[llvm-bugs] [Bug 25638] New: wrong code at -O2 and -O3 on x86_64-linux-gnu (in 64-bit mode)

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25638

            Bug ID: 25638
           Summary: wrong code at -O2 and -O3 on x86_64-linux-gnu (in
                    64-bit mode)
           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 (the compiled code hangs) by the current
clang trunk on x86_64-linux-gnu at -O2 and -O3 in 64-bit mode (but not in
32-bit mode).

This is a regression from 3.7.x. 


$ clang-trunk -v
clang version 3.8.0 (trunk 254011)
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/4.9.2
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.1.0
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/4.8.4
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 -m64 -Os small.c; ./a.out
$ clang-trunk -m32 -O2 small.c; ./a.out
$ clang-3.7.0 -m64 -O2 small.c; ./a.out
$ 
$ clang-trunk -m64 -O2 small.c
$ timeout -s 9 10 ./a.out
Killed
$ 


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


int a, b, c, d, e;

int
main ()
{
  for (;;)
    {
      int h[9][5];
      for (c = 0; c < 9; c++)
        for (e = 0; e < 5; e++)
          {
            while (b)
              a = 0;
            h[c][e] = 1;
          }
      if (h[0][4])
        for (; a; a++)
          b || d++;
      break;
    }
  return 0; 
}

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