[LLVMbugs] [Bug 19823] New: wrong code at -O2 and -O3 on x86_64-linux-gnu in 64-bit mode

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=19823

            Bug ID: 19823
           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: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The following code is miscompiled by the current clang trunk on
x86_64-linux-gnu at -O2 and -O3 in 64-bit mode (but not in 32-bit).  

This is a regression from 3.4.

$ clang-trunk -v
clang version 3.5.0 (trunk 209291)
Target: x86_64-unknown-linux-gnu
Thread model: posix
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/4.8.1
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.6
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/4.8.1
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
$ 
$ clang-trunk -m64 -Os small.c; a.out
1
$ clang-trunk -m32 -O2 small.c; a.out
1
$ clang-3.4 -m64 -O2 small.c; a.out
1
$ 
$ clang-trunk -m64 -O2 small.c; a.out
0
$ 


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


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

short a, b, *c = &a;
char d = 0;

int
main ()
{
  char *t = &d;

 lbl:
  b = (*t)--;
  *c = 1;
  if (d)
    goto lbl;

  printf ("%d\n", a);
  return 0;
}

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