decode.scm from Gdb at Krugle
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; Application independent decoder support.
; Copyright (C) 2000, 2004 Red Hat, Inc.
; This file is part of CGEN.
;
; This file provides utilities for building instruction set decoders.
; At present its rather limited, and is geared towards the simulator
; where the goal is hyper-efficiency [not that there isn't room for much
; improvement, but rather that that's what the current focus is].
;
; The CPU description file provides the first pass's bit mask with the
; `decode-assist' spec. This gives the decoder a head start on how to
; efficiently decode the instruction set. The rest of the decoder is
; determined algorithmically.
; ??? Need to say more here.
;
; The main entry point is decode-build-table.
;
; Main procedure call tree:
; decode-build-table
; -build-slots
; -build-decode-table-guts
; -build-decode-table-entry
; -build-slots
; -build-decode-table-guts
;
; -build-slots/-build-decode-table-guts are recursively called to construct a
; tree of "table-guts" elements, and then the application recurses on the
; result. For example see sim-decode.scm.
;
; FIXME: Don't create more than 3 shifts (i.e. no more than 3 groups).
; FIXME: Exits when insns are unambiguously determined, even if there are more
; opcode bits to examine.
; Decoder data structures and accessors.
; The set of instruction is internally recorded as a tree of two data
; structures: "table-guts" and "table-entry".
; [The choice of "table-guts" is historical, a better name will come to mind
; eventually.]
; Decoded tables data structure, termed "table guts".
; A simple data structure of 4 elements:
; bitnums: list of bits that have been used thus far to decode the insn
; startbit: bit offset in instruction of value in C local variable `insn'
; bitsize: size of value in C local variable `insn', the number
; of bits of the instruction read thus far
; entries: list of insns that match the decoding thus far,
; each entry in the list is a `dtable-entry' record
(define (dtable-guts-make bitnums startbit bitsize entries)
(vector bitnums startbit bitsize entries)
)
; Accessors.
(define (dtable-guts-bitnums tg) (vector-ref tg 0))
(define (dtable-guts-startbit tg) (vector-ref tg 1))
(define (dtable-guts-bitsize tg) (vector-ref tg 2))
(define (dtable-guts-entries tg) (vector-ref tg 3))
; A decoded subtable.
; A simple data structure of 3 elements:
; key: name to distinguish this subtable from others, used for lookup
; table: a table-guts element
; name: name of C variable containing the table
;
; The implementation uses a list so the lookup can use assv.
(define (subdtable-make key table name)
(list key table name)
)
; Accessors.
(define (subdtable-key st) (car st))
(define (subdtable-table st) (cadr st))
(define (subdtable-name st) (caddr st))
; List of decode subtables.
(define -decode-subtables nil)
(define (subdtable-lookup key) (assv key -decode-subtables))
; Add SUBTABLE-GUTS to the subtables list if not already present.
; Result is the subtable entry already present, or new entry.
; The key is computed so as to make comparisons possible with assv.
(define (subdtable-add subtable-guts name)
(let* ((key (string->symbol
(string-append
(numbers->string (dtable-guts-bitnums subtable-guts) " ")
" " (number->string (dtable-guts-bitsize subtable-guts))
(string-map
(lambda (elm)
(case (dtable-entry-type elm)
((insn)
(stringsym-append " " (obj:name (dtable-entry-value elm))))
((table)
(stringsym-append " " (subdtable-name (dtable-entry-value elm))))
((expr)
(stringsym-append " " (exprtable-name (dtable-entry-value elm))))
(else (error "bad dtable entry type:"
(dtable-entry-type elm)))))
(dtable-guts-entries subtable-guts)))))
(entry (subdtable-lookup key)))
(if (not entry)
(begin
(set! -decode-subtables (cons (subdtable-make key subtable-guts name)
-decode-subtables))
(car -decode-subtables))
entry))
)
; An instruction and predicate for final matching.
(define (exprtable-entry-make insn expr)
(vector insn expr (rtl-find-ifields expr))
)
; Accessors.
(define (exprtable-entry-insn entry) (vector-ref entry 0))
(define (exprtable-entry-expr entry) (vector-ref entry 1))
(define (exprtable-entry-iflds entry) (vector-ref entry 2))
; Return a pseudo-cost of processing exprentry X.
(define (exprentry-cost x)
(let ((expr (exprtable-entry-expr x)))
(case (rtx-name expr)
((member) (length (rtx-member-set expr)))
(else 4)))
)
; Sort an exprtable, optimum choices first.
; Basically an optimum choice is a cheaper choice.
(define (exprtable-sort expr-list)
(sort expr-list
(lambda (a b)
(let ((costa (exprentry-cost a))
(costb (exprentry-cost b)))
(< costa costb))))
)
; Return the name of the expr table for INSN-EXPRS,
; which is a list of exprtable-entry elements.
(define (-gen-exprtable-name insn-exprs)
(string-map (lambda (x)
(string-append (obj:str-name (exprtable-entry-insn x))
"-"
(rtx-strdump (exprtable-entry-expr x))))
insn-exprs)
)
; A set of instructions that need expressions to distinguish.
; Typically the expressions are ifield-assertion specs.
; INSN-EXPRS is a sorted list of exprtable-entry elements.
; The list is considered sorted in the sense that the first insn to satisfy
; its predicate is chosen.
(define (exprtable-make name insn-exprs)
(vector name insn-exprs)
)
; Accessors.
(define (exprtable-name etable) (vector-ref etable 0))
(define (exprtable-insns etable) (vector-ref etable 1))
; Decoded table entry data structure.
; A simple data structure of 3 elements:
; index: index in the parent table
; entry type indicator: 'insn or 'table or 'expr
; value: the insn or subtable or exprtable
(define (dtable-entry-make index type value)
(assert value)
(vector index type value)
)
; Accessors.
(define (dtable-entry-index te) (vector-ref te 0))
(define (dtable-entry-type te) (vector-ref te 1))
(define (dtable-entry-value te) (vector-ref te 2))
; Return #t if BITNUM is a good bit to use for decoding.
; MASKS is a list of opcode masks.
; MASK-LENS is a list of lengths of each value in MASKS.
; BITNUM is the number of the bit to test. It's value depends on LSB0?.
; It can be no larger than the smallest element in MASKS.
; E.g. If MASK-LENS consists of 16 and 32 and LSB0? is #f, BITNUM must
; be from 0 to 15.
; FIXME: This isn't quite right. What if LSB0? = #t? Need decode-bitsize.
; LSB0? is non-#f if bit number 0 is the least significant bit.
;
; FIXME: This is just a first cut, but the governing intent is to not require
; targets to specify decode tables, hints, or algorithms.
; Certainly as it becomes useful they can supply such information.
; The point is to avoid having to as much as possible.
;
; FIXME: Bit numbers shouldn't be considered in isolation.
; It would be better to compute use counts of all of them and then see
; if there's a cluster of high use counts.
(define (-usable-decode-bit? masks mask-lens bitnum lsb0?)
(let* ((has-bit (map (lambda (msk len)
(bit-set? msk (if lsb0? bitnum (- len bitnum 1))))
masks mask-lens)))
(or (all-true? has-bit)
; If half or more insns use the bit, it's a good one.
; FIXME: An empirical guess at best.
(>= (count-true has-bit) (quotient (length has-bit) 2))
))
)
; Compute population counts for each bit. Return it as a vector indexed by bit
; number. Rather than computing raw popularity, attempt to compute
; "disinguishing value" or inverse-entropy for each bit. The idea is that the
; larger the number for any particular bit slot, the more instructions it can
; be used to distinguish. Raw mask popularity is not enough -- popular masks
; may include useless "reserved" fields whose values don't change, and thus are
; useless in distinguishing.
(define (-distinguishing-bit-population masks mask-lens values lsb0?)
(let* ((max-length (apply max mask-lens))
(0-population (make-vector max-length 0))
(1-population (make-vector max-length 0))
(num-insns (length masks)))
; Compute the 1- and 0-population vectors
(for-each (lambda (mask len value)
(logit 5 " population count mask=" (number->hex mask) " len=" len "\n")
(for-each (lambda (bitno)
(let ((lsb-bitno (if lsb0? bitno (- len bitno 1))))
; ignore this bit if it's not set in the mask
(if (bit-set? mask lsb-bitno)
(let ((chosen-pop-vector (if (bit-set? value lsb-bitno)
1-population 0-population)))
(vector-set! chosen-pop-vector bitno
(+ 1 (vector-ref chosen-pop-vector bitno)))))))
(-range len)))
masks mask-lens values)
; Compute an aggregate "distinguishing value" for each bit.
(list->vector
(map (lambda (p0 p1)
(logit 4 p0 "/" p1 " ")
; The most useful bits for decoding are those with counts in both
; p0 and p1. These are the bits which distinguish one insn from
; another. Assign these bits a high value (greater than num-insns).
;
; The next most useful bits are those with counts in either p0
; or p1. These bits represent specializations of other insns.
; Assign these bits a value between 0 and (num-insns - 1). Note that
; p0 + p1 is guaranteed to be <= num-insns. The value 0 is assigned
; to bits for which p0 or p1 is equal to num_insns. These are bits
; which are always 1 or always 0 in the ISA and are useless for
; decoding purposes.
;
; Bits with no count in either p0 or p1 are useless for decoding
; and should never be considered. Assigning these bits a value of
; 0 ensures this.
(cond
((= (+ p0 p1) 0) 0)
((= (* p0 p1) 0) (- num-insns (+ p0 p1)))
(else (+ num-insns (sqrt (* p0 p1))))))
(vector->list 0-population) (vector->list 1-population))))
)
; Return a list (0 ... limit-1)
(define (-range limit)
(let loop ((i 0)
(indices (list)))
(if (= i limit) (reverse indices) (loop (+ i 1) (cons i indices))))
)
; Return a list (base ... base+size-1)
(define (-range2 base size)
(let loop ((i base)
(indices (list)))
(if (= i (+ base size)) (reverse indices) (loop (+ i 1) (cons i indices))))
)
; Return a copy of given vector, with all entries with given indices set
; to `value'
(define (-vector-copy-set-all vector indices value)
(let ((new-vector (make-vector (vector-length vector))))
(for-each (lambda (index)
(vector-set! new-vector index (if (memq index indices)
value
(vector-ref vector index))))
(-range (vector-length vector)))
new-vector)
)
; Return a list of indices whose counts in the given vector exceed the given
; threshold.
; Sort them in decreasing order of populatority.
(define (-population-above-threshold population threshold)
(let* ((unsorted
(find (lambda (index) (if (vector-ref population index)
(>= (vector-ref population index) threshold)
#f))
(-range (vector-length population))))
(sorted
(sort unsorted (lambda (i1 i2) (> (vector-ref population i1)
(vector-ref population i2))))))
sorted)
)
; Return the top few most popular indices in the population vector,
; ignoring any that are already used (marked by #f). Don't exceed
; `size' unless the clustering is just too good to pass up.
(define (-population-top-few population size)
(let loop ((old-picks (list))
(remaining-population population)
(count-threshold (apply max (map (lambda (value) (if value value 0))
(vector->list population)))))
(let* ((new-picks (-population-above-threshold remaining-population count-threshold)))
(logit 4 "-population-top-few"
" desired=" size
" picks=(" old-picks ") pop=(" remaining-population ")"
" threshold=" count-threshold " new-picks=(" new-picks ")\n")
(cond
; No point picking bits with population count of zero. This leads to
; the generation of layers of subtables which resolve nothing. Generating
; these tables can slow the build by several orders of magnitude.
((= 0 count-threshold)
(logit 2 "-population-top-few: count-threshold is zero!\n")
old-picks)
; No new matches?
((null? new-picks)
(if (null? old-picks)
(logit 2 "-population-top-few: No bits left to pick from!\n"))
old-picks)
; Way too many matches?
((> (+ (length new-picks) (length old-picks)) (+ size 3))
(list-take (+ 3 size) (append old-picks new-picks))) ; prefer old-picks
; About right number of matches?
((> (+ (length new-picks) (length old-picks)) (- size 1))
(append old-picks new-picks))
; Not enough? Lower the threshold a bit and try to add some more.
(else
(loop (append old-picks new-picks)
(-vector-copy-set-all remaining-population new-picks #f)
; Notice magic clustering decay parameter
; vvvv
(* 0.75 count-threshold))))))
)
; Given list of insns, return list of bit numbers of constant bits in opcode
; that they all share (or mostly share), up to MAX elements.
; ALREADY-USED is a list of bitnums we can't use.
; STARTBIT is the bit offset of the instruction value that C variable `insn'
; holds (note that this is independent of LSB0?).
; DECODE-BITSIZE is the number of bits of the insn that `insn' holds.
; LSB0? is non-#f if bit number 0 is the least significant bit.
;
; Nil is returned if there are none, meaning that there is an ambiguity in
; the specification up to the current word.
;
; We assume INSN-LIST matches all opcode bits before STARTBIT.
; FIXME: Revisit, as a more optimal decoder is sometimes achieved by doing
; a cluster of opcode bits that appear later in the insn, and then coming
; back to earlier ones.
;
; All insns are assumed to start at the same address so we handle insns of
; varying lengths - we only analyze the common bits in all of them.
;
; Note that if we get called again to compute further opcode bits, we
; start looking at STARTBIT again (rather than keeping track of how far in
; the insn word we've progressed). We could do this as an optimization, but
; we also have to handle the case where the initial set of decode bits misses
; some and thus we have to go back and look at them. It may also turn out
; that an opcode bit is skipped over because it doesn't contribute much
; information to the decoding process (see -usable-decode-bit?). As the
; possible insn list gets wittled down, the bit will become significant. Thus
; the optimization is left for later. Also, see preceding FIXME.
(define (decode-get-best-bits insn-list already-used startbit max decode-bitsize lsb0?)
(let* ((raw-population (-distinguishing-bit-population (map insn-base-mask insn-list)
(map insn-base-mask-length insn-list)
(map insn-value insn-list)
lsb0?))
; (undecoded (if lsb0?
; (-range2 startbit (+ startbit decode-bitsize))
; (-range2 (- startbit decode-bitsize) startbit)))
(used+undecoded already-used) ; (append already-used undecoded))
(filtered-population (-vector-copy-set-all raw-population used+undecoded #f))
(favorite-indices (-population-top-few filtered-population max))
(sorted-indices (sort favorite-indices (lambda (a b)
(if lsb0? (> a b) (< a b))))))
(logit 3
"Best decode bits (prev=" already-used " start=" startbit " decode=" decode-bitsize ")"
"=>"
"(" sorted-indices ")\n")
sorted-indices)
)
(define (OLDdecode-get-best-bits insn-list already-used startbit max decode-bitsize lsb0?)
(let ((masks (map insn-base-mask insn-list))
; ??? We assume mask lengths are repeatedly used for insns longer
; than the base insn size.
(mask-lens (map insn-base-mask-length insn-list))
(endbit (if lsb0?
-1 ; FIXME: for now (gets sparc port going)
(+ startbit decode-bitsize)))
(incr (if lsb0? -1 1)))
(let loop ((result nil)
(bitnum (if lsb0?
(+ startbit (- decode-bitsize 1))
startbit)))
(if (or (= (length result) max) (= bitnum endbit))
(reverse! result)
(if (and (not (memq bitnum already-used))
(-usable-decode-bit? masks mask-lens bitnum lsb0?))
(loop (cons bitnum result) (+ bitnum incr))
(loop result (+ bitnum incr))))
))
)
; Return list of decode table entry numbers for INSN's opcode bits BITNUMS.
; This is the indices into the decode table that match the instruction.
; LSB0? is non-#f if bit number 0 is the least significant bit.
;
; Example: If BITNUMS is (0 1 2 3 4 5), and the constant (i.e. opcode) part of
; the those bits of INSN is #b1100xx (where 'x' indicates a non-constant
; part), then the result is (#b110000 #b110001 #b110010 #b110011).
(define (-opcode-slots insn bitnums lsb0?)
(letrec ((opcode (insn-value insn))
(insn-len (insn-base-mask-length insn))
(decode-len (length bitnums))
(compute (lambda (val insn-len decode-len bl default)
;(display (list val insn-len decode-len bl)) (newline)
; Oh My God. This isn't tail recursive.
(if (null? bl)
0
(+ (if (or (and (>= (car bl) insn-len) (= default 1))
(and (< (car bl) insn-len)
(bit-set? val
(if lsb0?
(car bl)
(- insn-len (car bl) 1)))))
(integer-expt 2 (- (length bl) 1))
0)
(compute val insn-len decode-len (cdr bl) default))))))
(let* ((opcode (compute (insn-value insn) insn-len decode-len bitnums 0))
(opcode-mask (compute (insn-base-mask insn) insn-len decode-len bitnums 1))
(indices (missing-bit-indices opcode-mask (- (integer-expt 2 decode-len) 1))))
(logit 3 "insn =" (obj:name insn)
" insn-value=" (number->hex (insn-value insn))
" insn-base-mask=" (number->hex (insn-base-mask insn))
" insn-len=" insn-len
" decode-len=" decode-len
" opcode=" (number->hex opcode)
" opcode-mask=" (number->hex opcode-mask)
" indices=" indices "\n")
(map (lambda (index) (+ opcode index)) indices)))
)
; Subroutine of -build-slots.
; Fill slot in INSN-VEC that INSN goes into.
; BITNUMS is the list of opcode bits.
; LSB0? is non-#f if bit number 0 is the least significant bit.
;
; Example: If BITNUMS is (0 1 2 3 4 5) and the constant (i.e. opcode) part of
; the first six bits of INSN is #b1100xx (where 'x' indicates a non-constant
; part), then elements 48 49 50 51 of INSN-VEC are cons'd with INSN.
; Each "slot" is a list of matching instructions.
(define (-fill-slot! insn-vec insn bitnums lsb0?)
;(display (string-append "fill-slot!: " (obj:str-name insn) " ")) (display bitnums) (newline)
(let ((slot-nums (-opcode-slots insn bitnums lsb0?)))
;(display (list "Filling slot(s)" slot-nums "...")) (newline)
(for-each (lambda (slot-num)
(vector-set! insn-vec slot-num
(cons insn (vector-ref insn-vec slot-num))))
slot-nums)
*UNSPECIFIED*
)
)
; Given a list of constant bitnums (ones that are predominantly, though perhaps
; not always, in the opcode), record each insn in INSN-LIST in the proper slot.
; LSB0? is non-#f if bit number 0 is the least significant bit.
; The result is a vector of insn lists. Each slot is a list of insns
; that go in that slot.
(define (-build-slots insn-list bitnums lsb0?)
(let ((result (make-vector (integer-expt 2 (length bitnums)) nil)))
; Loop over each element, filling RESULT.
(for-each (lambda (insn)
(-fill-slot! result insn bitnums lsb0?))
insn-list)
result)
)
; Compute the name of a decode table, prefixed with PREFIX.
; INDEX-LIST is a list of pairs: list of bitnums, table entry number,
; in reverse order of traversal (since they're built with cons).
; INDEX-LIST may be empty.
(define (-gen-decode-table-name prefix index-list)
(set! index-list (reverse index-list))
(string-append
prefix
"table"
(string-map (lambda (elm) (string-append "_" (number->string elm)))
; CDR of each element is the table index.
(map cdr index-list)))
)
; Generate one decode table entry for INSN-VEC at INDEX.
; INSN-VEC is a vector of slots where each slot is a list of instructions that
; map to that slot (opcode value). If a slot is nil, no insn has that opcode
; value so the decoder marks it as being invalid.
; STARTBIT is the bit offset of the instruction value that C variable `insn'
; holds (note that this is independent of LSB0?).
; DECODE-BITSIZE is the number of bits of the insn that `insn' holds.
; INDEX-LIST is a list of pairs: list of bitnums, table entry number.
; LSB0? is non-#f if bit number 0 is the least significant bit.
; INVALID-INSN is an <insn> object to use for invalid insns.
; The result is a dtable-entry element (or "slot").
; ??? For debugging.
(define -build-decode-table-entry-args #f)
(define (-build-decode-table-entry insn-vec startbit decode-bitsize index index-list lsb0? invalid-insn)
(let ((slot (vector-ref insn-vec index)))
(logit 2 "Processing decode entry "
(number->string index)
" in "
(-gen-decode-table-name "decode_" index-list)
", "
(cond ((null? slot) "invalid")
((= 1 (length slot)) (insn-syntax (car slot)))
(else "subtable"))
" ...\n")
(cond
; If no insns map to this value, mark it as invalid.
((null? slot) (dtable-entry-make index 'insn invalid-insn))
; If only one insn maps to this value, that's it for this insn.
((= 1 (length slot))
; FIXME: Incomplete: need to check further opcode bits.
(dtable-entry-make index 'insn (car slot)))
; Otherwise more than one insn maps to this value and we need to look at
; further opcode bits.
(else
(logit 3 "Building subtable at index " (number->string index)
", decode-bitsize = " (number->string decode-bitsize)
", indices used thus far:"
(string-map (lambda (i) (string-append " " (number->string i)))
(apply append (map car index-list)))
"\n")
(let ((bitnums (decode-get-best-bits slot
(apply append (map car index-list))
startbit 4
decode-bitsize lsb0?)))
; If bitnums is nil, either there is an ambiguity or we need to read
; more of the instruction in order to distinguish insns in SLOT.
(if (and (null? bitnums)
(< startbit (apply min (map insn-length slot))))
(begin
; We might be able to resolve the ambiguity by reading more bits.
; We know from the < test that there are, indeed, more bits to
; be read.
(set! startbit (+ startbit decode-bitsize))
; FIXME: The calculation of the new decode-bitsize will
; undoubtedly need refinement.
(set! decode-bitsize
(min decode-bitsize
(- (apply min (map insn-length slot))
startbit)))
(set! bitnums (decode-get-best-bits slot
;nil ; FIXME: what to put here?
(apply append (map car index-list))
startbit 4
decode-bitsize lsb0?))))
; If bitnums is still nil there is an ambiguity.
(if (null? bitnums)
(begin
; Try filtering out insns which are more general cases of
; other insns in the slot. The filtered insns will appear
; in other slots as appropriate.
(set! slot (filter-non-specialized-ambiguous-insns slot))
(if (= 1 (length slot))
; Only 1 insn left in the slot, so take it.
(dtable-entry-make index 'insn (car slot))
; There is still more than one insn in 'slot',
; so there is still an ambiguity.
(begin
; If all insns are marked as DECODE-SPLIT, don't warn.
(if (not (all-true? (map (lambda (insn)
(obj-has-attr? insn 'DECODE-SPLIT))
slot)))
(message "WARNING: Decoder ambiguity detected: "
(string-drop1 ; drop leading comma
(string-map (lambda (insn)
(string-append ", " (obj:str-name insn)))
slot))
"\n"))
; Things aren't entirely hopeless. We've warned about
; the ambiguity. Now, if there are any identical insns,
; filter them out. If only one remains, then use it.
(set! slot (filter-identical-ambiguous-insns slot))
(if (= 1 (length slot))
; Only 1 insn left in the slot, so take it.
(dtable-entry-make index 'insn (car slot))
; Otherwise, see if any ifield-assertion
; specs are present.
; FIXME: For now we assume that if they all have an
; ifield-assertion spec, then there is no ambiguity (it's left
; to the programmer to get it right). This can be made more
; clever later.
; FIXME: May need to back up startbit if we've tried to read
; more of the instruction.
(let ((assertions (map insn-ifield-assertion slot)))
(if (not (all-true? assertions))
(begin
; Save arguments for debugging purposes.
(set! -build-decode-table-entry-args
(list insn-vec startbit decode-bitsize index index-list lsb0? invalid-insn))
(error "Unable to resolve ambiguity (maybe need some ifield-assertion specs?)")))
; FIXME: Punt on even simple cleverness for now.
(let ((exprtable-entries
(exprtable-sort (map exprtable-entry-make
slot
assertions))))
(dtable-entry-make index 'expr
(exprtable-make
(-gen-exprtable-name exprtable-entries)
exprtable-entries))))))))
; There is no ambiguity so generate the subtable.
; Need to build `subtable' separately because we
; may be appending to -decode-subtables recursively.
(let* ((insn-vec (-build-slots slot bitnums lsb0?))
(subtable
(-build-decode-table-guts insn-vec bitnums startbit
decode-bitsize index-list lsb0?
invalid-insn)))
(dtable-entry-make index 'table
(subdtable-add subtable
(-gen-decode-table-name "" index-list)))))))
)
)
)
; Given vector of insn slots, generate the guts of the decode table, recorded
; as a list of 3 elements: bitnums, decode-bitsize, and list of entries.
; Bitnums is recorded with the guts so that tables whose contents are
; identical but are accessed by different bitnums are treated as separate in
; -decode-subtables. Not sure this will ever happen, but play it safe.
;
; BITNUMS is the list of bit numbers used to build the slot table.
; STARTBIT is the bit offset of the instruction value that C variable `insn'
; holds (note that this is independent of LSB0?).
; For example, it is initially zero. If DECODE-BITSIZE is 16 and after
; scanning the first fetched piece of the instruction, more decoding is
; needed, another piece will be fetched and STARTBIT will then be 16.
; DECODE-BITSIZE is the number of bits of the insn that `insn' holds.
; INDEX-LIST is a list of pairs: list of bitnums, table entry number.
; Decode tables consist of entries of two types: actual insns and
; pointers to other tables.
; LSB0? is non-#f if bit number 0 is the least significant bit.
; INVALID-INSN is an <insn> object representing invalid insns.
(define (-build-decode-table-guts insn-vec bitnums startbit decode-bitsize index-list lsb0? invalid-insn)
(logit 2 "Processing decoder for bits"
(numbers->string bitnums " ")
" ...\n")
(dtable-guts-make
bitnums startbit decode-bitsize
(map (lambda (index)
(-build-decode-table-entry insn-vec startbit decode-bitsize index
(cons (cons bitnums index)
index-list)
lsb0? invalid-insn))
(iota (vector-length insn-vec))))
)
; Entry point.
; Return a table that efficiently decodes INSN-LIST.
; BITNUMS is the set of bits to initially key off of.
; DECODE-BITSIZE is the number of bits of the instruction that `insn' holds.
; LSB0? is non-#f if bit number 0 is the least significant bit.
; INVALID-INSN is an <insn> object representing the `invalid' insn (for
; instructions values that don't decode to any entry in INSN-LIST).
(define (decode-build-table insn-list bitnums decode-bitsize lsb0? invalid-insn)
; Initialize the list of subtables computed.
(set! -decode-subtables nil)
; ??? Another way to handle simple forms of ifield-assertions (like those
; created by insn specialization) is to record a copy of the insn for each
; possible value of the ifield and modify its ifield list with the ifield's
; value. This would then let the decoder table builder handle it normally.
; I wouldn't create N insns, but would rather create an intermediary record
; that recorded the necessary bits (insn, ifield-list, remaining
; ifield-assertions).
(let ((insn-vec (-build-slots insn-list bitnums lsb0?)))
(let ((table-guts (-build-decode-table-guts insn-vec bitnums
0 decode-bitsize
nil lsb0?
invalid-insn)))
table-guts))
)
See more files for this project here