[LLVMbugs] [Bug 18893] New: wrong code (hangs) at -Os and above on x86_64-linux-gnu (affecting clang 3.0 to trunk)

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=18893

            Bug ID: 18893
           Summary: wrong code (hangs) at -Os and above on
                    x86_64-linux-gnu (affecting clang 3.0 to trunk)
           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: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

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

It also affects all earlier clang versions from 3.0 to 3.4.   


$ clang-trunk -v
clang version 3.5 (trunk 201602)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.8.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.7
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/4.8.1
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multiilb: .;@m64
Candidate multiilb: 32;@m32
Selected multilib: .;@m64
$ 
$ clang-trunk -O1 small.c; a.out
$ clang-2.9 -Os small.c; a.out
$ 
$ clang-trunk -Os small.c
$ a.out
^C
$ 


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


int a, b;

int
fn1 (int p1, int p2)
{
  return p1 > 0 || (p2 < 0 && -p2) ? p1 : p1 + p2;
}

int
fn2 ()
{
  int c = -7;
  for (; c != 1; c = fn1 (c, 6))
    {
      b = (c <= 0);
      if (b)
    {
      if (c)
        for (; a;)
          ;
    }
      else
    return 0;
    }
  for (;;)
    ;
}

int
main ()
{
  fn2 ();
  return 0;
}

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