[Bug tree-optimization/53419] New: loop incorrectly optimized to endless loop at -O2 for table delimited by extern addresses (x86-32)

[gary at intrepid dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53419

             Bug #: 53419
           Summary: loop incorrectly optimized to endless loop at -O2 for
                    table delimited by extern addresses (x86-32)
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassig...@gcc.gnu.org
        ReportedBy: g...@intrepid.com
              Host: x86-32
            Target: x86-32


The attached test case, duplicated below yields an infinite loop when compiled
by GCC 4.8 trunk version 187666.  This appears to be a relatively recent
regression
(likely between 185454 and 186243).

typedef unsigned int size_t;

typedef void (*func_ptr_t) (void);
extern func_ptr_t init_array_begin[1];
extern func_ptr_t init_array_end[1];

void
per_thread_init (void)
{
  size_t n_init = (init_array_end - init_array_begin);
  int i;
  for (i = 0; i < n_init; ++i)
    {
      func_ptr_t init_func = init_array_begin[i];
      if (init_func)
        (*init_func) ();
    }
}

At -O1, the loop looks like this:

.L4:
        movl    init_array_begin(,%ebx,4), %eax
        testl   %eax, %eax
        je      .L3
        call    *%eax
.L3:
        addl    $1, %ebx
        cmpl    %esi, %ebx
        jne     .L4

At -O2 it looks like this.

.L5:
        movl    init_array_begin, %eax
        testl   %eax, %eax
        je      .L3
        call    *%eax
.L3:
        cmpl    $1, %ebx
        jne     .L5

The comparison "cmpl    $1, %ebx" is problematic, as well as the lack of
indexing the array.