[llvm-bugs] [Bug 24481] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24481

            Bug ID: 24481
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: su at cs.ucdavis.edu
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The following code is miscompiled by the current clang trunk on
x86_64-linux-gnu at -O2 and -O3 in both 32-bit and 64-bit modes.

This is a regression from 3.6.x. 


$ clang-trunk -v
clang version 3.8.0 (trunk 245195)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.1.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
$ 
$ clang-trunk -Os small.c; ./a.out
$ clang-3.6 -O2 small.c; ./a.out
$ 
$ clang-trunk -O2 small.c
$ ./a.out
Aborted (core dumped)
$ 


-------------------------------------------------


int a, b[10] = { 0, 0, 0, 0, 0, 0, 0, 1 }, c, e;

void
fn1 ()
{
  if (a) 
    fn1 ();
  for (e = 8; e >= 0; e--)
    {
      c = 2;
      for (; c < 8; c++)
    b[e] = b[c];
      b[7] = b[8];
      b[3] = 0;
    }
}

int
main ()
{
  for (;;)
    {
      fn1 ();
      break;
    }

  if (b[1] != 1) 
    __builtin_abort (); 

  return 0;
}

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