On Mon, 3 Mar 2014, Kai Tietz wrote: > 2014-03-03 12:33 GMT+01:00 Richard Biener <rguent...@suse.de>: > > On Fri, 28 Feb 2014, Kai Tietz wrote: > > > >> Hmm, this all reminds me about the approach Andrew Pinski and I came > >> up with two years ago. > > > > You are talking about the gimple folding interface? Yes, but it's > > more similar to what I proposed before that. > > Well, this interface was for rtl, gimple, and tree AFAIR. > > > >> So I doubt that we want to keep fold-const patterns similar to gimple > >> (forward-prop) ones. > >> Wouldn't it make more sense to move fold-const patterns completely > >> into gimple, and having a facility in FE to ask gimple to *pre*-fold a > >> given tree to see if a constant-expression can be achieved? > > > > That was proposed by somebody, yes. The FE would hand off an > > expression to 1) the gimplifier to gimplify it, then 2) to the > > gimple folder to simplify it. Not sure if that's a good design > > but yes, it mimics the awkward thing we do now (genericize for > > folding in fold_stmt), just the other way around - and it makes > > it very costly. > > Right, if we would redo step one, and two each time we visit same > statement again, then of course we would produce pretty high load. > By hashing this *pre*-computed gimple-expression I think the load of > such an approach would lower pretty much. Of course it is true that > we move gimplification-costs to FE. Nevertheless the avarage costs > should be in general the same as we have them now. > > > > Having a single meta-description of simplifications makes it > > possible to have the best of both worlds (no need to GENERICIZE > > back from GIMPLE and no need to GIMPLIFY from GENERIC) with > > a single point of maintainance. > > True. I am fully agreeing to the positive effects of a single > meta-description for this area. For sure it is worth to avoid the > re-doing of the same folding for GENERIC/GIMPLE again and again. > > > [the possibility to use offline verification tools for the > > transforms comes to my mind as well] > This is actually a pretty interesting idea. As it would allow us to > do testing for this area without side-effects by high-level passes, > target-properties, etc > > > If you think the .md style pattern definitions are too limiting > > can you think of sth more powerful without removing the possibility > > of implementing the matching with a state machine to make it O(1)? > > Well, I am not opposed to the use of .md style pattern defintions at > all. I see just some weaknesses on the current tree-folding > mechanism. > AST folder tries to fold into statementes by recursion into. This > causes pretty high load in different areas. Like stack-growth, > unnecessary repetition of operations on inner-statements, and complex > patterns for expression associative/distributive/commutative rules for > current operation-code.
Actually it doesn't recurse - it avoids recursion by requiring each sub-pattern to be already folded. > I am thinking about a model where we use just for the > base-fold-operations the .md-style pattern definitions. On top of this > model we set a layer implementing associative/commutative/distributive > properties for statements in an optimize form. Sure, that would be a pass using the match-and-simplify infrastructure. In fact the infrastructure alone only provides enough to do the bare folding. > By this we can do two different things with lower load. One hand we > can do "virtual" folding and avoid tree-rebuilding without need. On > the second hand we can use same pass for "normalize" tree structure of > expression-chains. > Additionally we can get rid of the than pretty useless reassociation > pass, which is IMHO just necessary by gimple-folders inability to do > that. Well ... you still need a pass that re-associates (or distributes or applies whatever properties) to feed the match-and-simplify infrastructure with proper input. So no, reassoc won't get and isn't useless. Richard.
