@include var-sb-ext-star-compiler-print-variable-alist-star.texinfo
+For information about muffling warnings signaled outside of the
+compiler, see @ref{Customization Hooks for Users}.
+
@c <!-- FIXME: How much control over error messages is in SBCL?
@c _ How much should be? How much of this documentation should
@c _ we save or adapt?
too mild for the standard condition classes, typically hints about how
efficiency might be improved. The @code{sb-ext:code-deletion-note}, a
subtype of @code{compiler-note}, is signalled when the compiler
-deletes user-supplied code, usually after proving that the code in
-question is unreachable.
+deletes user-supplied code after proving that the code in question is
+unreachable.
Future work for SBCL includes expanding this hierarchy of types to
allow more fine-grained control over emission of diagnostic messages.
@comment node-name, next, previous, up
@section Handling of Types
-The most unusual features of the SBCL compiler (which is very
-similar to the original CMUCL compiler, also known as @dfn{Python})
-is its unusually sophisticated understanding of the Common Lisp type
-system and its unusually conservative approach to the implementation
-of type declarations.
+One of the most important features of the SBCL compiler (similar to
+the original CMUCL compiler, also known as @dfn{Python}) is its fairly
+sophisticated understanding of the Common Lisp type system and its
+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
@subsection Declarations as Assertions
@findex safety
-The SBCL compiler treats type declarations differently from most
-other Lisp compilers. Under default compilation policy the compiler
-doesn't blindly believe type declarations, but considers them
-assertions about the program that should be checked: all type
-declarations that have not been proven to always hold are asserted at
-runtime.
+The SBCL compiler treats type declarations differently from most other
+Lisp compilers. Under default compilation policy the compiler doesn't
+blindly believe type declarations, but considers them assertions about
+the program that should be checked: all type declarations that have
+not been proven to always hold are asserted at runtime.
@quotation
@emph{Remaining bugs in the compiler's handling of types unfortunately
provide some exceptions to this rule, see @ref{Implementation
-Limitations}).}
+Limitations}.}
@end quotation
-There are three type checking policies available in SBCL,
-selectable via @code{optimize} declarations.
+CLOS slot types form a notable exception. Types declared using the
+@code{:type} slot option in @code{defclass} are asserted if and only
+if the class was defined in @emph{safe code} and the slot access
+location is in @emph{safe code} as well. This laxness does not pose
+any internal consistency issues, as the CLOS slot types are not
+available for the type inferencer, nor do CLOS slot types provide any
+efficiency benefits.
+
+There are three type checking policies available in SBCL, selectable
+via @code{optimize} declarations.
@table @strong
@item Full Type Checks
All declarations are considered assertions to be checked at runtime,
-and all type checks are precise.
+and all type checks are precise. The default compilation policy
+provides full type checks.
-Used when @code{(>= safety (max speed space compilation-speed)}. The
-default compilation policy provides full type checks.
+Used when @code{(or (>= safety 2) (>= safety speed 1))}.
@item Weak Type Checks
-Any or all type declarations may be believed without runtime
-assertions, and assertions that are done may be imprecise.
+Declared types may be simplified into faster to check supertypes: for
+example, @code{(or (integer -17 -7) (integer 7 17))} is simplified
+into @code{(integer -17 17)}.
-Used when @code{(< 0 safety (max speed space compilation-speed)}.
+@strong{Note}: it is relatively easy to corrupt the heap when weak
+type checks are used if the program contains type-errors.
+
+Used when @code{(and (< safety 2) (< safety speed))}
@item No Type Checks
All declarations are believed without assertions. Also disables
argument count and array bounds checking.
+@strong{Note}: any type errors in code where type checks are not
+performed are liable to corrupt the heap.
+
Used when @code{(= safety 0)}.
@end table
By default SBCL implements @code{eval} by calling the native code
compiler. SBCL also includes an interpreter for use in special cases
-where using the compiler is undesireable, for example due to compilation
+where using the compiler is undesirable, for example due to compilation
overhead. Unlike in some other Lisp implementations, in SBCL interpreted
code is not safer or more debuggable than compiled code.
projects / sbcl.git / blobdiff