[llvm-bugs] [Bug 49469] New: wrong code at -O1 and above on x86_64-linux-gnu

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49469

            Bug ID: 49469
           Summary: wrong code at -O1 and above on x86_64-linux-gnu
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: zhendong.su at inf.ethz.ch
                CC: llvm-bugs at lists.llvm.org

[537] % clangtk -v
clang version 13.0.0 (https://github.com/llvm/llvm-project.git
93afc3452cd4ebdb145e309f4d82f82e2f496479)
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/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/7.5.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
[538] % 
[538] % clangtk -O0 small.c; ./a.out
0
[539] % clangtk -O1 small.c; ./a.out
255
[540] % 
[540] % cat small.c
int printf (const char *, ...);

int c;
void d(unsigned f) { c = f; }

#pragma pack(1)
struct {
  unsigned : 15;
  signed : 21;
  signed : 26;
  signed a : 9;
} b;

int main() {
  unsigned long e = b.a;
  d(e >> 56);
  printf("%d\n", c);
  return 0;
}

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