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update performace section of gudie to mention unchecked ops

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svn: r16230
This commit is contained in:
Matthew Flatt 2009-10-03 15:07:41 +00:00
parent 9852bb16a2
commit 818c4b0409
1 changed files with 29 additions and 1 deletions
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30
collects/scribblings/guide/performance.scrbl
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@ -1,6 +1,7 @@
#lang scribble/doc
@(require scribble/manual
"guide-utils.ss")
"guide-utils.ss"
(for-label scheme/unsafe/ops))
@title[#:tag "performance"]{Performance}
@ -262,6 +263,33 @@ generational garbage collector (described later in @secref["gc-perf"])
makes allocation for short-lived results reasonably cheap. Fixnums, in
contrast are never boxed, so they are especially cheap to use.
The @schememodname[scheme/unsafe/ops] library provides fixnum- and
flonum-specific operations, and combinations of unchecked flonum
operations allow the @tech{JIT} compiler to generate code that avoids
boxing and unboxing intermediate results. Currently, only expressions
involving a combination of unchecked flonum operations,
@scheme[unsafe-fx->fl], constants, and variable references are
optimized to avoid boxing. See also @secref["unchecked-unsafe"],
especially the warnings about unsafety.
@; ----------------------------------------------------------------------
@section[#:tag "unchecked-unsafe"]{Unchecked, Unsafe Operations}
The @schememodname[scheme/unsafe/ops] library provides functions that
are like other functions in @schememodname[scheme/base], but they
assume (instead of checking) that provided arguments are of the right
type. For example, @scheme[unsafe-vector-ref] accesses an element from
a vector without checking that its first argument is actually a vector
and without checking that the given index is in bounds. For tight
loops that use these functions, avoiding checks can sometimes speed
the computation, though the benefits vary for different unchecked
functions and different contexts.
Beware that, as ``unsafe'' in the library and function names suggest,
misusing the exports of @schememodname[scheme/unsafe/ops] can lead to
crashes or memory corruption.
@; ----------------------------------------------------------------------
@section[#:tag "gc-perf"]{Memory Management}