[llvm-bugs] [Bug 30838] New: wrong code at -O3 on x86_64-linux-gnu (in both 32-bit and 64-bit modes)

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30838

            Bug ID: 30838
           Summary: wrong code at -O3 on x86_64-linux-gnu (in both 32-bit
                    and 64-bit modes)
           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

Interestingly, although fn2 isn't invoked, removing it seems to hide the bug. 


$ clang -v
clang version 4.0.0 (trunk 285491)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /usr/local/clang-trunk/bin
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/4.7.4
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
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/6.0.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.4.0
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
$
$ clang -O2 small.c; ./a.out
10
$ clang-3.8 -O3 small.c; ./a.out
10
$
$ clang -O3 small.c; ./a.out
0
$


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


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

short a, b, *c = &a;
static char d, e = 5;
int f, *g = &f, h, i, j = 6, k;

int fn1 (int p1)
{ 
  for (; j < 7; j++)
    if (b || d < 2)
      for (; e; e++)
        { 
          *g = 10;
          h = *c;
        }
  return p1;
}

void fn2 ()
{ 
  d++;
}

int main ()
{ 
  int l[1];
  for (; k < 1; k++)
    if (j)
      l[0] = 1;
  fn1 (l[0]);
  l[i] = 0;
  printf ("%d\n", f);
  return 0;
}

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