On Fri, Sep 15, 2017 at 10:56:04AM -0600, Jeff Law wrote: > Yes, these are some of the things we kicked around. One of the most > interesting conclusions was that for these target issues we'd really > like a target.pd file to handle this class of transformations just prior > to rtl expansion.
But often combine will need to know about which bits you actually care about (and other passes too, but combine is the biggie). > Essentially early type promotion/demotion would be concerned with cases > where we can eliminate operations in a target independent manner and > narrow operands as much as possible. Late promotion/demotion would deal > with stuff like the target's desire to work on specific sized hunks in > specific contexts. > > I'm greatly oversimplifying here. Type promotion/demotion is fairly > complex to get right. Yeah :-( Maybe the best thing is to promote really early, but to keep track of which bits matter. And then adjust some passes to take that into account. Not a trivial amount of work. Segher
