Re: Moving statements from one BB to other BB.

["Richard Guenther" <richard dot guenther at gmail dot com>]

On Fri, Apr 18, 2008 at 9:34 AM, Sandeep Maram <sandeep244@gmail.com> wrote:
> Hi,
>
>  Consider the 2 for loops given below.
>
>  for (i = 0; i < N ; i++)
>     {
>       a[i]= 1;
>     }
>
>  for (i = 0; i < N ; i++)
>     {
>       j = j+a[i];
>     }
>
>  The headers of these 2 loops are bb_3, bb_6 respectively. They are as follows.
>
>  bb_3 (preds = {bb_4 bb_2 }, succs = {bb_4 bb_5 })
>   {
>   <bb 3>:
>     # a_27 = PHI <a_18(4), a_17(D)(2)>
>     # i_24 = PHI <i_7(4), 0(2)>
>     # a_18 = VDEF <a_27> { a }
>     a[i_24] = 1;
>     i_7 = i_24 + 1;
>     if (i_7 <= 999)
>       goto <bb 4>;
>     else
>       goto <bb 5>;
>
>   }
>
>  bb_6 (preds = {bb_5 bb_7 }, succs = {bb_7 bb_8 })
>   {
>   <bb 6>:
>     # j_26 = PHI <0(5), j_12(7)>
>     # i_25 = PHI <0(5), i_13(7)>
>     # VUSE <a_18> { a }
>
>     D.1189_10 = a[i_25];
>     D.1190_11 = (unsigned int) D.1189_10;
>     j_12 = D.1190_11 + j_26;
>     i_13 = i_25 + 1;
>     if (i_13 <= 999)
>       goto <bb 7>;
>     else
>       goto <bb 8>;
>
>   }
>
>  I am transfering statements from bb_6 to bb_3 using code -
>
>  for (bsi = bsi_start (loop_a->header); !bsi_end_p (bsi);)
>     {
>         if ((TREE_CODE (bsi_stmt (bsi)) != LABEL_EXPR) && (TREE_CODE
>  (bsi_stmt (bsi)) != COND_EXPR))
>            {
>             bsi_move_before (&bsi, &bsi_b_last);
>            }
>         else
>            {
>             bsi_next (&bsi);
>            }
>      }
>
>  After this the two BBs look like -
>
>  bb_6 (preds = {bb_5 bb_7 }, succs = {bb_7 bb_8 })
>   {
>   <bb 6>:
>     # j_26 = PHI <0(5), j_12(7)>
>     # i_25 = PHI <0(5), i_13(7)>
>     if (i_13 <= 999)
>       goto <bb 7>;
>     else
>       goto <bb 8>;
>
>   }
>
>  bb_3 (preds = {bb_4 bb_2 }, succs = {bb_4 bb_5 })
>   {
>   <bb 3>:
>     # a_27 = PHI <a_18(4), a_17(D)(2)>
>     # i_24 = PHI <i_7(4), 0(2)>
>     # a_18 = VDEF <a_27> { a }
>     a[i_24] = 1;
>     # VUSE <a_18> { a }
>
>     D.1189_10 = a[i_25];
>     D.1190_11 = (unsigned int) D.1189_10;
>     j_12 = D.1190_11 + j_26;
>     i_13 = i_25 + 1;
>     i_7 = i_24 + 1;
>     if (i_7 <= 999)
>       goto <bb 4>;
>     else
>       goto <bb 5>;
>
>   }
>
>  The iterator variables for the 2 loops are i_24, i_25 respectively. So
>  when the statements are transferred, there is a problem with i_25 in
>  bb_3 . How can I change the name of i_25 to i_24 ?

You can for example add a copy instruction

  i_25 = i_24

at the top of the new loop (but you have to remove the existing definition
for i_25 in the old loop then).  Otherwise you can change and walk
operands of trees using the immediate use iterators and SET_USE.
Just grep for it and you'll find example uses.

Richard.