[Bug tree-optimization/58742] New: pointer arithmetic simplification

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58742

            Bug ID: 58742
           Summary: pointer arithmetic simplification
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org

int* f(int*b,int*e){
  return b+(e-b);
}

gives the following .optimized dump:

  e.0_2 = (long int) e_1(D);
  b.1_4 = (long int) b_3(D);
  _5 = e.0_2 - b.1_4;
  _6 = _5 /[ex] 4;
  _7 = (long unsigned int) _6;
  _8 = _7 * 4;
  _9 = b_3(D) + _8;
  return _9;

I believe the desired result is obvious enough...

At the asm level, this gives:

    subq    %rdi, %rsi
    movq    %rsi, %rax
    andq    $-4, %rax
    addq    %rdi, %rax

which seems to indicate that the exactness of the division was somehow lost
(otherwise the andq would disappear, and sub+add could then be simplified).