[LLVMbugs] [Bug 20172] New: wrong code by LTO at -O1 (only) on x86_64-linux-gnu

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20172

            Bug ID: 20172
           Summary: wrong code by LTO at -O1 (only) 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: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The current clang trunk miscompile the following code when using LTO at -O1
(only) on x86_64-linux-gnu in both 32-bit and 64-bit modes.

This is a regression from clang 3.4. 

$ clang-trunk -v
clang version 3.5.0 (trunk 212004)
Target: x86_64-unknown-linux-gnu
Thread model: posix
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 multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
$ 
$ clang-trunk -O1 small.c; a.out
0
$ clang-3.4 -flto -O1 small.c; a.out
0
$ 
$ clang-trunk -flto -O1 small.c; a.out
1
$ 


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


int printf (const char *, ...);

int a[2], b = 1, c = 1, e = 1, f, g, h, n, o, r;
short d, q;
volatile int j;

int
fn1 (int p)
{
  return p;
}

int
fn2 (unsigned short p1, int p2)
{
  return p2 || p1 >> 1 ? p1 : p1 << 1;
}

void
fn3 (int p)
{
  q = fn2 (e, 0);
  if (fn1 (q))
    r = g = r != 1;
  if (fn2 (p && fn1 (0), c))
    {
      c = 4;
      n = o = n ? 0 : n + 1;
      for (; o;)
    j ? 0 : j;
      g = 0;
    }
}

int
fn4 ()
{
  if (b)
    for (;; b++)
      {
    if (fn1 (1))
      {
        fn3 (1);
        fn3 (0);
        if ((d = c) | fn2 (h, 0))
          return 0;
      }
    else
      {
        f = g;
        h = fn1 (0);
        e = c = 0;
      }
      }
  return 0;
}

int
main ()
{
  a[0] = 1;
  fn4 ();
  printf ("%d\n", a[d]);
  return 0;
}

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