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Re: Interest in integer auto-upcasting pass for normalization and optimization?


On Sat, May 9, 2009 at 10:07 PM, Albert Cohen <Albert.Cohen@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.


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