[plt-dev] performance-oriented unsafe operations (v4.2.1.8)

From: Doug Williams (m.douglas.williams at gmail.com)
Date: Sun Sep 6 16:22:34 EDT 2009


These look like good things for me to use for efficiency in the science
collection. Will these these be in the upcoming 4.2.2 release?

I assume that if I use these and introduce a dependency on 4.2.2 or later
that I should also bump the version number of the science collection in
PLaneT - even if the interface remains the same. Does that make sense?

I guess the term 'experiment' here confuses me. Is this something that might
go away again? Should I wait until everyone is comfortable with the results?


On Sun, Sep 6, 2009 at 12:27 PM, Matthew Flatt <mflatt at cs.utah.edu> wrote:

> Version (now in SVN) starts an experiment with
> performance-oriented unsafe operations.
> The new `scheme/unsafe/ops' library provides operations such as
> `unsafe-vector-ref' and `unsafe-fl+' (for inexact real addition) that
> are inlined by the JIT without contract checks. If a call violates the
> contract of a function from `scheme/unsafe/ops', arbitrarily bad
> things can happen. As long as contracts are satisfied, however,
> performance should be a little better than with corresponding safe
> operations.
> As part of the experiment, the expansion of `for' with `in-range' and
> `in-vector' now uses unsafe operations (in a safe way). For example,
> `in-vector' uses `unsafe-vector-ref' as well as a fixnum comparison
> for detecting when it has reached the end of a vector; it also uses
> unsafe fixnum arithmetic to increment the index as long as the step is
> 1 or -1 (in which case no overflow can occur during the
> increment). These improvements make a 10-20% difference in a tight
> loop over a vector.
> Future possible directions include making the JIT specialize
> combinations of inexact operations to avoid boxing intermediate
> results.
> The exports of `scheme/unsafe/ops' are protected, so that a code
> inspector can prevent access to unsafe operations by untrusted code.
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