[llvm-bugs] [Bug 24801] New: wrong code at -O2 and -O3 on x86_64-linux-gnu (in 64-bit mode)

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=24801

            Bug ID: 24801
           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: llvm-bugs at lists.llvm.org
    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 mode).

This is a regression from 3.7.x. 

It might be related to PR 24481, which is however also miscompiled in 32-bit
mode. 


$ 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 -m64 -Os small.c; ./a.out
$ clang-trunk -m32 -O2 small.c; ./a.out
$ clang-3.7.0 -m64 -O2 small.c; ./a.out
$ 
$ clang-trunk -m64 -O2 small.c
$ ./a.out
Aborted
$ 


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


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

void
fn1 ()
{
  int d[] = { 0 };
  for (i = c[0]; i < 4; i++)
    c[i] = 0;
}

int
main ()
{
  for (; i < 4; i++)
    a[i] = i;
  fn1 ();
  for (i = 0; i < 4; i++)
    b = a[b ^ c[i]];

  if (b)
    __builtin_abort ();

  return 0;
}

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