[erlang-questions] Stack space used by "body-recursive calls".

[Björn Gustavsson]

On Wed, Jun 30, 2010 at 10:02 AM, Kresten Krab Thorup <krab@REDACTED> wrote:
> One of my challenges in Erjang is that list comprehensions end up chewing stack space.  And today I got a glimpse of hope reading this:
>
> According to http://www.erlang.org/doc/efficiency_guide/myths.html
>
> In R12B and later releases, there is an optimization that will in many cases reduces the number of words used on the stack in body-recursive calls, so that a body-recursive list function and tail-recursive function that calls lists:reverse/1<../man/lists.html#reverse-1> at the end will use exactly the same amount of memory. lists:map/2, lists:filter/2, list comprehensions, and many other recursive functions now use the same amount of space as their tail-recursive equivalents.
>
> Can someone tell me, what exactly is the optimization that was done in R12B? It must be something in the runtime system that recognizes these conditions and does something smart; I'd love to be able to be equally smart.

No, it's a compiler optimization - stack trimming. List comprehensions and
other body recursive calls used to eat even more stack space before R12.
(The only support that was added to the run-time system was the trim/2
instruction to do the actual stack trimming.)

-- 
Björn Gustavsson, Erlang/OTP, Ericsson AB