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Re: [PATCH] Remove basic_block->loop_depth

On Tue, 14 Aug 2012, Ulrich Weigand wrote:

> Richard Guenther wrote:
> > Accessing loop_depth (bb->loop_father) isn't very expensive.  The
> > following removes the duplicate info in basic-blocks which is not
> > properly kept up-to-date at the moment.
> Looks like this broke SPU build, since spu_machine_dependent_reorg
> accesses ->loop_depth.  According to comments in the code, this
> was done because of concerns that loop_father may no longer be set up
> this late in compilation, so I'm wondering whether just replacing
> this by loop_depth (bb->loop_father) would work here ...

Well, if loops are no longer set up (thus ->loop_father is NULL) then
the loop_depth information was stale and possibly wrong.

>           /* If this branch is a loop exit then propagate to previous
>              fallthru block. This catches the cases when it is a simple
>              loop or when there is an initial branch into the loop. */
>           if (prev && (loop_exit || simple_loop)
>               && prev->loop_depth <= bb->loop_depth)
>             prop = prev;
>           /* If there is only one adjacent predecessor.  Don't propagate
>              outside this loop.  This loop_depth test isn't perfect, but
>              I'm not sure the loop_father member is valid at this point.  */
>           else if (prev && single_pred_p (bb)
>                    && prev->loop_depth == bb->loop_depth)
>             prop = prev;
> Any suggestions?

If SPU md reorg would like to look at loop structures it should
compute them.  Simply call flow_loops_find, which hopefully works
in CFG RTL mode (which I think is the mode available from md reorg?).

I was simply throwing away loops after RTL loop optimizers not only
because IRA for some weird reason decides to re-compute them in
non-standard ways and because loop verification fails between
ira / reload passes.  So the other way would be to preserve loops
for a longer period.


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