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[sbcl.git] / doc / compiler.xml

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<chapter id="compiler"><title>The Compiler</title>

<para>This chapter will discuss most compiler issues other than
efficiency, including compiler error messages, the &SBCL; compiler's
unusual approach to type safety in the presence of type declarations,
the effects of various compiler optimization policies, and the way
that inlining and open coding may cause optimized code to differ from
a naive translation. Efficiency issues are sufficiently varied and
separate that they have <link linkend="efficiency">their own
chapter</link>.</para>

<sect1 id="error-messages"><title>Error Messages</title>
<!--INDEX {error messages}{compiler}-->
<!--INDEX {compiler error messages}-->

<para>The compiler supplies a large amount of source location
information in error messages. The error messages contain a lot of
detail in a terse format, so they may be confusing at first. Error
messages will be illustrated using this example program:
<programlisting>(defmacro zoq (x)
`(roq (ploq (+ ,x 3))))

(defun foo (y)
(declare (symbol y))
(zoq y))</programlisting>
The main problem with this program is that it is trying to add
<literal>3</literal> to a symbol. Note also that the functions
<function>roq</function> and <function>ploq</function> aren't defined anywhere.
</para>

<sect2><title>The Parts of the Error Message</title>

<para>When processing this program, the compiler will produce this
warning: 

<screen>file: /tmp/foo.lisp

in: DEFUN FOO
(ZOQ Y)
--> ROQ PLOQ + 
==>
Y
caught WARNING:
Result is a SYMBOL, not a NUMBER.</screen>
In this example we see each of the six possible parts of a compiler error
message:
<orderedlist>
  <listitem><para><computeroutput>File: /tmp/foo.lisp</computeroutput>
  This is the name of the file that the compiler read the relevant
  code from.  The file name is displayed because it may not be
  immediately obvious when there is an error during compilation of a
  large system, especially when
  <function>with-compilation-unit</function> is used to delay
  undefined warnings.</para></listitem>
  <listitem><para><computeroutput>in: DEFUN FOO</computeroutput> This is the
  definition top level form responsible for the error. It is
  obtained by taking the first two elements of the enclosing form
  whose first element is a symbol beginning with <quote><literal>def</literal></quote>.
  If there is no such enclosing <quote><literal>def</literal></quote> form, then the 
  outermost form is used.  If there are multiple <literal>def</literal>
  forms, then they are all printed from the outside in, separated by
  <literal>=></literal>'s.  In this example, the problem was in the
  <function>defun</function> for <function>foo</function>.</para></listitem>
  <listitem><para><computeroutput>(ZOQ Y)</computeroutput> This is the
  <emphasis>original source</emphasis> form responsible for the error.
  Original source means that the form directly appeared in the
  original input to the compiler, i.e. in the lambda passed to
  <function>compile</function> or in the top level form read from the
  source file. In this example, the expansion of the <function>zoq</function>
  macro was responsible for the error.</para></listitem>
  <listitem><para><computeroutput>--> ROQ PLOQ +</computeroutput> This is the
  <emphasis>processing path</emphasis> that the compiler used to produce
  the errorful code.  The processing path is a representation of
  the evaluated forms enclosing the actual source that the
  compiler encountered when processing the original source.
  The path is the first element of each form, or the form itself
  if the form is not a list.  These forms result from the
  expansion of macros or source-to-source transformation done
  by the compiler.  In this example, the enclosing evaluated forms
  are the calls to <function>roq</function>, <function>ploq</function> and
  <function>+</function>.  These calls resulted from the expansion of
  the <function>zoq</function> macro.</para></listitem>
  <listitem><para><computeroutput>==> Y</computeroutput> This is the
  <emphasis>actual source</emphasis> responsible for the error. If
  the actual source appears in the explanation, then
  we print the next enclosing evaluated form, instead of
  printing the actual source twice.  (This is the form
  that would otherwise have been the last form of the processing
  path.) In this example, the problem is with the evaluation of
  the reference to the variable <varname>y</varname>.</para></listitem>
  <listitem><para>
  <computeroutput>caught WARNING: Result is a SYMBOL, not a NUMBER.</computeroutput>
  This is the <emphasis>explanation</emphasis> of the problem. In this
  example, the problem is that <varname>y</varname> evaluates to a symbol,
  but is in a context where a number is required (the argument
  to <function>+</function>).</para></listitem>
</orderedlist>

Note that each part of the error message is distinctively marked:

<itemizedlist>
  <listitem><para> <computeroutput>file:</computeroutput> and <computeroutput>in:</computeroutput>
  mark the file and definition, respectively.</para></listitem>
  <listitem><para> The original source is an indented form with no
  prefix.</para></listitem>
  <listitem><para> Each line of the processing path is prefixed with
  <computeroutput>--></computeroutput></para></listitem>
  <listitem><para> The actual source form is indented like the original
  source, but is marked by a preceding <computeroutput>==></computeroutput> line.
  </para></listitem>
  <listitem><para> The explanation is prefixed with the error
  severity, which can be <computeroutput>caught ERROR:</computeroutput>,
  <computeroutput>caught WARNING:</computeroutput>,
  <computeroutput>caught STYLE-WARNING:</computeroutput>, or
  <computeroutput>note:</computeroutput>. </para></listitem>
</itemizedlist>
</para>

<para>Each part of the error message is more specific than the preceding
one.  If consecutive error messages are for nearby locations, then the
front part of the error messages would be the same.  In this case, the
compiler omits as much of the second message as in common with the
first.  For example:
<screen>file: /tmp/foo.lisp

in: DEFUN FOO
(ZOQ Y)
--> ROQ
==>
(PLOQ (+ Y 3))
caught STYLE-WARNING:
undefined function: PLOQ

==>
(ROQ (PLOQ (+ Y 3)))
caught STYLE-WARNING:
undefined function: ROQ</screen>
In this example, the file, definition and original source are
identical for the two messages, so the compiler omits them in the
second message.  If consecutive messages are entirely identical, then
the compiler prints only the first message, followed by:
<computeroutput>[Last message occurs <replaceable>repeats</replaceable> times]</computeroutput>
where <replaceable>repeats</replaceable> is the number of times the message
was given.</para>

<para>If the source was not from a file, then no file line is printed.
If the actual source is the same as the original source, then the
processing path and actual source will be omitted. If no forms
intervene between the original source and the actual source, then the
processing path will also be omitted.</para>

</sect2>

<sect2><title>The Original and Actual Source</title>

<para>The <emphasis>original source</emphasis> displayed will almost always be
a list. If the actual source for an error message is a symbol, the
original source will be the immediately enclosing evaluated list form.
So even if the offending symbol does appear in the original source,
the compiler will print the enclosing list and then print the symbol
as the actual source (as though the symbol were introduced by a
macro.)</para>

<para>When the <emphasis>actual source</emphasis> is displayed
(and is not a symbol), it will always
be code that resulted from the expansion of a macro or a source-to-source
compiler optimization.  This is code that did not appear in the original
source program; it was introduced by the compiler.</para>

<para>Keep in mind that when the compiler displays a source form
in an error message, it always displays the most specific (innermost)
responsible form.  For example, compiling this function
<programlisting>(defun bar (x)
(let (a)
(declare (fixnum a))
(setq a (foo x))
a))</programlisting>
gives this error message
<screen>in: DEFUN BAR
(LET (A) (DECLARE (FIXNUM A)) (SETQ A (FOO X)) A)
caught WARNING: The binding of A is not a FIXNUM:
NIL</screen>
This error message is not saying <quote>there is a problem somewhere in
this <function>let</function></quote> &mdash; it is saying that there is a
problem with the <function>let</function> itself. In this example, the problem
is that <varname>a</varname>'s <literal>nil</literal> initial value is not a
<type>fixnum</type>.</para>

</sect2>

<sect2><title>The Processing Path</title>
<!--INDEX processing path-->
<!--INDEX macroexpansion-->
<!--INDEX source-to-source transformation-->

<para>The processing path is mainly useful for debugging macros, so if
you don't write macros, you can probably ignore it. Consider this
example:

<programlisting>(defun foo (n)
(dotimes (i n *undefined*)))
</programlisting>

Compiling results in this error message:

<screen>in: DEFUN FOO
(DOTIMES (I N *UNDEFINED*))
--> DO BLOCK LET TAGBODY RETURN-FROM
==>
(PROGN *UNDEFINED*)
caught STYLE-WARNING:
undefined variable: *UNDEFINED*</screen>

Note that <function>do</function> appears in the processing path. This is because
<function>dotimes</function> expands into:

<programlisting>(do ((i 0 (1+ i)) (#:g1 n))
((>= i #:g1) *undefined*)
(declare (type unsigned-byte i)))</programlisting>

The rest of the processing path results from the expansion
of <function>do</function>:

<programlisting>
  (block nil
  (let ((i 0) (#:g1 n))
  (declare (type unsigned-byte i))
  (tagbody (go #:g3)
  #:g2    (psetq i (1+ i))
  #:g3    (unless (>= i #:g1) (go #:g2))
  (return-from nil (progn *undefined*)))))
</programlisting>

In this example, the compiler descended into the <function>block</function>,
<function>let</function>, <function>tagbody</function> and <function>return-from</function> to
reach the <function>progn</function> printed as the actual source. This is a
place where the <quote>actual source appears in explanation</quote> rule
was applied. The innermost actual source form was the symbol
<varname>*undefined*</varname> itself, but that also appeared in the
explanation, so the compiler backed out one level.</para>

</sect2>

<sect2><title>Error Severity</title>
<!--INDEX severity of compiler errors -->
<!--INDEX compiler error severity -->

<para>There are four levels of compiler error severity:
<wordasword>error</wordasword>, <wordasword>warning</wordasword>, <wordasword>style
warning</wordasword>, and <wordasword>note</wordasword>. The first three levels correspond
to condition classes which are defined in the &ANSI; standard for
&CommonLisp; and which have special significance to the
<function>compile</function> and <function>compile-file</function> functions. These
levels of compiler error severity occur when the compiler handles
conditions of these classes. The fourth level of compiler error
severity, <wordasword>note</wordasword>, is used for problems which are too mild
for the standard condition classes, typically hints about how
efficiency might be improved.</para>

</sect2>

<sect2><title>Errors During Macroexpansion</title>
<!--INDEX {macroexpansion}{errors during}-->

<para>The compiler handles errors that happen during macroexpansion,
turning them into compiler errors. If you want to debug the error (to
debug a macro), you can set <varname>*break-on-signals*</varname> to
<literal>error</literal>. For example, this definition:

<programlisting>(defun foo (e l)
(do ((current l (cdr current))
((atom current) nil))
(when (eq (car current) e) (return current))))</programlisting>

gives this error:

<screen>in: DEFUN FOO
(DO ((CURRENT L #) (# NIL)) (WHEN (EQ # E) (RETURN CURRENT)) )
caught ERROR: 
(in macroexpansion of (DO # #))
(hint: For more precise location, try *BREAK-ON-SIGNALS*.)
DO step variable is not a symbol: (ATOM CURRENT)</screen>
</para>

</sect2>

<sect2><title>Read Errors</title>
<!--INDEX {read errors}{compiler}-->

<para>&SBCL;'s compiler (unlike &CMUCL;'s) does not attempt to recover
from read errors when reading a source file, but instead just reports
the offending character position and gives up on the entire source
file.</para>

</sect2>

<!-- FIXME: How much control over error messages is in SBCL?
     _     How much should be? How much of this documentation should
     _     we save or adapt? 
     _ 
     _ %%\node Error Message Parameterization,  , Read Errors, Interpreting Error Messages
     _ \subsection{Error Message Parameterization}
     _ \cpsubindex{error messages}{verbosity}
     _ \cpsubindex{verbosity}{of error messages}
     _ 
     _ There is some control over the verbosity of error messages.  See also
     _ \varref{undefined-warning-limit}, \code{*efficiency-note-limit*} and
     _ \varref{efficiency-note-cost-threshold}.
     _ 
     _ \begin{defvar}{}{enclosing-source-cutoff}
     _ 
     _   This variable specifies the number of enclosing actual source forms
     _   that are printed in full, rather than in the abbreviated processing
     _   path format.  Increasing the value from its default of \code{1}
     _   allows you to see more of the guts of the macroexpanded source,
     _   which is useful when debugging macros.
     _ \end{defvar}
     _ 
     _ \begin{defvar}{}{error-print-length}
     _   \defvarx{error-print-level}
     _ 
     _   These variables are the print level and print length used in
     _   printing error messages.  The default values are \code{5} and
     _   \code{3}.  If null, the global values of \code{*print-level*} and
     _   \code{*print-length*} are used.
     _ \end{defvar}
     _ 
     _ \begin{defmac}{extensions:}{define-source-context}{%
     _     \args{\var{name} \var{lambda-list} \mstar{form}}}
     _ 
     _   This macro defines how to extract an abbreviated source context from
     _   the \var{name}d form when it appears in the compiler input.
     _   \var{lambda-list} is a \code{defmacro} style lambda-list used to
     _   parse the arguments.  The \var{body} should return a list of
     _   subforms that can be printed on about one line.  There are
     _   predefined methods for \code{defstruct}, \code{defmethod}, etc.  If
     _   no method is defined, then the first two subforms are returned.
     _   Note that this facility implicitly determines the string name
     _   associated with anonymous functions.
     _ \end{defmac}
     _ 
     _ -->

</sect1>

<sect1 id="compiler-types"><title>The Compiler's Handling of Types</title>

<para>The most unusual features of the &SBCL; compiler (which is
very similar to the original &CMUCL; compiler, also known as
&Python;) is its unusually sophisticated understanding of the
&CommonLisp; type system and its unusually conservative approach to
the implementation of type declarations. These two features reward the
use of type declarations throughout development, even when high
performance is not a concern. (Also, as discussed <link
linkend="efficiency">in the chapter on performance</link>, the use of
appropriate type declarations can be very important for performance as
well.)</para>

<para>The &SBCL; compiler, like the related compiler in &CMUCL;,
treats type declarations much differently than other Lisp compilers.
By default (<emphasis>i.e.</emphasis>, at ordinary levels of the
<parameter>safety</parameter> compiler optimization parameter), the compiler
doesn't blindly believe most type declarations; it considers them
assertions about the program that should be checked.</para>

<para>The &SBCL; compiler also has a greater knowledge of the
&CommonLisp; type system than other compilers.  Support is incomplete
only for the <type>not</type>, <type>and</type> and <type>satisfies</type>
types.
<!-- FIXME: See also sections \ref{advanced-type-stuff}
     and \ref{type-inference}, once we snarf them from the
     CMU CL manual. -->
</para>

<sect2 id="compiler-impl-limitations"><title>Implementation Limitations</title>

<para>
  Ideally, the compiler would consider <emphasis>all</emphasis> type declarations to
  be assertions, so that adding type declarations to a program, no
  matter how incorrect they might be, would <emphasis>never</emphasis> cause
  undefined behavior. As of &SBCL; version 0.8.1, the compiler is known to
  fall short of this goal in two areas:
  <itemizedlist>
    <listitem><para><function>Proclaim</function>ed constraints on argument and
    result types of a function are supposed to be checked by the
    function. If the function type is proclaimed before function
    definition, type checks are inserted by the compiler, but the
    standard allows the reversed order, in which case the compiler
    will trust the declaration.</para></listitem>
    <listitem><para>The compiler cannot check types of an unknown number
    of values; if the number of generated values is unknown, but the
    number of consumed is known, only consumed values are
    checked.</para></listitem>
    <listitem><para>There are a few poorly characterized but apparently
    very uncommon situations where a type declaration in an unexpected
    location will be trusted and never checked by the
    compiler.</para></listitem>
</itemizedlist></para>

<para>These are important bugs, but are not necessarily easy to fix,
so they may, alas, remain in the system for a while.</para>

</sect2>

<sect2><title>Type Errors at Compile Time</title>
<!--INDEX compile time type errors-->
<!--INDEX type checking}{at compile time}-->

<para>If the compiler can prove at compile time that some portion of
the program cannot be executed without a type error, then it will give
a warning at compile time. It is possible that the offending code
would never actually be executed at run-time due to some higher level
consistency constraint unknown to the compiler, so a type warning
doesn't always indicate an incorrect program. For example, consider
this code fragment:

<programlisting>(defun raz (foo)
(let ((x (case foo
(:this 13)
(:that 9)
(:the-other 42))))
(declare (fixnum x))
(foo x)))
</programlisting>

Compilation produces this warning:

<screen>in: DEFUN RAZ
(CASE FOO (:THIS 13) (:THAT 9) (:THE-OTHER 42))
--> LET COND IF COND IF COND IF
==>
(COND)
caught WARNING: This is not a FIXNUM:
NIL</screen>

In this case, the warning means that if <varname>foo</varname> isn't any of
<literal>:this</literal>, <literal>:that</literal> or <literal>:the-other</literal>, then
<varname>x</varname> will be initialized to <literal>nil</literal>, which the
<type>fixnum</type> declaration makes illegal. The warning will go away if
<function>ecase</function> is used instead of <function>case</function>, or if
<literal>:the-other</literal> is changed to <literal>t</literal>.</para>

<para>This sort of spurious type warning happens moderately often in
the expansion of complex macros and in inline functions. In such
cases, there may be dead code that is impossible to correctly execute.
The compiler can't always prove this code is dead (could never be
executed), so it compiles the erroneous code (which will always signal
an error if it is executed) and gives a warning.</para>

<para>
  Type warnings are inhibited when the
  <parameter>sb-ext:inhibit-warnings</parameter> optimization quality is
  <literal>3</literal>. (See <link linkend="compiler-policy">the section
  on compiler policy</link>.) This can be used in a local declaration
  to inhibit type warnings in a code fragment that has spurious
warnings.</para>

</sect2>

<sect2 id="precisetypechecking"><title>Precise Type Checking</title>
<!--INDEX precise type checking-->
<!--INDEX {type checking}{precise}-->

<para>With the default compilation policy, all type declarations are
precisely checked, except in a few situations where they are simply
ignored instead. Precise checking means that the check is done as
though <function>typep</function> had been called with the exact type
specifier that appeared in the declaration. In &SBCL;, adding type
declarations makes code safer. (Except that as noted <link
linkend="compiler-impl-limitations">elsewhere</link>, remaining bugs
in the compiler's handling of types unfortunately provide some
exceptions to this rule.)</para>

<para>If a variable is declared to be
<type>(integer 3 17)</type> then its value must always be an integer
between <literal>3</literal> and <literal>17</literal>. If multiple type
declarations apply to a single variable, then all the declarations
must be correct; it is as though all the types were intersected
producing a single <type>and</type> type specifier.</para>

<para>Argument and result type declarations are automatically
enforced. If you declare the type of a function argument, a type check
will be done when that function is called. In a function call, the
called function does the argument type checking.</para>

<para>The types of structure slots are also checked. The value of a
structure slot must always be of the type indicated in any
<literal>:type</literal> slot option. </para>

<para>In traditional &CommonLisp; compilers, not all type assertions
are checked, and type checks are not precise. Traditional compilers
blindly trust explicit type declarations, but may check the argument
type assertions for built-in functions. Type checking is not precise,
since the argument type checks will be for the most general type legal
for that argument. In many systems, type declarations suppress what
little type checking is being done, so adding type declarations makes
code unsafe. This is a problem since it discourages writing type
declarations during initial coding. In addition to being more error
prone, adding type declarations during tuning also loses all the
benefits of debugging with checked type assertions.</para>

<para>To gain maximum benefit from the compiler's type checking, you
should always declare the types of function arguments and structure
slots as precisely as possible. This often involves the use of
<type>or</type>, <type>member</type>, and other list-style type specifiers.</para>

</sect2>

<sect2 id="weakened-type-checking"><title>Weakened Type Checking</title>
<!--INDEX weakened type checking-->
<!--INDEX {type checking}{weakened}-->

<para>At one time, &CMUCL; supported another level of type checking,
<quote>weakened type checking</quote>, when the value for the
<parameter>speed</parameter> optimization quality is greater than
<parameter>safety</parameter>, and <parameter>safety</parameter> is not <literal>0</literal>.
The &CMUCL; manual still has a description of it, but even the CMU CL
code no longer corresponds to the manual. Some of this partial safety
checking lingers on in SBCL, but it's not a supported feature, and 
should not be relied on. If you ask the compiler to optimize
<parameter>speed</parameter> to a higher level than <parameter>safety</parameter>,
your program is performing without a safety net, because &SBCL; may
at its option believe any or all type declarations with either partial
or nonexistent runtime checking.</para>

</sect2>

<sect2><title>Getting Existing Programs to Run</title>
<!--INDEX {existing programs}{to run}-->
<!--INDEX {types}{portability}-->
<!--INDEX {compatibility with other Lisps}
     (should also have an entry in the non-&ANSI;-isms section)-->

<para>Since &SBCL;'s compiler, like &CMUCL;'s compiler, does much more
comprehensive type checking than most Lisp compilers, &SBCL; may
detect type errors in programs that have been debugged using other
compilers. These errors are mostly incorrect declarations, although
compile-time type errors can find actual bugs if parts of the program
have never been tested.</para>

<para>Some incorrect declarations can only be detected by run-time
type checking. It is very important to initially compile a program
with full type checks (high <parameter>safety</parameter> optimization) and
then test this safe version. After the checking version has been
tested, then you can consider weakening or eliminating type checks.
<emphasis>This applies even to previously debugged
programs,</emphasis> because the &SBCL; compiler does much more type
inference than other &CommonLisp; compilers, so an incorrect
declaration can do more damage.</para>

<para>The most common problem is with variables whose constant initial
value doesn't match the type declaration. Incorrect constant initial
values will always be flagged by a compile-time type error, and they
are simple to fix once located. Consider this code fragment:

<programlisting>(prog (foo)
(declare (fixnum foo))
(setq foo ...)
...)</programlisting>

Here <varname>foo</varname> is given an initial value of <literal>nil</literal>, but
is declared to be a <type>fixnum</type>.  Even if it is never read, the
initial value of a variable must match the declared type.  There are
two ways to fix this problem. Change the declaration

<programlisting>(prog (foo)
(declare (type (or fixnum null) foo))
(setq foo ...)
...)</programlisting>

or change the initial value

<programlisting>(prog ((foo 0))
(declare (fixnum foo))
(setq foo ...)
...)</programlisting>

It is generally preferable to change to a legal initial value rather
than to weaken the declaration, but sometimes it is simpler to weaken
the declaration than to try to make an initial value of the
appropriate type.</para>

<para>Another declaration problem occasionally encountered is
incorrect declarations on <function>defmacro</function> arguments. This can happen
when a function is converted into a macro. Consider this macro:

<programlisting>(defmacro my-1+ (x)
(declare (fixnum x))
`(the fixnum (1+ ,x)))</programlisting>

Although legal and well-defined &CommonLisp; code, this meaning of
this definition is almost certainly not what the writer intended. For
example, this call is illegal:

<programlisting>(my-1+ (+ 4 5))</programlisting>

This call is illegal because the argument to the macro is
<literal>(+ 4 5)</literal>, which is a <type>list</type>, not a
<type>fixnum</type>.  Because of
macro semantics, it is hardly ever useful to declare the types of
macro arguments.  If you really want to assert something about the
type of the result of evaluating a macro argument, then put a
<function>the</function> in the expansion:

<programlisting>(defmacro my-1+ (x)
`(the fixnum (1+ (the fixnum ,x))))</programlisting>

In this case, it would be stylistically preferable to change this
macro back to a function and declare it inline. 
<!--FIXME: <xref>inline-expansion, once we crib the 
     relevant text from the CMU CL manual.-->
</para>

<para>
  Some more subtle problems are caused by incorrect declarations that
  can't be detected at compile time.  Consider this code:

  <programlisting>(do ((pos 0 (position #\a string :start (1+ pos))))
  ((null pos))
  (declare (fixnum pos))
  ...)</programlisting>

  Although <varname>pos</varname> is almost always a <varname>fixnum</varname>, it is
  <literal>nil</literal> at the end of the loop. If this example is compiled
  with full type checks (the default), then running it will signal a
  type error at the end of the loop. If compiled without type checks,
  the program will go into an infinite loop (or perhaps
  <function>position</function> will complain because <literal>(1+ nil)</literal> isn't
  a sensible start.) Why? Because if you compile without type checks,
  the compiler just quietly believes the type declaration. Since the
  compiler believes that <varname>pos</varname> is always a <type>fixnum</type>, it
  believes that <varname>pos</varname> is never <literal>nil</literal>, so
  <literal>(null pos)</literal> is never true, and the loop exit test is
  optimized away. Such errors are sometimes flagged by unreachable code
  notes, but it is still important to initially compile and test any
  system with full type checks, even if the system works fine when
compiled using other compilers.</para>

<para>In this case, the fix is to weaken the type declaration to
<type>(or fixnum null)</type>.
<footnote><para>Actually, this declaration is 
unnecessary in &SBCL;, since it already knows that <function>position</function>
returns a non-negative <type>fixnum</type> or <literal>nil</literal>.
</para></footnote>

Note that there is usually little performance penalty for weakening a
declaration in this way. Any numeric operations in the body can still
assume that the variable is a <type>fixnum</type>, since <literal>nil</literal>
is not a legal numeric argument. Another possible fix would be to say:

<programlisting>(do ((pos 0 (position #\a string :start (1+ pos))))
((null pos))
(let ((pos pos))
(declare (fixnum pos))
...))</programlisting>

This would be preferable in some circumstances, since it would allow a
non-standard representation to be used for the local <varname>pos</varname>
variable in the loop body.
<!-- FIXME: <xref>ND-variables, once we crib the text from the 
     CMU CL manual. -->
</para>

</sect2>

</sect1>

<sect1 id="compiler-policy"><title>Compiler Policy</title>

<para>As of version 0.6.4, &SBCL; still uses most of the &CMUCL; code
for compiler policy. The &CMUCL; code has many features and high-quality
documentation, but the two unfortunately do not match. So this area of
the compiler and its interface needs to be cleaned up. Meanwhile, here
is some rudimentary documentation on the current behavior of the
system.</para>

<para>Compiler policy is controlled by the <parameter>optimize</parameter>
declaration. The compiler supports the &ANSI; optimization qualities,
and also an extension <parameter>sb-ext:inhibit-warnings</parameter>.</para>

<para>Ordinarily, when the <parameter>speed</parameter> quality is high, the
compiler emits notes to notify the programmer about its inability to
apply various optimizations. Setting
<parameter>sb-ext:inhibit-warnings</parameter> to a value at least as large as
the <parameter>speed</parameter> quality inhibits this notification. This can
be useful to suppress notes about code which is known to be
unavoidably inefficient. (For example, the compiler issues notes about
having to use generic arithmetic instead of fixnum arithmetic, which
is not helpful for code which by design supports arbitrary-sized
integers instead of being limited to fixnums.)</para>

<note><para>The basic functionality of the <parameter>optimize
inhibit-warnings</parameter> extension will probably be supported in all future
versions of the system, but it will probably be renamed when the
compiler and its interface are cleaned up. The current name is
misleading, because it mostly inhibits optimization notes, not
warnings. And making it an optimization quality is misleading, because
it shouldn't affect the resulting code at all. It may become a
declaration identifier with a name like
<parameter>sb-ext:inhibit-notes</parameter>, so that what's currently written

<programlisting>(declaim (optimize (sb-ext:inhibit-warnings 2)))</programlisting>

would become something like

<programlisting>(declaim (sb-ext:inhibit-notes 2))</programlisting>

</para></note>

<para> (In early versions of SBCL, a <parameter>speed</parameter> value of zero
was used to enable byte compilation, but since version 0.7.0, SBCL
only supports native compilation.)</para>

<para>When <parameter>safety</parameter> is zero, almost all runtime checking
of types, array bounds, and so forth is suppressed.</para>

<para>When <parameter>safety</parameter> is less than <parameter>speed</parameter>, any
and all type checks may be suppressed. At some point in the past,
&CMUCL; had <link linkend="weakened-type-checking">a more nuanced
interpretation of this</link>. However, &SBCL; doesn't support that
interpretation, and setting <parameter>safety</parameter> less than
<parameter>speed</parameter> may have roughly the same effect as setting
<parameter>safety</parameter> to zero.</para>

<para>The value of <parameter>space</parameter> mostly influences the
compiler's decision whether to inline operations, which tend to
increase the size of programs. Use the value <literal>0</literal> with
caution, since it can cause the compiler to inline operations so
indiscriminately that the net effect is to slow the program by causing
cache misses or swapping.</para>

<!-- FIXME: old CMU CL compiler policy, should perhaps be adapted
     _    for SBCL. (Unfortunately, the CMU CL docs are out of sync with the
     _    CMU CL code, so adapting this requires not only reformatting
     _    the documentation, but rooting out code rot.)
     _
     _<sect2 id="compiler-policy"><title>Compiler Policy</1000
     _  INDEX {policy}{compiler}
     _  INDEX compiler policy
     _
     _<para>The policy is what tells the compiler <emphasis>how</emphasis> to
     _compile a program. This is logically (and often textually) distinct
     _from the program itself. Broad control of policy is provided by the
     _<parameter>optimize</parameter> declaration; other declarations and variables
     _control more specific aspects of compilation.</para>
     _
     _\begin{comment}
     _* The Optimize Declaration::
     _* The Optimize-Interface Declaration::
     _\end{comment}
     _
     _%%\node The Optimize Declaration, The Optimize-Interface Declaration, Compiler Policy, Compiler Policy
     _\subsection{The Optimize Declaration}
     _\label{optimize-declaration}
     _\cindex{optimize declaration}
     _\cpsubindex{declarations}{\code{optimize}}
     _
     _The \code{optimize} declaration recognizes six different
     _\var{qualities}.  The qualities are conceptually independent aspects
     _of program performance.  In reality, increasing one quality tends to
     _have adverse effects on other qualities.  The compiler compares the
     _relative values of qualities when it needs to make a trade-off; i.e.,
     _if \code{speed} is greater than \code{safety}, then improve speed at
     _the cost of safety.
     _
     _The default for all qualities (except \code{debug}) is \code{1}.
     _Whenever qualities are equal, ties are broken according to a broad
     _idea of what a good default environment is supposed to be.  Generally
     _this downplays \code{speed}, \code{compile-speed} and \code{space} in
     _favor of \code{safety} and \code{debug}.  Novice and casual users
     _should stick to the default policy.  Advanced users often want to
     _improve speed and memory usage at the cost of safety and
     _debuggability.
     _
     _If the value for a quality is \code{0} or \code{3}, then it may have a
     _special interpretation.  A value of \code{0} means ``totally
     _unimportant'', and a \code{3} means ``ultimately important.''  These
     _extreme optimization values enable ``heroic'' compilation strategies
     _that are not always desirable and sometimes self-defeating.
     _Specifying more than one quality as \code{3} is not desirable, since
     _it doesn't tell the compiler which quality is most important.
     _
     _
     _These are the optimization qualities:
     _\begin{Lentry}
     _
     _\item[\code{speed}] \cindex{speed optimization quality}How fast the
     _  program should is run.  \code{speed 3} enables some optimizations
     _  that hurt debuggability.
     _
     _\item[\code{compilation-speed}] \cindex{compilation-speed optimization
     _    quality}How fast the compiler should run.  Note that increasing
     _  this above \code{safety} weakens type checking.
     _
     _\item[\code{space}] \cindex{space optimization quality}How much space
     _  the compiled code should take up.  Inline expansion is mostly
     _  inhibited when \code{space} is greater than \code{speed}.  A value
     _  of \code{0} enables indiscriminate inline expansion.  Wide use of a
     _  \code{0} value is not recommended, as it may waste so much space
     _  that run time is slowed.  \xlref{inline-expansion} for a discussion
     _  of inline expansion.
     _
     _\item[\code{debug}] \cindex{debug optimization quality}How debuggable
     _  the program should be.  The quality is treated differently from the
     _  other qualities: each value indicates a particular level of debugger
     _  information; it is not compared with the other qualities.
     _  \xlref{debugger-policy} for more details.
     _
     _\item[\code{safety}] \cindex{safety optimization quality}How much
     _  error checking should be done.  If \code{speed}, \code{space} or
     _  \code{compilation-speed} is more important than \code{safety}, then
     _  type checking is weakened (\pxlref{weakened-type-checks}).  If
     _  \code{safety} if \code{0}, then no run time error checking is done.
     _  In addition to suppressing type checks, \code{0} also suppresses
     _  argument count checking, unbound-symbol checking and array bounds
     _  checks.
     _
     _\item[\code{extensions:inhibit-warnings}] \cindex{inhibit-warnings
     _    optimization quality}This is a CMU extension that determines how
     _  little (or how much) diagnostic output should be printed during
     _  compilation.  This quality is compared to other qualities to
     _  determine whether to print style notes and warnings concerning those
     _  qualities.  If \code{speed} is greater than \code{inhibit-warnings},
     _  then notes about how to improve speed will be printed, etc.  The
     _  default value is \code{1}, so raising the value for any standard
     _  quality above its default enables notes for that quality.  If
     _  \code{inhibit-warnings} is \code{3}, then all notes and most
     _  non-serious warnings are inhibited.  This is useful with
     _  \code{declare} to suppress warnings about unavoidable problems.
     _\end{Lentry}
     _
     _%%\node The Optimize-Interface Declaration,  , The Optimize Declaration, Compiler Policy
     _\subsection{The Optimize-Interface Declaration}
     _\label{optimize-interface-declaration}
     _\cindex{optimize-interface declaration}
     _\cpsubindex{declarations}{\code{optimize-interface}}
     _
     _The \code{extensions:optimize-interface} declaration is identical in
     _syntax to the \code{optimize} declaration, but it specifies the policy
     _used during compilation of code the compiler automatically generates
     _to check the number and type of arguments supplied to a function.  It
     _is useful to specify this policy separately, since even thoroughly
     _debugged functions are vulnerable to being passed the wrong arguments.
     _The \code{optimize-interface} declaration can specify that arguments
     _should be checked even when the general \code{optimize} policy is
     _unsafe.
     _
     _Note that this argument checking is the checking of user-supplied
     _arguments to any functions defined within the scope of the
     _declaration, \code{not} the checking of arguments to \llisp{}
     _primitives that appear in those definitions.
     _
     _The idea behind this declaration is that it allows the definition of
     _functions that appear fully safe to other callers, but that do no
     _internal error checking.  Of course, it is possible that arguments may
     _be invalid in ways other than having incorrect type.  Functions
     _compiled unsafely must still protect themselves against things like
     _user-supplied array indices that are out of bounds and improper lists.
     _See also the \kwd{context-declarations} option to
     _\macref{with-compilation-unit}.
     _
     _(end of section on compiler policy)
     _-->

</sect1>

<sect1 id="open-coding"><title>Open Coding and Inline Expansion</title>
<!--INDEX open-coding-->
<!--INDEX inline expansion-->
<!--INDEX static functions-->

<para>Since &CommonLisp; forbids the redefinition of standard
functions, the compiler can have special knowledge of these standard
functions embedded in it. This special knowledge is used in various
ways (open coding, inline expansion, source transformation), but the
implications to the user are basically the same:
<itemizedlist>
  <listitem><para> Attempts to redefine standard functions may
  be frustrated, since the function may never be called. Although
  it is technically illegal to redefine standard functions, users
  sometimes want to implicitly redefine these functions when they
  are debugging using the <function>trace</function> macro.  Special-casing
  of standard functions can be inhibited using the
  <parameter>notinline</parameter> declaration.</para></listitem>
  <listitem><para> The compiler can have multiple alternate
  implementations of standard functions that implement different
  trade-offs of speed, space and safety.  This selection is
  based on the <link linkend="compiler-policy">compiler policy</link>.
  </para></listitem>
</itemizedlist>
</para>

<para>When a function call is <emphasis>open coded</emphasis>, inline code whose
effect is equivalent to the function call is substituted for that
function call. When a function call is <emphasis>closed coded</emphasis>, it
is usually left as is, although it might be turned into a call to a
different function with different arguments. As an example, if
<function>nthcdr</function> were to be open coded, then

<programlisting>(nthcdr 4 foobar)</programlisting>

might turn into

<programlisting>(cdr (cdr (cdr (cdr foobar))))</programlisting>

or even

<programlisting>(do ((i 0 (1+ i))
(list foobar (cdr foobar)))
((= i 4) list))</programlisting>

If <function>nth</function> is closed coded, then

<programlisting>
  (nth x l)
</programlisting>

might stay the same, or turn into something like

<programlisting>
  (car (nthcdr x l))
</programlisting>
</para>

<para>In general, open coding sacrifices space for speed, but some
functions (such as <function>car</function>) are so simple that they are always
open-coded. Even when not open-coded, a call to a standard function
may be transformed into a different function call (as in the last
example) or compiled as <emphasis>static call</emphasis>. Static function call
uses a more efficient calling convention that forbids
redefinition.</para>

</sect1>

</chapter>