10.4 Compiler control

There are ways to control the nature of compiled code via the declare special form and proclaim function. See later in this chapter for fuller discussion of these two forms.

In particular there are a set of optimize qualities which take integral values from 0 to 3, in order to control the trade-offs between code size, speed, compilation time, debuggability of the resulting code, and the safety of the code (whether type checks are omitted). For example:

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

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

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:

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:

or

and reduce debugging information to a minimum by

Normally code is interruptible, but when going for the extreme levels of speed and "undebuggability" this ceases to be the case unless you also ensure it thus:

The levels of safety have the following implications:

• 0 implies no type checking upon reading or writing from defstructs, arrays and objects in general, nor any checking of array index bounds.
• 1 implies no type checking upon reading from defstructs, arrays and objects in general, nor any checking of array index bounds when reading. However, array index bounds are checked when writing.
• 2 implies type checking when writing, but not when reading. Other than this the compiler generates generally safe code, but allows type and fixnum-safety declarations to take effect. Array index bounds are checked for both reading and writing.
• 3 (default) implies complete type and bounds checking, and disallows fixnum-safety and type declarations from taking any effect.

The levels of fixnum-safety have the following implications:

• 0 implies no type checking of arguments to numeric operations, which are assumed to be fixnums. Also the result is assumed, without checking, to not overflow - this level means single machine instructions can be generated for most common integer operations, but risks generating values that may confuse the garbage collector.
• 1 implies that numeric operations do not check their argument types (assumed fixnum), but do signal an error if the result would have been out of range.
• 2 implies that numeric operations signal an error if their arguments are non-fixnum, and also check for overflow.
• 3 (default) implies complete conformance to the semantics of Common Lisp numbers, so that types other than integers are handled in compiled code.

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:

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.

For example, 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))