Re: Optimization for static local variables

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Jun 14, 2017 at 1:14 PM, Prachi Godbole
<Prachi.Godbole@imgtec.com> wrote:
> I'm developing a solution to optimize away intermediate stores (loads) for static local variables which are assigned to before referenced on every path through a function.
>
> Currently GCC eliminates all loads/stores in a straight line code but not in control structures. AFAIU, passes like sccvn, lim (store motion in particular) sink etc. try to propagate the assignment exprs through SSA vars instead of through memory thereby eliminating almost all loads and some stores but still not all.
>
> For example:
>
> void some_function()
> {
>     static some_type sum;
>     ...
>     for( i = 0 ; i < n ; i++ )
>          {
>              sum = matrix [i][n] ;
>
>              for( j = 0 ; j < i ; j++ )
>                  {
>                      sum -= matrix [i][j] * matrix [j][n] ;
>                  }
>
>             matrix [i][n] = sum ;
>         }
>     ...
> }
>
> Resulting SSA:
>
> ...
>   <bb 14> [2.25%]:
>   _10 = matrix[0][n_13(D)];
>   sum = _10;
>   goto <bb 10>; [100.00%]
> ...
>   <bb 4> [12.75%]:
>   _1 = matrix[i_16][n_13(D)];
>   if (i_16 > 0)
>     goto <bb 6>; [100.00%]
>   else
>     goto <bb 9>; [0.00%]
> ...
>   <bb 6> [85.00%]:
>   # j_24 = PHI <j_18(6), 0(4)>
>   # sum_lsm.4_27 = PHI <_6(6), _1(4)>
>   _3 = matrix[i_16][j_24];
>   _4 = matrix[j_24][n_13(D)];
>   _5 = _3 * _4;
>   _6 = sum_lsm.4_27 - _5;
>   j_18 = j_24 + 1;
>   if (i_16 > j_18)
>     goto <bb 6>; [85.00%]
>   else
>     goto <bb 7>; [15.00%]
> ...
>   <bb 7> [12.75%]:
>   # _40 = PHI <_6(6)>
>   goto <bb 9>; [100.00%]
>
>   <bb 9> [12.75%]:
>   # sum_lsm.4_19 = PHI <_40(7), _1(4)>
>
>   <bb 10> [15.00%]:
>   # i_20 = PHI <i_16(9), 0(14)>
>   # sum_lsm.4_8 = PHI <sum_lsm.4_19(9), _10(14)>
>   # sum_lsm.5_22 = PHI <1(9), 0(14)>
>   matrix[i_20][n_13(D)] = sum_lsm.4_8;
>   i_16 = i_20 + 1;
>   if (n_13(D) > i_16)
>     goto <bb 4>; [85.00%]
>   else
>     goto <bb 11>; [15.00%]
>
>   <bb 11> [2.25%]:
>   # sum_lsm.4_39 = PHI <sum_lsm.4_8(10)>
>   # sum_lsm.5_38 = PHI <sum_lsm.5_22(10)>
>   if (sum_lsm.5_38 != 0)
>     goto <bb 12>; [0.00%]
>   else
>     goto <bb 13>; [100.00%]
>
>   <bb 12> [0.00%]:
>   sum = sum_lsm.4_39;
> ...
>
> Although all intermediate loads are eliminated here, store to sum before and after the 'j' loop (see bb 14 and bb 12) still remain which can be eliminated.
>
> My solution would reuse a lot of data structures and routines from the sink pass. So my question is, can I add this optimization into the sink pass itself or would it require a new pass and if yes, what would be the ideal place for it.
> Any other pointers are more than welcome.

So the idea is that for any store to such variable that is dominated
by a kill the function exit doesn't count as a use, right?
(unless the variable escapes the scope of the function, for example
via a nested function -- which might be the case that
is very difficult to detect).

I'm not sure what part of the sink pass is suitable for this but as
sinking walks post dominators backwards which also DSE
does it would fit DSE as well, no?  You'd pre-compute blocks
containing kills (or do two backwards passes) and then
special case GIMPLE_RETURN in dse_classify_store where previously
ref_maybe_used_by_stmt_p covers those.

nested fn case:

void foo (void)
{
  static int i = 0;
  int __attribute__((noinline)) bar () { return i; }
  int j = bar ();
  i = 1;
}

I think we don't mark 'i' TREE_ADDRESSABLE (escaped) here so we simply
treat 'i' as a global variable accessible
from other scopes.

Richard.

> Thanks,
> Prachi