[llvm-bugs] [Bug 36629] New: Clang miscompiles with -O0.

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36629

            Bug ID: 36629
           Summary: Clang miscompiles with -O0.
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: Vsevolod.Livinskij at frtk.ru
                CC: llvm-bugs at lists.llvm.org

Created attachment 20013
  --> https://bugs.llvm.org/attachment.cgi?id=20013&action=edit
Reproducer.

Reduced reproducer (see full in attachment):
>$ cat repr.cpp
#include <stdio.h>
#include <vector>

struct struct_1 {
  signed char member_1_0 : 23;
  unsigned char : 16;
  unsigned char member_1_1 : 25;
};

const unsigned char aa = 60;
const unsigned char bb = 246;
signed char cc = 36;
signed char dd = 97;
unsigned char ee = 178;
struct_1 struct_obj_1;
short a = 42;

void tf_0_foo() {
  ee = a % (unsigned char)((bb && aa ? ~0 : 0) % struct_obj_1.member_1_0 ^ cc);
  dd = struct_obj_1.member_1_1;
}

int main() {
  struct_obj_1.member_1_0 = -122;
  struct_obj_1.member_1_1 = 98;
  tf_0_foo();
  printf("%d\n", (int)ee);
}

Error:
>$ clang++ -fsanitize=undefined -fno-sanitize-recover=undefined -Werror=uninitialized -Werror=implicitly-unsigned-literal -O0 -w repr.cpp ; ./a.out
42
>$  clang++ -w -O0 repr.cpp ; ./a.out
5

LLVM version:
>$ clang++ -v
clang version 7.0.0 (trunk 326547)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /llvm/bin-trunk/bin
Found candidate GCC installation: /usr/lib/gcc/i686-redhat-linux/4.8.2
Found candidate GCC installation: /usr/lib/gcc/i686-redhat-linux/4.8.5
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.5
Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.8.5
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64

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