Hi! On Wed, Apr 03, 2024 at 01:07:41PM +0200, Richard Biener wrote: > The following avoids re-walking and re-combining the instructions > between i2 and i3 when the pattern of i2 doesn't change. > > Bootstrap and regtest running ontop of a reversal of > r14-9692-g839bc42772ba7a.
Please include that in the patch (or series, preferably). > It brings down memory use frmo 9GB to 400MB and compile-time from > 80s to 3.5s. r14-9692-g839bc42772ba7a does better in both metrics > but has shown code generation regressions across acrchitectures. > > OK to revert r14-9692-g839bc42772ba7a? No. The patch solved a very real problem. How does your replacement handle that? You don't say. It looks like it only battles symptoms a bit, instead :-( We had this before: 3->2 combinations that leave an instruction identical to what was there before. This was just a combination with context as well. The only reason this wasn't a huge problem then already was because this is a 3->2 combination, even if it really is a 2->1 one it still is beneficial in all the same cases. But in the new case it can iterate indefinitely -- well not quite, but some polynomial number of times, for a polynomial at least of degree three, possibly more :-( With this patch you need to show combine still is linear. I don't think it is, but some deeper analysis might show it still is. ~ - ~ - ~ What should *really* be done is something that has been on the wish list for decades: an uncse pass. The things that combine no longer works on after my patch are actually 1->1 combinations (which we never do currently, although we probably should); or alternatively, an un-CSE followed by a 2->1 combination. We can do the latter of course, but we need to do an actual uncse first! Somewhere before combine, and then redo a CSE after it. An actual CSE, not doing ten gazillion other things. Segher
