On Dec 13, 2010, at 09:43 , Alain Frisch wrote: > On 12/12/2010 08:09 PM, Benedikt Meurer wrote: >> The boxing involved is relevant, but boxing in general is not the >> issue. In this special case, the "let nlen, n = if..." code requires >> heap allocation, because of the way the pattern is compiled. This could >> be fixed by moving the condition out of the code and using two if's to >> select n/nlen separately (doesn't speed up that much). Fixing the >> pattern compiler to handle these cases might be interesting for general >> benefit. > > Instead of duplicating the conditional, you could also push the assignments > to bound variables down the expression. For instance: > > let (x, y) = if b then (u, v) else (v, u) in ... > > can be replaced, conceptually, by: > > let x = <dummy> in > let y = <dummy> in > if b then (x <- u; y <- v) else (x <- v; y <- u); > ... > > and similarly when the bound expression is a pattern matching. > > > I've played with this a few months ago and could observe important speedups > (27%, 20%) on two micro-benchmarks. > > The diff is really small: > > http://caml.inria.fr/cgi-bin/viewcvs.cgi/ocaml/branches/inplace_let/bytecomp/matching.ml?rev=10475&sortby=date&r2=10475&r1=10474
Nice. But it would be even better to avoid the dummy, in your example let x = u in let y = v in if b then x <- v; y <- u This does not only avoid the dummy, but would also allow lowering to "cmovcc" instructions in the backend selector (atleast for x86-32/64). > -- Alain Benedikt _______________________________________________ Caml-list mailing list. Subscription management: http://yquem.inria.fr/cgi-bin/mailman/listinfo/caml-list Archives: http://caml.inria.fr Beginner's list: http://groups.yahoo.com/group/ocaml_beginners Bug reports: http://caml.inria.fr/bin/caml-bugs
