Re: [tree-ssa] [RFC] loop versioning

[Zdenek Dvorak <rakdver at atrey dot karlin dot mff dot cuni dot cz>]

Hello,

> >>tree_duplicate_bb () is more suitable to copy loop header. For
> >>other basic blocks inside loop, while duplicating a basic block
> >>I may not have corresponding edge available.
> >
> >it is trivial to update tree_duplicate_bb to not require the edge
> >(similarly as cfg_layout_duplicate_bb does not).
> 
> true, but based on your example, you can see that its purpose is
> different.

ummm.... looking at your code, it does something very similar to loop
unswitching.  It might be a good idea to use the code we already have
for this (see loop-unswitch.c and related functions in cfgloop*.c and
cfglayout.c).  It would save you time with handling many technical
details (updating dominators and loop structures, handling irreducible
regions).  It would of course be needed to modify the code a bit so that
it works both over rtl and trees (using cfghooks.h), but it should be
quite straightforward to do.

> >Updating SSA after code duplication is nontrivial (in general 
> >equivalent
> >to doing a full rewrite for the definitions in the duplicated code).
> 
> yup, but I'm willing to give it a shot.

Fine, that definitely would be useful to have.  I would however stronly
recommend you to have a look on functions in tree-ssa.c, so that you
don't end up unnecesarily writing the same code again.

> What if I rename it to tree_duplicate_head_bb () and implement
> new duplicate_bb() as Jan suggested ?

Please avoid creating unnecessary code duplication.  If you need to
extend functionality provided by tree_duplicate_bb in some way, it would
be probably preferable to either extend it, or use it as a subroutine and
just add the extra functionality you need.

As for Honza's suggestions, I assume he was speaking about something like
cfglayout.c:copy_bbs.

Zdenek