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Re: Slowness when using lambda expressions

From: Elias Mårtenson
Subject: Re: Slowness when using lambda expressions
Date: Mon, 30 Mar 2020 19:51:23 +0800

Thank you for the clarification. I can see that the problem isn't that the lambda function is slow. It's the +/ variant that is really fast. :-)

Of course, as you say, in real code you'd never write it the way I did. :-)


On Mon, 30 Mar 2020 at 17:53, Dr. Jürgen Sauermann <mail@jürgen-sauermann.de> wrote:
Hi Elias,

in GNU APL (and I suppose also in other APLs) lambda expressions are not
macros (or inline functions) but fully-fledged defined functions.
That is,

{⍺+⍵}/ ⍳1000000 is NOT equivalent to:  +/ ⍳1000000


 Z←A + B

and then:

{⍺+⍵}/ ⍳1000000 is equivalent to: FOO/⍳1000000

The 12 seconds are mainly spent for one million calls
of FOO, each call passing two scalar arguments A and B
and returning a scalar Z. It also wraps every Cell (i.e. every
ravel item) of A, B, and Z into a scalar value A, B, and Z.
Ravel cells are rather light-weight while values are more heavy
(each scalar value has, for example, its own shape vector).
And finally: each call of FOO pushes and pops an )SI entry and
a value stack entry for each A, B, and Z. These operations
are pretty fast but become noticeable if you do them very often.

In your example one call of FOO takes 12 micro-seconds which
is, IMHO, not too bad.

You could have avoided this overhead by computing:

{+/ ⍵} ⍳1000000 instead of: {⍺+⍵}/  ⍳1000000

Best Regards,
Jürgen Sauermann

On 3/30/20 3:39 AM, Elias Mårtenson wrote:
The following _expression_ takes about 12 seconds to compute on my laptop:

{⍺+⍵}/ ⍳1000000

However, the equivalent _expression_ without the lambda _expression_ is immediate (i.e. thr prompt returns before I have time to notice that it even started calculating):

+/ ⍳1000000

What is causing the large difference in performance?


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