Re: Optimization for static local variables

["Bin.Cheng" <amker dot cheng at gmail dot com>]

On Wed, Jun 14, 2017 at 12:14 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.
It looks like dead store elimination to sum.  Maybe static local
variable can be better handled there?

Thanks,
bin
>
> 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.
>
> Thanks,
> Prachi