[llvm-bugs] [Bug 26843] New: wrong code at -Os and above on x86_64-linux-gnu

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=26843

            Bug ID: 26843
           Summary: wrong code at -Os and above 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 on
x86_64-linux-gnu at -Os and above in both the 32-bit and 64-bit modes.  

This is a regression from 3.7.x. 


$ clang-trunk -v
clang version 3.9.0 (trunk 262590)
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/5.3.0
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/4.8.5
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/5.3.0
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 -O1 small.c; ./a.out
$ clang-3.7 -Os small.c; ./a.out
$ 
$ clang-trunk -Os small.c
$ ./a.out
Aborted (core dumped)
$ 


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


int a, b[4], c; 

void
fn1 ()
{
  b[1] = 1;
  if (a)
    for (a = 1; a < 4; a++)
      b[a] = 0;
  a = 1;
}

int
main ()
{
  for (; c < 2; c++)
    fn1 ();

  if (b[1] != 0) 
    __builtin_abort (); 

  return 0;
}

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