FWIW, the optimization coach confirms both unrollings, and the fact that neither `build-string1` nor `test1` (you wrote `test2`, was that a mistake?) are ever inlined.
Reaching out for the decompiler would not have been necessary. :) Vincent At Mon, 4 Aug 2014 06:47:34 +0100, Matthew Flatt wrote: > > Well... a function call is expensive relative to some things, such as > adding fixnums to produce a fixnum. > > > My read of your initial results is that calling an unknown function is > similar to the cost of one iteration in a loop that sets a character in > a string. > > More precisely, decompiling the program shows that the compiler unrolls > the loop in `test1` by 4 iterations, so one call to an unknown function > is the same cost as 4 generic `<`s on fixnums, 4 generic `+ 1`s on > fixnums, 4 `integer->char`s, 4 `string-set!s`, and one jump to a known > function (to recur). > > The `build-string1` loop is similarly unrolled 4 times, and the call to > `build-string1` is not inlined in `test3`, so we really can compare the > unknown function call in `test3` to the overall loop overhead plus > `string-set!`s of `test1`. And the fact that neither `build-string1` > nor `test2 is inlined is consistent with `test2` and `test3` having the > same performance. > > If I change the `for` body of `test1` with a constant, then the time is > cut in half. So, a `string-set!` takes up about half of the time. > > > Putting that all together, it looks like a call to an unknown function > takes about twice the time of a `string-set!` --- which (due to various > tag and bounds checks for the string assignment) costs the same as a > small pile of generic arithmetic on fixnums. That could be considered > expensive in a tight loop. > > I would hesitate to say that higher order functions are always slow, > because many uses do more work around the call than set one character > in a string. > > > At Mon, 04 Aug 2014 07:42:53 +0400, Roman Klochkov wrote: > > > unknown function call is expensive > > > > So, higher order function always slow. > > Thus, to improve performance, one should use for/fold, for/list, ... and > > never > > use map, foldl, build-string, ... with lambda. > > Is it correct? > > > > Sun, 3 Aug 2014 13:15:57 -0400 от Matthias Felleisen <matth...@ccs.neu.edu>: > > > > > >Because build-string calls an unknown function 1000 x 100000 times, and an > > unknown function call is expensive. > > > > > >Racket could possible collapse all modules and perform additional > > >in-lining > > optimizations eventually, which may help here. But it doesn't yet. > > > > > >-- Matthias > > > > > > > > > > > >On Aug 3, 2014, at 5:15 AM, Roman Klochkov wrote: > > >>Are higher order function always slow? > > >>... > > >>--- > > >> > > >>So I see, that build-string version is about two times slower, than > > set-in-the-loop. Why so much? I expected about 10-20% difference. > > >>-- > > >>Roman Klochkov ____________________ > > >> Racket Users list: > > >> http://lists.racket-lang.org/users > > > > > > > > > -- > > Roman Klochkov > > ____________________ > > Racket Users list: > > http://lists.racket-lang.org/users > > ____________________ > Racket Users list: > http://lists.racket-lang.org/users ____________________ Racket Users list: http://lists.racket-lang.org/users
