[llvm-bugs] [Bug 37820] New: Clang miscompiles with optimization level -O1 and higher

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37820

            Bug ID: 37820
           Summary: Clang miscompiles with optimization level -O1 and
                    higher
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: LLVM Codegen
          Assignee: unassignedclangbugs at nondot.org
          Reporter: Vsevolod.Livinskij at frtk.ru
                CC: llvm-bugs at lists.llvm.org

Created attachment 20433
  --> https://bugs.llvm.org/attachment.cgi?id=20433&action=edit
Reproducer.

Clang produces incorrect code with -O1 and higher.
Opt-bisect-limit points to X86 DAG->DAG Instruction Selection on function.

Reproducer:
>$ cat func.c
extern unsigned long a;
extern long b, c;
void foo() {
  b = (unsigned int)(a >> ((-16384L & c) + 4503359447364223024));
}

>$ cat driver.c
#include <stdio.h>

unsigned long a = 10144615248613568396UL;
long b, c = -4503359447364216586L;

void foo();

int main() {
  foo();
  printf("%lu\n", b);
}

Error:
>$ clang -O0 driver.c func.c ; ./a.out
36040
>$ clang -O1 driver.c func.c ; ./a.out
3640036552

LLVM version:
clang version 7.0.0 (trunk 334837)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /llvm/bin-trunk/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.7
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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