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Date: Wed, 05 Mar 2008 07:45:00 -0000
Subject: [Bug tree-optimization/35467]  New: Missed optimization in dependency analysis for arrays allocated by new()
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The code below demonstrates a missed optimization opportunity that
can have an impact on code. 
If arrays are allocated by new(), the static alias analysis is unable to
prove independency:
#include <stdlib.h>
int
foo (void)
{
        int *a = new int [1000];
        int *b = new int [1000];
        int x;

        a[10] = 5;
        b[10] = 6;
        x = a[10];
        return x;
}

=cut
While if the arrays are allocated by malloc(), the independency has been
proved:

#include <stdlib.h>
int
foo (void)
{
        int *a = (int*) malloc (sizeof(int) * 1000);
        int *b = (int*) malloc (sizeof(int) * 1000);
        int x;

        a[10] = 5;
        b[10] = 6;
        x = a[10];
        free (a);
        free (b);
        return x;
}

=cut

;; Function int foo() (_Z3foov)

Analyzing Edge Insertions.
int foo() ()
{
  int * b;
  int * a;
  void * D.2486;
  void * D.2485;

<bb 2>:
  D.2485 = malloc (4000);
  a = (int *) D.2485;
  D.2486 = malloc (4000);
  b = (int *) D.2486;
  *(a + 40) = 5;
  *(b + 40) = 6;
  free (a);
  free (b);
  return 5;

}

The ability to prove dependency in this kind of c++ source code can enable
vectorization for many loops.


-- 
           Summary: Missed optimization in dependency analysis for arrays
                    allocated by new()
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: victork at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35467