[llvm-bugs] [Bug 26860] New: wrong code at -O3 on x86_64-linux-gnu

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26860

            Bug ID: 26860
           Summary: wrong code at -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 -O3 in both the 32-bit and 64-bit modes.  

This is a regression from 3.7.x. 


$ clang-trunk -v
clang version 3.9.0 (trunk 262797)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/tools/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.9.3
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.3.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.7
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/4.8.5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.3.0
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 -O2 small.c; ./a.out
$ clang-3.7 -O3 small.c; ./a.out
$ 
$ clang-trunk -O3 small.c
$ ./a.out
Aborted (core dumped)
$ 


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


int a[4], b[4], c, d; 
char e;

void
fn1 ()
{
  for (;; c++)
    {
      for (;;)
    {
      e = b[3];
      if (e)
        {
          b[c] = 0;
          break;
        }
      return;
    }
      a[3] = d = 1; 
      for (; d < 4; d++)
    {
      a[d] = 0; 
      if (b[3])
        break;
    }
    }
}

int
main ()
{
  b[3] = 1; 
  fn1 ();

  if (a[3])
    __builtin_abort (); 

  return 0;
}

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