On Sat, May 9, 2009 at 10:07 PM, Albert Cohen <albert.co...@inria.fr> wrote: > Sebastian Pop and I have been discussing the option of designing a new pass, > based on vrp, to normalize integer types towards a canonical supertype > typically a machine word, equivalent to signed long, or to truncate to a > smaller-size word when it makes sense. This would be a very simple pass (on > top of not-so-simple vrp), but arguably a quite regression-prone one as well > (due to aliases/escape and common C standard violations). > > The pass could be parameterized with three different objectives, depending > on where it is scheduled in the pass manager. > > (1) canonicalize to the supertype aggressively, to facilitate the > application of further passes like autovect which require very precise > understanding of the type conversions; > (2) compress the types to increase vectorization factor and reduce register > pressure (assuming the target supports sub-word register allocation with > register aliases); > (3) optimize the types to minimize the dynamic number of casts that result > in actual ASM instructions. > > Graphite and the vectorizer would clearly benefit from such a pass, at least > if it implemented objective (1). > > I wonder if some of this is already implemented somewhere, or if someone > played with it in the past, or is interesting in contributing. > > Nothing is planned yet on our side, and temporary fixes exist in the short > term (as far as Graphite and the vectorizer are concerned), but it would > potentially be of great help.
This is certainly one useful transformation based on value-range information. The choice of a canonical type is of course at least target dependent. I suppose you want to do this on register variables only? Did you think about promoting function arguments and returns as well as part of an IPA pass? I don't understand how register variable promotion/demotion will help graphite though - I had the impression graphite can only work on memory. No? Thanks, Richard.
