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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Issue FIXNUM-NON-PORTABLE Writeup</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="../Issues/iss157_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="../Issues/iss158.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="../Issues/iss159_w.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>Issue FIXNUM-NON-PORTABLE Writeup</H2>
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<PRE><B>Status:</B> Passed Jan 89 X3J13, as amended<P>
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<B>Issue:</B> <A HREF="iss158.htm">FIXNUM-NON-PORTABLE</A><P>
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<B>References:</B> CLtL p. 14, 34, 43, 231<P>
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<B>Category:</B> CHANGE, CLARIFICATION<P>
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<P>
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<B>Edit History:</B> Version 1, 11-Jul-88, Sandra Loosemore<P>
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Version 2, 15-Sep-88, Masinter<P>
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Version 3, 23-Sep-88, Masinter<P>
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Version 4, 7-Dec-88, Masinter (two proposals)<P>
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Version 5, 16-Mar-89, Masinter (incorporate amendments)<P>
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Version 6, 17-Mar-89, Masinter (incorporate amendments correctly)<P>
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<P>
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<B>Problem Description: <P>
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</B><P>
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Implementations of Common Lisp are required to support two disjoint<P>
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subsets of integers, fixnums and bignums, with the promise that<P>
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fixnums have a more efficient representation. However, nothing is<P>
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guaranteed about the range of integers which are fixnums: "Exactly<P>
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which integers are fixnums is implementation-dependent; typically they<P>
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will be those integers in the range -2**n to 2**n - 1, inclusive, for<P>
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some n not less than 15."<P>
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<P>
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There are few uses of the <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>fixnum</B></A> type that are portable, given the<P>
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current definition. In particular, many programmers use <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> type<P>
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declarations where they really mean "small integer".<P>
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<P>
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While most Common Lisp implementations have a <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> range<P>
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which is a subset of integers represeted and operated on most<P>
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efficiently, many also have several other subranges. The<P>
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partitioning of <A REL=DEFINITION HREF="../Body/t_intege.htm#integer"><B>INTEGER</B></A> into <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A> and <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> is merely<P>
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confusing in these implementations, and not useful.<P>
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<P>
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CLtL p14 and p34 disagree about <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A>. One says that<P>
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<A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> and <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A> are an exhaustive partition of the<P>
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integer space, the other says they might not be!<P>
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<P>
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Proposal: FIXNUM-NONPORTABLE:TIGHTEN-DEFINITION<P>
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<P>
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(1) Change the description of the type <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> to reflect that it is<P>
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required to be a supertype of (<A REL=DEFINITION HREF="../Body/t_sgn_by.htm#signed-byte"><B>SIGNED-BYTE</B></A> 16).<P>
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<P>
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(2) Define <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A> to be exactly (<A REL=DEFINITION HREF="../Body/a_and.htm#and"><B>AND</B></A> <A REL=DEFINITION HREF="../Body/t_intege.htm#integer"><B>INTEGER</B></A> (<A REL=DEFINITION HREF="../Body/a_not.htm#not"><B>NOT</B></A> <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A>))<P>
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<P>
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(3) <A REL=DEFINITION HREF="../Body/f_provid.htm#require"><B>require</B></A> that (<A REL=DEFINITION HREF="../Body/f_eq_sle.htm#LTEQ"><B><=</B></A> <A REL=DEFINITION HREF="../Body/v_ar_dim.htm#array-dimension-limit"><B>ARRAY-DIMENSION-LIMIT</B></A> <A REL=DEFINITION HREF="../Body/v_most_p.htm#most-positive-fixnum"><B>MOST-POSITIVE-FIXNUM</B></A>)<P>
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<P>
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<B>Example:<P>
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</B><P>
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Consider an implementation with three numeric representations:<P>
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<P>
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Fast (<A REL=DEFINITION HREF="../Body/t_intege.htm#integer"><B>INTEGER</B></A> -1024 1023)<P>
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Immediate 29 bits<P>
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Extended Multi-precision<P>
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<P>
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Such an implementation would have to define<P>
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<A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> to be (<A REL=DEFINITION HREF="../Body/a_or.htm#or"><B>OR</B></A> Fast Immediate). <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A><P>
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would then refer to multi-precision integers. <P>
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<P>
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<B>Rationale:<P>
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</B><P>
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Many programmers already use <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> to mean "small integer"; this<P>
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proposal makes this usage portable. <P>
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<P>
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However, there is little portable use for the type <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A>, and it<P>
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is inconsistent with many current implementation techniques.<P>
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Removing it is an incompatible change for a weak reason.<P>
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<P>
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<B>Current Practice:<P>
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</B><P>
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Xerox Common Lisp has 17-bit fixnums. Most other Common Lisp<P>
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implementations have <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>fixnum</B></A> ranges of 24 bits or larger. We know<P>
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of no implementation that currently violates the proposed minimum <P>
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size.<P>
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<P>
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Several existing Common Lisp implementations have more than two <P>
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representations for integers, such that the FIXNUM/BIGNUM distinction<P>
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is confusing; they define <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A> to cover all of the larger number<P>
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types.<P>
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<P>
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<B>Cost to implementors:<P>
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</B><P>
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Slight. All implementations we know of already define FIXNUMs to be at<P>
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least 16 bits.<P>
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<P>
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<B>Cost to users:<P>
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</B><P>
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Slight. <P>
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<P>
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<B>Benefits:<P>
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</B><P>
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The <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> type specifier would have a portable interpretation.<P>
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<P>
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The language would be less confusing.<P>
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<P>
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<B>Discussion:<P>
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</B><P>
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There was little consensus on whether to leave <A REL=DEFINITION HREF="../Body/t_bignum.htm#bignum"><B>BIGNUM</B></A> in the language.<P>
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<P>
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Earlier discussion of a related proposal contained several other more<P>
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controversial components (adding a constant MAX-INTEGER-LENGTH, allowing <P>
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<A REL=DEFINITION HREF="../Body/v_most_p.htm#most-positive-fixnum"><B>MOST-POSITIVE-FIXNUM</B></A> to be <A REL=DEFINITION HREF="../Body/a_nil.htm#nil"><B>NIL</B></A> as well as an integer.) This proposal<P>
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is an attempt to address the part that cleanup committee seemed to agree<P>
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on.<P>
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<P>
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It is possible that an implementation have a single representation for all<P>
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integers, and no way to identify any efficient range of integers. Those<P>
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implementations might need to set <A REL=DEFINITION HREF="../Body/v_most_p.htm#most-positive-fixnum"><B>MOST-POSITIVE-FIXNUM</B></A><P>
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and <A REL=DEFINITION HREF="../Body/v_most_p.htm#most-negative-fixnum"><B>MOST-NEGATIVE-FIXNUM</B></A> to arbitrary values, consistent with <P>
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the requirement that (<A REL=DEFINITION HREF="../Body/t_sgn_by.htm#signed-byte"><B>SIGNED-BYTE</B></A> 16) is a subtype of <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A>.<P>
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<P>
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Other alternatives considered (and not necessarily mutually exclusive<P>
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with this proposal):<P>
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<P>
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remove the <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> type specifier entirely, while leaving a way<P>
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to query what is the most efficient range of integers<P>
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<P>
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leave the range of FIXNUMs unconstrained and introduce a <P>
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SMALL-INTEGER type with a fixed range (but no promises about<P>
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efficiency) . <P>
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<P>
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It might be possible to specify the required performance behavior<P>
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of FIXNUMs more concretely, e.g., specify that the basic integer operations<P>
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use algorithms that are not proportional to the size of the data; it <P>
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should be just about as fast to add numbers in the middle of the <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>fixnum</B></A> <P>
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range as it is to add, say, 10 and 11. This might be a useful way to<P>
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describe<P>
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the intent of the <A REL=DEFINITION HREF="../Body/t_fixnum.htm#fixnum"><B>FIXNUM</B></A> range, if not its specification.<P>
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<P>
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</PRE>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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