Making struct bitfield readers fast

db4
Daniel Ehrenberg 2009-10-07 23:51:18 -05:00
parent 8ba295d8a8
commit 3179dacb3e
2 changed files with 46 additions and 17 deletions

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@ -0,0 +1,29 @@
! Copyright (C) 2009 Daniel Ehrenberg
! See http://factorcode.org/license.txt for BSD license.
USING: kernel sequences math fry locals math.order alien.accessors ;
IN: classes.struct.bit-accessors
! Bitfield accessors are little-endian on all platforms
! Why not? It's platform-dependent in C
: ones-between ( start end -- n )
[ 2^ 1 - ] bi@ swap bitnot bitand ;
:: read-bits ( offset bits -- quot: ( byte-array -- n ) shift-amount offset' bits' )
offset 8 /mod :> start-bit :> i
start-bit bits + 8 min :> end-bit
start-bit end-bit ones-between :> mask
end-bit start-bit - :> used-bits
! The code generated for this isn't optimal
! To improve the code, algebraic simplifications should
! have interval information available
[ i alien-unsigned-1 mask bitand start-bit neg shift ]
used-bits
i 1 + 8 *
bits used-bits - ;
: bit-reader ( offset bits -- quot: ( alien -- n ) )
read-bits dup zero? [ 3drop ] [
bit-reader swap '[ _ _ bi _ shift bitor ]
] if ;

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@ -6,7 +6,8 @@ combinators.smart cpu.architecture definitions functors.backend
fry generalizations generic.parser kernel kernel.private lexer
libc locals macros make math math.order parser quotations
sequences slots slots.private specialized-arrays vectors words
summary namespaces assocs vocabs.parser math.functions bit-arrays ;
summary namespaces assocs vocabs.parser math.functions
classes.struct.bit-accessors bit-arrays ;
QUALIFIED: math
IN: classes.struct
@ -89,23 +90,14 @@ MACRO: <struct-boa> ( class -- quot: ( ... -- struct ) )
: pad-struct-slots ( values class -- values' class )
[ struct-slots [ initial>> ] map over length tail append ] keep ;
: bits@ ( slot -- beginning end )
[ offset>> ] [ bits>> ] bi dupd + ;
QUALIFIED: math.bits
: bytes>bits ( byte-array -- bit-array )
[ 8 math.bits:<bits> ] { } map-as ?{ } join ;
: read-bits ( beginning end byte-array -- n )
! This is absurdly inefficient
bytes>bits subseq bit-array>integer ;
: sign-extend ( n bits -- n' )
! formula from:
! http://guru.multimedia.cx/fast-sign-extension/
1 - -1 swap shift [ + ] keep bitxor ; inline
: sign-extender ( signed? bits -- quot )
'[ _ [ _ sign-extend ] when ] ;
GENERIC: (reader-quot) ( slot -- quot )
M: struct-slot-spec (reader-quot)
@ -113,10 +105,10 @@ M: struct-slot-spec (reader-quot)
[ offset>> [ >c-ptr ] swap suffix ] bi prepend ;
M: struct-bit-slot-spec (reader-quot)
[ bits@ ] [ signed?>> ] [ bits>> ] tri '[
[ _ _ ] dip (underlying)>> read-bits
_ [ _ sign-extend ] when
] ;
[ [ offset>> ] [ bits>> ] bi bit-reader ]
[ [ signed?>> ] [ bits>> ] bi sign-extender ]
bi compose
[ >c-ptr ] prepose ;
GENERIC: (writer-quot) ( slot -- quot )
@ -124,6 +116,11 @@ M: struct-slot-spec (writer-quot)
[ type>> c-setter ]
[ offset>> [ >c-ptr ] swap suffix ] bi prepend ;
QUALIFIED: math.bits
: bytes>bits ( byte-array -- bit-array )
[ 8 math.bits:<bits> ] { } map-as ?{ } join ;
: (write-bits) ( value offset end byte-array -- )
! This is absurdly inefficient
[
@ -131,6 +128,9 @@ M: struct-slot-spec (writer-quot)
replace-slice ?{ } like underlying>>
] keep 0 swap copy ;
: bits@ ( slot -- beginning end )
[ offset>> ] [ bits>> ] bi dupd + ;
M: struct-bit-slot-spec (writer-quot) ( slot -- quot )
bits@ '[ [ _ _ ] dip (underlying)>> (write-bits) ] ;