-@node Efficiency, Beyond The ANSI Standard, The Debugger, Top
+@node Efficiency
@comment node-name, next, previous, up
@chapter Efficiency
+@cindex Efficiency
+
+@menu
+* Slot access::
+* Dynamic-extent allocation::
+* Modular arithmetic::
+* Global and Always-Bound variables::
+* Miscellaneous Efficiency Issues::
+@end menu
+
+@node Slot access
+@comment node-name, next, previous, up
+@section Slot access
+@cindex Slot access
+
+@subsection Structure object slot access
+
+Structure slot accessors are efficient only if the compiler is able to
+open code them: compiling a call to a structure slot accessor before
+the structure is defined, declaring one @code{notinline}, or passing
+it as a functional argument to another function causes severe
+performance degradation.
+
+@subsection Standard object slot access
+
+The most efficient way to access a slot of a @code{standard-object} is
+by using @code{slot-value} with a constant slot name argument inside a
+@code{defmethod} body, where the variable holding the instance is a
+specializer parameter of the method and is never assigned to. The cost
+is roughly 1.6 times that of an open coded structure slot accessor.
+
+Second most efficient way is to use a CLOS slot accessor, or
+@code{slot-value} with a constant slot name argument, but in
+circumstances other than specified above. This may be up to 3 times as
+slow as the method described above.
+
+Example:
+
+@lisp
+(defclass foo () ((bar)))
+
+;; Fast: specializer and never assigned to
+(defmethod quux ((foo foo) new)
+ (let ((old (slot-value foo 'bar)))
+ (setf (slot-value foo 'bar) new)
+ old))
+
+;; Slow: not a specializer
+(defmethod quux ((foo foo) new)
+ (let* ((temp foo)
+ (old (slot-value temp 'bar)))
+ (setf (slot-value temp 'bar) new)
+ old))
+
+;; Slow: assignment to FOO
+(defmethod quux ((foo foo) new)
+ (let ((old (slot-value foo 'bar)))
+ (setf (slot-value foo 'bar) new)
+ (setf foo new)
+ old))
+@end lisp
+
+Note that when profiling code such as this, the first few calls to the
+generic function are not representative, as the dispatch mechanism is
+lazily set up during those calls.
+
+@node Dynamic-extent allocation
+@comment node-name, next, previous, up
+@section Dynamic-extent allocation
+@cindex @code{dynamic-extent} declaration
+@cindex declaration, @code{dynamic-extent}
+
+SBCL has fairly extensive support for performing allocation on the
+stack when a variable is declared @code{dynamic-extent}. The
+@code{dynamic-extent} declarations are not verified, but are simply
+trusted as long as @code{sb-ext:*stack-allocate-dynamic-extent*} is
+true.
+
+@include var-sb-ext-star-stack-allocate-dynamic-extent-star.texinfo
+
+If dynamic extent constraints specified in the Common Lisp standard
+are violated, the best that can happen is for the program to have
+garbage in variables and return values; more commonly, the system will
+crash.
+
+In particular, it is important to realize that dynamic extend is
+contagious:
+
+@lisp
+(let* ((a (list 1 2 3))
+ (b (cons a a)))
+ (declare (dynamic-extent b))
+ ;; Unless A is accessed elsewhere as well, SBCL will consider
+ ;; it to be otherwise inaccessible -- it can only be accessed
+ ;; through B, after all -- and stack allocate it as well.
+ ;;
+ ;; Hence returning (CAR B) here is unsafe.
+ ...)
+@end lisp
+
+This allows stack allocation of complex structures. As a notable
+exception to this, SBCL does not as of 1.0.48.21 propagate
+dynamic-extentness through @code{&rest} arguments -- but another
+conforming implementation might, so portable code should not rely on
+this.
+
+@lisp
+(declaim (inline foo))
+(defun foo (fun &rest arguments)
+ (declare (dynamic-extent arguments))
+ (apply fun arguments))
+
+(defun bar (a)
+ ;; SBCL will heap allocate the result of (LIST A), and stack allocate
+ ;; only the spine of the &rest list -- so this is safe, but unportable.
+ ;;
+ ;; Another implementation, including earlier versions of SBCL might consider
+ ;; (LIST A) to be otherwise inaccessible and stack-allocate it as well!
+ (foo #'car (list a)))
+@end lisp
+
+There are many cases when @code{dynamic-extent} declarations could be
+useful. At present, SBCL implements stack allocation for
+
+@itemize
+
+@item
+@code{&rest} lists, when these are declared @code{dynamic-extent}.
+
+@item
+@findex @cl{cons}
+@findex @cl{list}
+@findex @cl{list*}
+@findex @cl{vector}
+@code{cons}, @code{list}, @code{list*}, and @code{vector} when the
+result is bound to a variable declared @code{dynamic-extent}.
+
+@item
+@findex @cl{make-array}
+simple forms of @code{make-array}, whose result is bound to a variable
+declared @code{dynamic-extent}: stack allocation is possible only if
+the resulting array is known to be both simple and one-dimensional,
+and has a constant @code{:element-type}.
+
+@cindex Safety optimization quality
+@strong{Note}: stack space is limited, so allocation of a large vector
+may cause stack overflow. For this reason potentially large vectors,
+which might circumvent stack overflow detection, are stack allocated
+only in zero @code{safety} policies.
+
+@item
+@findex @cl{flet}
+@findex @cl{labels}
+@cindex @code{safety} optimization quality
+@cindex optimization quality, @code{safety}
+closures defined with @code{flet} or @code{labels}, with a bound
+@code{dynamic-extent} declaration. Blocks and tags are also allocated
+on the heap, unless all non-local control transfers to them are
+compiled with zero @code{safety}.
+
+@item
+user-defined structures when the structure constructor defined using
+@code{defstruct} has been declared @code{inline} and the result of the
+call to the constructor is bound to a variable declared
+@code{dynamic-extent}.
+
+@strong{Note}: structures with ``raw'' slots can currently be
+stack-allocated only on x86 and x86-64.
+
+@item
+all of the above when they appear as initial parts of another
+stack-allocated object.
+
+@end itemize
+
+Examples:
+
+@lisp
+;;; Declaiming a structure constructor inline before definition makes
+;;; stack allocation possible.
+(declaim (inline make-thing))
+(defstruct thing obj next)
+
+;;; Stack allocation of various objects bound to DYNAMIC-EXTENT
+;;; variables.
+(let* ((list (list 1 2 3))
+ (nested (cons (list 1 2) (list* 3 4 (list 5))))
+ (vector (make-array 3 :element-type 'single-float))
+ (thing (make-thing :obj list
+ :next (make-thing :obj (make-array 3)))))
+ (declare (dynamic-extent list nested vector thing))
+ ...)
+
+;;; Stack allocation of arguments to a local function is equivalent
+;;; to stack allocation of local variable values.
+(flet ((f (x)
+ (declare (dynamic-extent x))
+ ...))
+ ...
+ (f (list 1 2 3))
+ (f (cons (cons 1 2) (cons 3 4)))
+ ...)
+
+;;; Stack allocation of &REST lists
+(defun foo (&rest args)
+ (declare (dynamic-extent args))
+ ...)
+@end lisp
+
+Future plans include
+
+@itemize
+
+@item
+Automatic detection of the common idiom of calling quantifiers with a
+closure, even when the closure is not declared @code{dynamic-extent}.
+
+@end itemize
+
+@node Modular arithmetic
+@comment node-name, next, previous, up
+@section Modular arithmetic
+@cindex Modular arithmetic
+@cindex Arithmetic, modular
+@cindex Arithmetic, hardware
+@findex @cl{logand}
+Some numeric functions have a property: @var{N} lower bits of the
+result depend only on @var{N} lower bits of (all or some)
+arguments. If the compiler sees an expression of form @code{(logand
+@var{exp} @var{mask})}, where @var{exp} is a tree of such ``good''
+functions and @var{mask} is known to be of type @code{(unsigned-byte
+@var{w})}, where @var{w} is a ``good'' width, all intermediate results
+will be cut to @var{w} bits (but it is not done for variables and
+constants!). This often results in an ability to use simple machine
+instructions for the functions.
+
+Consider an example.
+
+@lisp
+(defun i (x y)
+ (declare (type (unsigned-byte 32) x y))
+ (ldb (byte 32 0) (logxor x (lognot y))))
+@end lisp
+
+The result of @code{(lognot y)} will be negative and of type
+@code{(signed-byte 33)}, so a naive implementation on a 32-bit
+platform is unable to use 32-bit arithmetic here. But modular
+arithmetic optimizer is able to do it: because the result is cut down
+to 32 bits, the compiler will replace @code{logxor} and @code{lognot}
+with versions cutting results to 32 bits, and because terminals
+(here---expressions @code{x} and @code{y}) are also of type
+@code{(unsigned-byte 32)}, 32-bit machine arithmetic can be used.
+
+As of SBCL 0.8.5 ``good'' functions are @code{+}, @code{-};
+@code{logand}, @code{logior}, @code{logxor}, @code{lognot} and their
+combinations; and @code{ash} with the positive second
+argument. ``Good'' widths are 32 on HPPA, MIPS, PPC, Sparc and x86 and
+64 on Alpha. While it is possible to support smaller widths as well,
+currently this is not implemented.
+
+@node Global and Always-Bound variables
+@comment node-name, next, previous, up
+@section Global and Always-Bound variables
+
+@include macro-sb-ext-defglobal.texinfo
+
+@deffn {Declaration} @sbext{global}
+
+Syntax: @code{(sb-ext:global symbol*)}
+
+Only valid as a global proclamation.
+
+Specifies that the named symbols cannot be proclaimed or locally
+declared @code{special}. Proclaiming an already special or constant
+variable name as @code{global} signal an error. Allows more efficient
+value lookup in threaded environments in addition to expressing
+programmer intention.
+@end deffn
+
+@deffn {Declaration} @sbext{always-bound}
+
+Syntax: @code{(sb-ext:always-bound symbol*)}
+
+Only valid as a global proclamation.
+
+Specifies that the named symbols are always bound. Inhibits
+@code{makunbound} of the named symbols. Proclaiming an unbound symbol
+as @code{always-bound} signals an error. Allows the compiler to elide
+boundness checks from value lookups.
+@end deffn
+
+@node Miscellaneous Efficiency Issues
+@comment node-name, next, previous, up
+@section Miscellaneous Efficiency Issues
FIXME: The material in the CMUCL manual about getting good
performance from the compiler should be reviewed, reformatted in
points to keep in mind.
@itemize
-
+
@item
+@findex @cl{let}
+@findex @cl{let*}
+@findex @cl{setq}
+@findex @cl{setf}
The CMUCL manual doesn't seem to state it explicitly, but Python has a
mental block about type inference when assignment is involved. Python
is very aggressive and clever about inferring the types of values
@c <!-- FIXME: Python dislikes assignments, but not in type
@c inference. The real problems are loop induction, closed over
@c variables and aliases. -->
-
+
@item
Since the time the CMUCL manual was written, CMUCL (and thus SBCL) has
gotten a generational garbage collector. This means that there are
some efficiency implications of various patterns of memory usage which
aren't discussed in the CMUCL manual. (Some new material should be
written about this.)
-
+
@item
SBCL has some important known efficiency problems. Perhaps the most
important are
-
+
@itemize @minus
-
-@item
-There is only limited support for the ANSI @code{dynamic-extent}
-declaration. @xref{Dynamic-extent allocation}.
-
+
@item
The garbage collector is not particularly efficient, at least on
platforms without the generational collector (as of SBCL 0.8.9, all
except x86).
-
+
@item
Various aspects of the PCL implementation of CLOS are more inefficient
than necessary.
-
+
@end itemize
@end itemize
hand-coding has been done as of SBCL version 0.6.3 include
@itemize
-
+
@item
@code{(reduce #'f x)} where the type of @code{x} is known at compile
time
-
+
@item
various bit vector operations, e.g. @code{(position 0
some-bit-vector)}
sources. Such code is often reasonably straightforward to write;
search the sources for the string ``@code{deftransform}'' to find many
examples (some straightforward, some less so).
-
-@menu
-* Dynamic-extent allocation::
-* Modular arithmetic::
-@end menu
-
-@node Dynamic-extent allocation, Modular arithmetic, Efficiency, Efficiency
-@comment node-name, next, previous, up
-@section Dynamic-extent allocation
-@cindex Dynamic-extent declaration
-
-SBCL has limited support for performing allocation on the stack when a
-variable is declared @code{dynamic-extent}. The @code{dynamic-extent}
-declarations are not verified, but are simply trusted; if the
-constraints in the Common Lisp standard are violated, the best that
-can happen is for the program to have garbage in variables and return
-values; more commonly, the system will crash.
-
-As a consequence of this, the condition for performing stack
-allocation is stringent: either of the @code{speed} or @code{space}
-optimization qualities must be higher than the maximum of
-@code{safety} and @code{debug} at the point of the allocation. For
-example:
-
-@lisp
-(locally
- (declare (optimize speed (safety 1) (debug 1)))
- (defun foo (&rest rest)
- (declare (dynamic-extent rest))
- (length rest)))
-@end lisp
-
-Here the @code{&rest} list will be allocated on the stack. Note that
-it would not be in the following situation:
-
-@lisp
-(defun foo (&rest rest)
- (declare (optimize speed (safety 1) (debug 1)))
- (declare (dynamic-extent rest))
- (length rest))
-@end lisp
-
-because both the allocation of the @code{&rest} list and the variable
-binding are outside the scope of the @code{optimize} declaration.
-
-There are many cases when dynamic-extent declarations could be useful.
-At present, SBCL implements
-
-@itemize
-
-@item
-Stack allocation of @code{&rest} lists, where these are declared
-@code{dynamic-extent}.
-
-@end itemize
-
-Future plans include
-
-@itemize
-
-@item
-Stack allocation of closures, where these are declared
-@code{dynamic-extent};
-
-@item
-Stack allocation of @code{list}, @code{list*} and @code{cons}
-(including following chains during initialization, and also for
-binding mutation), where the allocation is declared
-@code{dynamic-extent};
-
-@item
-Automatic detection of the common idiom of applying a function to some
-defaults and a @code{&rest} list, even when this is not declared
-@code{dynamic-extent};
-
-@item
-Automatic detection of the common idiom of calling quantifiers with a
-closure, even when the closure is not declared @code{dynamic-extent}.
-
-@end itemize
-
-@node Modular arithmetic, , Dynamic-extent allocation, Efficiency
-@comment node-name, next, previous, up
-@section Modular arithmetic
-@cindex Modular arithmetic
-@cindex Arithmetic, modular
-@cindex Arithmetic, hardware
-
-Some numeric functions have a property: @var{N} lower bits of the
-result depend only on @var{N} lower bits of (all or some)
-arguments. If the compiler sees an expression of form @code{(logand
-@var{exp} @var{mask})}, where @var{exp} is a tree of such ``good''
-functions and @var{mask} is known to be of type @code{(unsigned-byte
-@var{w})}, where @var{w} is a ``good'' width, all intermediate results
-will be cut to @var{w} bits (but it is not done for variables and
-constants!). This often results in an ability to use simple machine
-instructions for the functions.
-
-Consider an example.
-
-@lisp
-(defun i (x y)
- (declare (type (unsigned-byte 32) x y))
- (ldb (byte 32 0) (logxor x (lognot y))))
-@end lisp
-
-The result of @code{(lognot y)} will be negative and of type
-@code{(signed-byte 33)}, so a naive implementation on a 32-bit
-platform is unable to use 32-bit arithmetic here. But modular
-arithmetic optimizer is able to do it: because the result is cut down
-to 32 bits, the compiler will replace @code{logxor} and @code{lognot}
-with versions cutting results to 32 bits, and because terminals
-(here---expressions @code{x} and @code{y}) are also of type
-@code{(unsigned-byte 32)}, 32-bit machine arithmetic can be used.
-
-
-As of SBCL 0.8.5 ``good'' functions are @code{+}, @code{-};
-@code{logand}, @code{logior}, @code{logxor}, @code{lognot} and their
-combinations; and @code{ash} with the positive second
-argument. ``Good'' widths are 32 on HPPA, MIPS, PPC, Sparc and X86 and
-64 on Alpha. While it is possible to support smaller widths as well,
-currently it is not implemented.
projects / sbcl.git / blobdiff