[PATCH PR64705]Don't aggressively expand induction variable's base

[Bin Cheng]

Hi,
As comments in the PR, root cause is GCC aggressively expand induction
variable's base.  This patch avoids that by adding new parameter to
expand_simple_operations thus we can stop expansion whenever ssa var
referred by IV's step is encountered.  As comments in patch, we could stop
expanding at each ssa var referred by IV's step, but that's expensive and
not likely to happen, this patch only extracts the first ssa var and skips
expanding accordingly.
For the new test case, currently the loop body is bloated as below after
IVOPT:

<bb 5>:
  # ci_28 = PHI <ci_12(D)(4), ci_17(6)>
  # ivtmp.13_31 = PHI <ivtmp.13_25(4), ivtmp.13_27(6)>
  ci_17 = ci_28 + 1;
  _1 = (void *) ivtmp.13_31;
  MEM[base: _1, offset: 0B] = 0;
  ivtmp.13_27 = ivtmp.13_31 + _26;
  _34 = (unsigned long) _13;
  _35 = (unsigned long) flags_8(D);
  _36 = _34 - _35;
  _37 = (unsigned long) step_14;
  _38 = _36 - _37;
  _39 = ivtmp.13_27 + _38;
  _40 = _39 + 3;
  iter_33 = (long int) _40;
  if (len_16(D) >= iter_33)
    goto <bb 6>;
  else
    goto <bb 7>;

<bb 6>:
  goto <bb 5>;

And it can be improved by this patch as below:

<bb 5>:
  # steps_28 = PHI <steps_12(D)(4), steps_17(6)>
  # iter_29 = PHI <iter_15(4), iter_21(6)>
  steps_17 = steps_28 + 1;
  _31 = (sizetype) iter_29;
  MEM[base: flags_8(D), index: _31, offset: 0B] = 0;
  iter_21 = step_14 + iter_29;
  if (len_16(D) >= iter_21)
    goto <bb 6>;
  else
    goto <bb 7>;

<bb 6>:
  goto <bb 5>;


I think this is a corner case, it only changes several files' assembly code
slightly in spec2k6.  Among these files, only one of them is regression.  I
looked into the regression and thought it was because of passes after IVOPT.
The IVOPT dump is at least not worse than the original version.  

Bootstrap and test on x86_64 and AArch64, so is it OK?

2015-02-09  Bin Cheng  <bin.ch...@arm.com>

        PR tree-optimization/64705
        * tree-ssa-loop-niter.h (expand_simple_operations): New parameter.
        * tree-ssa-loop-niter.c (expand_simple_operations): New parameter.
        (tree_simplify_using_condition_1, refine_bounds_using_guard)
        (number_of_iterations_exit): Pass new argument to
        expand_simple_operations.
        * tree-ssa-loop-ivopts.c (extract_single_var_from_expr): New.
        (find_bivs, find_givs_in_stmt_scev): Pass new argument to
        expand_simple_operations.  Call extract_single_var_from_expr.
        (difference_cannot_overflow_p): Pass new argument to
        expand_simple_operations.

gcc/testsuite/ChangeLog
2015-02-09  Bin Cheng  <bin.ch...@arm.com>

        PR tree-optimization/64705
        * gcc.dg/tree-ssa/pr64705.c: New test.