[Gcl-devel] Re: Improvement to GCL's EXPT?

[Camm Maguire]

Greetings!

You are of course correct -- wrote the previous too quickly.

In 2.6.6, the only way at present that I can see is to define foo as
follows:

(defun foo (x) (declare ((integer 0 31) x)) (the fixnum (expt 2 (the
fixnum x))))

This will work out of the box in 2.6.6 due to the following optimizer:


(push '((fixnum fixnum) fixnum #.(flags)(LAMBDA (LOC1 LOC2)
                                          (IF
                                           (AND (CONSP LOC1)
                                            (EQ (CAR LOC1) 'FIXNUM-LOC)
                                            (CONSP (CADR LOC1))
                                            (EQ (CAADR LOC1)
                                             'FIXNUM-VALUE)
                                            (EQUAL (CADDR (CADR LOC1))
                                             2))
                                           (WT "(1<<(" LOC2 "))")
                                           (WT "fixnum_expt(" LOC1 #\,
                                            LOC2 #\)))))
   (get 'expt 'inline-always))


This crude use of fixnum is limiting and imprecise of course.  2.7.0
is the first GCL to introduce integer range type inferencing, so in
this version one can simply eval in the compiler package:

(push '(((integer 2 2) (integer 0 #.(integer-length
   most-positive-fixnum))) fixnum #.(flags rfa) "(1<<(#1))") (get
   'expt 'inline-always))

Depending on how nfix is declared, your existing code will likely not
have to be modified.

Analogs of course can be added for the other special cases, including
making a ratio from a shifted fixnum in the case of a negative second
arg in range, etc.  I don't see any other special first args other
than -2 -1 0 1 2.  2.7.0 also has the ability to pass situations like
this off to a special inliner so that only one entry in the table is
needed. 

Will try to finalize this and the boxed version enhancement.  If all
passes through Paul's random tester, can then commit.

Take care,

Matt Kaufmann <address@hidden> writes:

> Howdy --
> 
> Interesting.... But this seems to break when the exponent is not an integer.
> An example transcript is below, showing that (expt 2 3/2) works fine until
> after we adopt your modification.
> 
> -- Matt
>   sundance:~> gcl
>   GCL (GNU Common Lisp)  2.6.5 CLtL1    Aug 18 2004 10:07:03
>   Source License: LGPL(gcl,gmp), GPL(unexec,bfd)
>   Binary License:  GPL due to GPL'ed components: (BFD UNEXEC)
>   Modifications of this banner must retain notice of a compatible license
>   Dedicated to the memory of W. Schelter
> 
>   Use (help) to get some basic information on how to use GCL.
> 
>   >(in-package 'compiler)
> 
>   #<"COMPILER" package>
> 
>   COMPILER>(defun foo (x)  (expt 2 x))
> 
>   FOO
> 
>   COMPILER>(foo 3/2)
> 
>   2.8284271247461898
> 
>   COMPILER>(compile 'foo)
> 
>   Compiling gazonk0.lsp.
>   End of Pass 1.  
>   End of Pass 2.  
>   OPTIMIZE levels: Safety=0 (No runtime error checking), Space=0, Speed=3
>   Finished compiling gazonk0.lsp.
>   Loading gazonk0.o
>   start address -T 0x848a8d0 Finished loading gazonk0.o
>   #<compiled-function FOO>
> 
>   COMPILER>(foo 3/2)
> 
>   2.8284271247461898
> 
>   COMPILER>(defun expt-2-to-ash (form env)
>    (declare (ignore env))
>    (if (eql (cadr form) 2) `(ash 1 ,(caddr form)) form))
> 
>   EXPT-2-TO-ASH
> 
>   COMPILER>(si::putprop 'expt (function expt-2-to-ash) 'compiler-macro)
> 
>   (LAMBDA-BLOCK EXPT-2-TO-ASH (FORM ENV)
>     (DECLARE (IGNORE ENV))
>     (IF (EQL (CADR FORM) 2) (LIST 'ASH 1 (CADDR FORM)) FORM))
> 
>   COMPILER>(defun foo (x)  (expt 2 x))
> 
>   FOO
> 
>   COMPILER>(foo 3/2)
> 
>   2.8284271247461898
> 
>   COMPILER>(compile 'foo)
> 
>   Compiling gazonk0.lsp.
>   End of Pass 1.  
>   End of Pass 2.  
>   OPTIMIZE levels: Safety=0 (No runtime error checking), Space=0, Speed=3
>   Finished compiling gazonk0.lsp.
>   Loading gazonk0.o
>   start address -T 0x83d3660 Finished loading gazonk0.o
>   #<compiled-function FOO>
> 
>   COMPILER>(foo 3/2)
> 
>   Error: 3/2 is not of type INTEGER.
>   Fast links are on: do (si::use-fast-links nil) for debugging
>   Error signalled by FOO.
>   Broken at FOO.  Type :H for Help.
>   COMPILER>>
> 
> 
> 

-- 
Camm Maguire                                            address@hidden
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"The earth is but one country, and mankind its citizens."  --  Baha'u'llah