[Bug tree-optimization/48172] New: incorrect vectorization of loop in GCC 4.5.* with -O3

["geir at cray dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48172

           Summary: incorrect vectorization of loop in GCC 4.5.* with -O3
           Product: gcc
           Version: 4.5.2
            Status: UNCONFIRMED
          Severity: critical
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: geir@cray.com


When the following test case is compiled with -O3, the program fails to compute
correct array values.  Compiling with -O2 allows the program to execute
correctly.

Looking at the disassembly of the program, the compiler appears to be
vectorizing a loop (lines 25-27 in this example) incorrectly.  Turning off loop
vectorization allows correct code generation ("cc -O3 -fno-tree-vectorize"
works)

Problem was duplicated using gcc 4.5.0, 4.5.1, and 4.5.2.  Problem does not
occur using gcc 4.4.4 or 4.3.1.

The CPU being used is a 12-core Magny-Cours Opteron.

test case:

$ cat vec.c
// Compile with gcc -O3 to see failures
// Compile with gcc -O3 -fno-tree-vectorize to get correct results
#include <inttypes.h>
#include <stdio.h>
#include <string.h>

#define ASIZE 1028
#define HALF (ASIZE/2)

int main() {
  uint32_t array[ASIZE];
  int failures;
  int i;

  memset(array, 0, sizeof(array));

  // initialize first half of the array
  for (i = 0; i < HALF; i++) {
    array[i] = i;
  }

  // fill second half of array in by summing earlier elements of the array
  // gcc 4.5.1 and 4.5.2 incorrectly vectorize this loop!  aray[1025] is left
  // at 0 for ASIZE=1028
  for (i = 0; i < HALF-1; i++) {
    array[HALF+i] = array[2*i] + array[2*i + 1];
  }

  // see if we have any failures
  failures = 0;
  for (i = 0; i < HALF - 1; i++) {
    if (array[HALF+i] != array[2*i] + array[2*i + 1]) {
      printf("COMPILER BUG: array[%d] should be %d but is %d\n",
             HALF+i, array[2*i] + array[2*i + 1], array[HALF+i]);
      ++failures;
    }
  }

  if (failures == 0) {
    printf("pass\n");
  }

  return 0;
}

$$ gcc --version
gcc (GCC) 4.5.2 20101216 (Cray Inc.)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -O3 vec.c ; ./a.out
COMPILER BUG: array[1025] should be 98177 but is 0
$ gcc -O3 -fno-tree-vectorize vec.c ; ./a.out 
pass
$ gcc -O2 vec.c ; ./a.out
pass
$

  GCC 4.4.4 works:

$ gcc --version
gcc (GCC) 4.4.4 20100429 (Cray Inc.)
Copyright (C) 2010 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ gcc -O3 vec.c ; ./a.out
pass
$