[LLVMbugs] [Bug 20355] New: wrong code at -O2 and -O3 on x86_64-linux-gnu

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=20355

            Bug ID: 20355
           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: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The current clang trunk miscompiles the following code at -O2 and -O3 on
x86_64-linux-gnu in both 32-bit and 64-bit modes.

This is a regression from 3.4.x. 

$ clang-trunk -v
clang version 3.5.0 (trunk 213212)
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 -Os small.c; a.out
-1
$ clang-3.4 -O2 small.c; a.out
-1
$ 
$ clang-trunk -O2 small.c; a.out
0
$ 

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

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

int a[4] = { -1, 0, 0, 0 }; 
static int b, c;

int
main ()
{
  for (; b < 4; b++)
    {
      short t = a[b];
      a[b] = c = t % 3;
    }
  printf ("%d\n", a[0]);
  return 0;
}

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