[llvm-bugs] [Bug 24802] New: wrong code at -O1 and above on x86_64-linux-gnu

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24802

            Bug ID: 24802
           Summary: wrong code at -O1 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 -O1 and above in both 32-bit and 64-bit modes.

This is a regression from 3.7.x. 


$ clang-trunk -v
clang version 3.8.0 (trunk 247514)
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/4.9.2
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/5.1.0
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/4.8.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.9.2
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.1.0
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 -O0 small.c; ./a.out
$ clang-3.7.0 -O1 small.c; ./a.out
$ 
$ clang-trunk -O1 small.c
$ ./a.out
Aborted
$ 


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


static int a[3];
int b;

int
main ()
{
  int c[1] = {0};
  a[2] = 1;
  for (b = 0; b < 3; b++)
    a[b] = 0;

  if (a[2] != 0) 
    __builtin_abort (); 

  return 0;
}

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