[llvm-bugs] [Bug 26256] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26256

            Bug ID: 26256
           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 (and 3.7.x) 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 3.9.0 (trunk 258416)
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/5.2.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.7
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/4.9.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.2.1
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
0
$ clang-3.7 -O2 small.c
$ ./a.out
0
$


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


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

int a = 8, b, d, e, f;
short c;

void
fn1 ()
{ 
  char g = c, h = c = c | 195;
  if (d)
    ;
  if ((!a && c > d) || c < d)
    for (g = 0; g < 20; g++)
      e ^= f;
  a = 2 & g;
  if (h >= c || g)
    printf ("%d\n", b);
  if (a)
    { 
      fn1 ();
      for (;;)
        ;
    }
}

int
main ()
{ 
  fn1 ();
  return 0;
}

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