

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=35507

            Bug ID: 35507
           Summary: [Clang-Optimization] wrong code for union assignment
                    optimization
           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-svn319460-1~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 {
  int a;
} b;

int *c = (int*)&b;

int main() {
  int *d = (int*)&b;
  *d = 2;
  *c = 1;
  printf("%d\n", b.a);
}

$ clang -O0 small.c; ./a.out
1

$ clang -O1 small.c; ./a.out
2

$ clang -O2 small.c; ./a.out
2

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

The output is different while the optimization is enabled.

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