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Chez Scheme: update docs for expand/optimize and run-cp0

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Mention that it runs cptypes.
This commit is contained in:
Matthew Flatt 2021-03-08 09:44:50 -07:00
parent 75ba7ff5bc
commit 5a2b6aab1e
1 changed files with 22 additions and 12 deletions
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34
racket/src/ChezScheme/csug/system.stex
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@ -1898,7 +1898,9 @@ an expression.
or unannotated value.
\scheme{expand/optimize} expands the expression in environment \var{env}
and passes the expression through the source optimizer \scheme{cp0}
(unless \scheme{cp0} is disabled via \scheme{run-cp0}).
and a type-inferring optimizer \scheme{cptypes}
(unless \scheme{cp0} and \scheme{cptypes} are disabled via \scheme{run-cp0},
or unless \scheme{cptypes} is disabled via \scheme{enable-type-recovery}).
It also simplifies \scheme{letrec} and \scheme{letrec*} expressions within
the expression and makes their undefined checks explicit.
It returns an object representing the expanded, simplified, and optimized form.
@ -1906,7 +1908,7 @@ If no environment is provided, it defaults to the environment
returned by \scheme{interaction-environment}.
\scheme{expand/optimize} is primarily useful for understanding what
\scheme{cp0} does and does not optimize.
\scheme{cp0} plus \scheme{cptypes} does and does not optimize.
Many optimizations are performed later in the compiler,
so \scheme{expand/optimize} does not give a complete picture of
optimizations performed.
@ -2864,9 +2866,10 @@ When the parameter is set to \scheme{#f}, the message is not printed.
\endentryheader
\noindent
These parameters control the operation of \scheme{cp0}, a source
optimization pass that runs after macro expansion and prior
These parameters control the operation of \scheme{cp0} plus \scheme{cptypes}, source
optimization passes that run after macro expansion and prior
to most other compiler passes.
\scheme{cp0} performs procedure inlining, in which the code of one
procedure is inlined at points where it is called by other procedures,
as well as copy propagation, constant folding, useless code
@ -2874,6 +2877,13 @@ elimination, and several related optimizations.
The algorithm used by the optimizer is described in detail in the paper
``Fast and effective procedure inlining''~\cite{waddell:sas97}.
\scheme{cptypes} performs additional optimizations based on type
information about primitives, such as the fact that \scheme{+} always
returns a number; since it primarily removes unneecssary run-time
checks, \scheme{cptypes} primarly benefits compilation in safe mode.
The \scheme{cptypes} pass can be independently disabled through the
\scheme{enable-type-recovery} parameter.
When \scheme{cp0} is enabled, the programmer can count on the compiler
to fold constants, eliminate unnecessary \scheme{let} bindings, and
eliminate unnecessary and inaccessible code.
@ -2903,9 +2913,9 @@ if \scheme{e} turns out to be an unassigned variable, and count on
the entire \scheme{case} expression to be folded if \scheme{e} turns
out to be a constant.
It is possible to see what \scheme{cp0} does with an expression
It is possible to see what \scheme{cp0} plus \scheme{cptypes} does with an expression
via the procedure \index{\scheme{expand/optimize}}\scheme{expand/optimize},
which expands its argument and passes the result through \scheme{cp0}, as
which expands its argument and passes the result through \scheme{cp0} and \scheme{cptypes}, as
illustrated by the following transcript.
\schemedisplay
@ -2914,9 +2924,9 @@ illustrated by the following transcript.
'(lambda (x)
(case x [(a) 1] [(b c) 2] [(d) 3] [else 4])))
(lambda (x)
(if (#2%memv x '(a))
(if (#2%member x '(a))
1
(if (#2%memv x '(b c)) 2 (if (#2%memv x '(d)) 3 4))))
(if (#2%member x '(b c)) 2 (if (#2%member x '(d)) 3 4))))
> (expand/optimize
'(+ (let ([f (lambda (x)
(case x [(a) 1] [(b c) 2] [(d) 3] [else 4]))])
@ -2928,7 +2938,7 @@ illustrated by the following transcript.
In the first example, the \scheme{let} expression produced by \scheme{case}
is eliminated, and in the second, the entire expression is optimized down
to the constant \scheme{17}.
Although not shown by \scheme{expand/optimize}, the \scheme{memv} calls
Although not shown by \scheme{expand/optimize}, the \scheme{member} calls
in the output code for the first example will be replaced by calls to the
less expensive \scheme{eq?} by a later pass of the compiler.
Additional examples are given in the description
@ -2937,9 +2947,9 @@ of \scheme{expand/optimize}.
The value of \scheme{run-cp0} must be a procedure.
Whenever the compiler is invoked on a Scheme form, the value \var{p}
of this parameter is called to determine whether and how
\scheme{cp0} is run.
\var{p} receives two arguments: \var{cp0}, the entry point into
\scheme{cp0}, and \var{x}, the form being compiled.
\scheme{cp0} and \scheme{cptypes} are run.
\var{p} receives two arguments: \var{cp0}, the entry point into the
\scheme{cp0} plus \scheme{cptypes} passes, and \var{x}, the form being compiled.
The default value of \scheme{run-cp0} simply invokes \var{cp0} on
\var{x}, then \var{cp0} again on the result.
The second run is useful in some cases because the first run