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9.5 Compiler control

There are ways to control the nature of compiled code via the declare special form and proclaim function. See 9.6 Declare, proclaim, and declaim for fuller discussion of these two forms.

In particular there are a set of optimize qualities which take integral values from 0 to 3. These control the trade-offs between size, speed, retention of debug information, optimizations and safety (that is, type checks) in the resulting code, and also compilation time. For example:

(proclaim '(optimize (speed 3) (safety 0) (debug 0)))

tells the compiler to concentrate on code speed rather than anything else, and:

(proclaim '(optimize (safety 3)))

ensures that the compiler never takes liberties with Lisp semantics and produces code that checks for every kind of error that can be signaled.

The important declarations to the compiler are type declarations and optimize declarations. To declare that the type of the value of a variable can be relied upon to be unchanging (and hence allow the compiler to omit various checks in the code), say:

(declare (type the-type variable * )

Optimize declarations have various qualities, and these take values from 0 to 3. The names are safety, fixnum-safety, float, sys:interruptable, debug, speed, compilation-speed, and space.

Most of the qualities default to 1 (but safety and fixnum-safety default to 3 and interruptable defaults to 0). You can either associate an optimize quality with a new value (with local lexical scope if in declare, and global scope if proclaim), or just give it by itself, which implies the value 3 (taken to mean "maximum" in some loose sense).

Thus you ensure code is at maximum safety by:

(proclaim '(optimize (safety 3)))


(proclaim '(optimize safety))

and reduce debugging information to a minimum by:

(proclaim '(optimize (debug 0)))

Normally code is interruptible, but when aiming for maximum speed and minimum safety and debug information code is not interruptible unless you ensure it thus:

(proclaim '(optimize (debug 0) (safety 0) (speed 3) interruptable))

The levels of safety have the following implications:

The levels of fixnum-safety have the following implications:

Additionally if the level of float (really this should be called "float-safety") is 0 then the compiler reduces allocation during float calculations.

The effects of combining these qualities is summarized below:

Combining debug and safety levels in the compiler
Keyword settingsOperations


Array access optimizations


Dumps symbol names for arglist


Ensure debugger knows values of args (and variables when source level debugging is on) and can find the exact subform in the Editor.


Does not generate any debug info at all


Avoids make-instance and
find-class optimizations


Avoids gethash and puthash optimizations


Avoids ldb and dpb optimizations


Avoids an optimization to last


Be careful when multiple value counts are wrong


Do not check array indices during write


Do not check array indices during read


Inline map functions (unless debug>2)


Optimize (merge) tail calls

debug<2 and safety<2

Self calls


Check get special


Do not check types during write


Do not check types during read


Check structure access


Inline structure readers, with no type check


Inline structure writers, with no type check


Check number of args

safety>=1 or

Check stack overflow


Ensures the thing being funcalled is a function

safety<3 and

Fixnum-only arithmetic with errors for
non fixnum arguments.

safety<3 and

No fixnum overflow checks

safety<3 and

No fixnum arithmetic checks at all


char= checks for arguments of type character


Ensures symbols in progv


Avoids "ad hoc" predicate type transforms


Reuse virtual registers in very large functions

debug=3 and safety=3

(declare (type foo x)) and
(the foo x) ensure a type check


Optimize floating point calculations

The other optimize qualities are: speed — the attention to fast code, space — the degree of compactness, compilation-speed — speed of compilation, interruptable — whether code must be interruptible when unsafe.

Note that if you compile code with a low level of safety, you may get segmentation violations if the code is incorrect (for example, if type checking is turned off and you supply incorrect types). You can check this by interpreting the code rather than compiling it.

9.5.1 Examples of compiler control

The following function, compiled with safety = 2, does not check the type of its argument because it merely reads:

(defun foo (x)
  (declare (optimize (safety 2)))
  (car x))

However the following function, also compiled with safety = 2, does check the type of its argument because it writes:

(defun set-foo (x y)
  (declare (optimize (safety 2)))
  (setf (car x) y))

As another example, interpreted code and code compiled at at low safety does not check type declarations. To make LispWorks check declarations, you need to compile your code after doing:

(declaim (optimize (safety 3) (debug 3)))

This last example shows how to copy efficiently bytes from a typed-aref vector (see make-typed-aref-vector) to an (unsigned-byte 8) array. type and safety declarations cause the compiler to inline the code that deals specifically with (unsigned-byte 8). This code was developed after an application was found to have a bottleneck in the original version of this function:

(defun copy-typed-aref-vector-to-byte-vector
       (byte-vector typed-vector length)
  (declare (optimize (safety 0))
           (type (simple-array (unsigned-byte 8) 1) byte-vector)
           (fixnum length))
  (dotimes (index length)
    (declare (type fixnum index))
    (setf (aref byte-vector index)
          (sys:typed-aref '(unsigned-byte 8) 
                          typed-vector index))))

LispWorks® User Guide and Reference Manual - 01 Dec 2021 19:30:19