Re: high level loop optimizations in gcc-4.1.2

["Daniel Berlin" <dberlin at dberlin dot org>]

On 8/15/07, ranjith kumar <ranjit_kumar_b4u@yahoo.co.uk> wrote:
> Hi,
>      I know that linear loop transformations on tree
> representation can be enabled by -ftree-loop-linear
> flag.
>
> It is said that these transformations include
> loop-interchange, reversal,skewing,scaling.
> These transformations are done by apllying different
> transformation matrices. For example for
> loop-interchange transfomation matrix is (0 1)
>                                          (1 0)
>
> 1)Where can I find example programs for which
> gcc-4.1.2 can do these transformations.
ltrans-* in the testsuite

>
> 2) I have had a look at internal files. It seems that
> gcc-4.1.2 can do only loop-interchange only.
> Am I right?
>
No.
It can do the others, but nothing *asks it to*.
This is very different than not being able to do it.

> 3)If gcc-4.1.2 can do other transformations, what are
> they? and how gcc-4.1.2 selects which particular
> transformation matrix to apply??( Gcc can apply any
> integer non-singular matrix theoretically.)

tree-loop-linear has code to try to figure out a good matrix, but as
you said, it only currenly decides to do interchanges.

(4.2 or 4.3 has better code here, but it still only chooses
interchange matrices)