[Bug tree-optimization/79578] Unnecessary instructions in generated code

[law at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79578

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at redhat dot com

--- Comment #5 from Jeffrey A. Law <law at redhat dot com> ---
So at the first DSE pass the IL looks like:

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  if (b_3(D) != 0B)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [0.00%]
;;    succ:       3
;;                4

;;   basic block 3, loop depth 0
;;    pred:       2
  # .MEM_9 = VDEF <.MEM_5(D)>
  MEM[(struct A *)b_3(D)] = {};
;;    succ:       4

;;   basic block 4, loop depth 0
;;    pred:       3
;;                2
  # .MEM_2 = PHI <.MEM_9(3), .MEM_5(D)(2)>
  # .MEM_12 = VDEF <.MEM_2>
  MEM[(struct A *)b_3(D)].a = 1;
  # .MEM_13 = VDEF <.MEM_12>
  MEM[(struct A *)b_3(D)].b = 2;
  # VUSE <.MEM_13>
  return b_3(D);
;;    succ:       EXIT

The initialization looks like a fully dead store to me.  And it is.  The reason
it's not caught is we currently require pointer equality on the address
expression.  Using operand_equal_p instead results in the initialization being
eliminated by DSE1.  I think the only question is whether or not to fix this
during stage4 or wait for gcc-8.  It's a 1-liner ;-)

Of course once the dead store is gone BB3 is pointless and we can eliminate the
test resulting in:

        movq    %rdi, %rax
        movl    $131073, (%rdi)
        ret