1 ; Application independent decoder support.
2 ; Copyright (C) 2000 Red Hat, Inc.
3 ; This file is part of CGEN.
5 ; This file provides utilities for building instruction set decoders.
6 ; At present its rather limited, and is geared towards the simulator
7 ; where the goal is hyper-efficiency [not that there isn't room for much
8 ; improvement, but rather that that's what the current focus is].
10 ; The CPU description file provides the first pass's bit mask with the
11 ; `decode-assist' spec. This gives the decoder a head start on how to
12 ; efficiently decode the instruction set. The rest of the decoder is
13 ; determined algorithmically.
14 ; ??? Need to say more here.
16 ; The main entry point is decode-build-table.
18 ; Main procedure call tree:
21 ; -build-decode-table-guts
22 ; -build-decode-table-entry
24 ; -build-decode-table-guts
26 ; -build-slots/-build-decode-table-guts are recursively called to construct a
27 ; tree of "table-guts" elements, and then the application recurses on the
28 ; result. For example see sim-decode.scm.
30 ; FIXME: Don't create more than 3 shifts (i.e. no more than 3 groups).
31 ; FIXME: Exits when insns are unambiguously determined, even if there are more
32 ; opcode bits to examine.
34 ; Decoder data structures and accessors.
35 ; The set of instruction is internally recorded as a tree of two data
36 ; structures: "table-guts" and "table-entry".
37 ; [The choice of "table-guts" is historical, a better name will come to mind
40 ; Decoded tables data structure, termed "table guts".
41 ; A simple data structure of 4 elements:
42 ; bitnums: list of bits that have been used thus far to decode the insn
43 ; startbit: bit offset in instruction of value in C local variable `insn'
44 ; bitsize: size of value in C local variable `insn', the number
45 ; of bits of the instruction read thus far
46 ; entries: list of insns that match the decoding thus far,
47 ; each entry in the list is a `dtable-entry' record
49 (define (dtable-guts-make bitnums startbit bitsize entries)
50 (vector bitnums startbit bitsize entries)
54 (define (dtable-guts-bitnums tg) (vector-ref tg 0))
55 (define (dtable-guts-startbit tg) (vector-ref tg 1))
56 (define (dtable-guts-bitsize tg) (vector-ref tg 2))
57 (define (dtable-guts-entries tg) (vector-ref tg 3))
60 ; A simple data structure of 3 elements:
61 ; key: name to distinguish this subtable from others, used for lookup
62 ; table: a table-guts element
63 ; name: name of C variable containing the table
65 ; The implementation uses a list so the lookup can use assv.
67 (define (subdtable-make key table name)
72 (define (subdtable-key st) (car st))
73 (define (subdtable-table st) (cadr st))
74 (define (subdtable-name st) (caddr st))
76 ; List of decode subtables.
77 (define -decode-subtables nil)
79 (define (subdtable-lookup key) (assv key -decode-subtables))
81 ; Add SUBTABLE-GUTS to the subtables list if not already present.
82 ; Result is the subtable entry already present, or new entry.
83 ; The key is computed so as to make comparisons possible with assv.
85 (define (subdtable-add subtable-guts name)
86 (let* ((key (string->symbol
88 (numbers->string (dtable-guts-bitnums subtable-guts) " ")
89 " " (number->string (dtable-guts-bitsize subtable-guts))
92 (case (dtable-entry-type elm)
94 (string-append " " (obj:name (dtable-entry-value elm))))
96 (string-append " " (subdtable-name (dtable-entry-value elm))))
98 (string-append " " (exprtable-name (dtable-entry-value elm))))
99 (else (error "bad dtable entry type:"
100 (dtable-entry-type elm)))))
101 (dtable-guts-entries subtable-guts)))))
102 (entry (subdtable-lookup key)))
105 (set! -decode-subtables (cons (subdtable-make key subtable-guts name)
107 (car -decode-subtables))
111 ; An instruction and predicate for final matching.
113 (define (exprtable-entry-make insn expr)
114 (vector insn expr (rtl-find-ifields expr))
119 (define (exprtable-entry-insn entry) (vector-ref entry 0))
120 (define (exprtable-entry-expr entry) (vector-ref entry 1))
121 (define (exprtable-entry-iflds entry) (vector-ref entry 2))
123 ; Return a pseudo-cost of processing exprentry X.
125 (define (exprentry-cost x)
126 (let ((expr (exprtable-entry-expr x)))
127 (case (rtx-name expr)
128 ((member) (length (rtx-member-set expr)))
132 ; Sort an exprtable, optimum choices first.
133 ; Basically an optimum choice is a cheaper choice.
135 (define (exprtable-sort expr-list)
138 (let ((costa (exprentry-cost a))
139 (costb (exprentry-cost b)))
143 ; Return the name of the expr table for INSN-EXPRS,
144 ; which is a list of exprtable-entry elements.
146 (define (-gen-exprtable-name insn-exprs)
147 (string-map (lambda (x)
148 (string-append (obj:name (exprtable-entry-insn x))
150 (rtx-strdump (exprtable-entry-expr x))))
154 ; A set of instructions that need expressions to distinguish.
155 ; Typically the expressions are ifield-assertion specs.
156 ; INSN-EXPRS is a sorted list of exprtable-entry elements.
157 ; The list is considered sorted in the sense that the first insn to satisfy
158 ; its predicate is chosen.
160 (define (exprtable-make name insn-exprs)
161 (vector name insn-exprs)
166 (define (exprtable-name etable) (vector-ref etable 0))
167 (define (exprtable-insns etable) (vector-ref etable 1))
169 ; Decoded table entry data structure.
170 ; A simple data structure of 3 elements:
171 ; index: index in the parent table
172 ; entry type indicator: 'insn or 'table or 'expr
173 ; value: the insn or subtable or exprtable
175 (define (dtable-entry-make index type value)
177 (vector index type value)
181 (define (dtable-entry-index te) (vector-ref te 0))
182 (define (dtable-entry-type te) (vector-ref te 1))
183 (define (dtable-entry-value te) (vector-ref te 2))
185 ; Return #t if BITNUM is a good bit to use for decoding.
186 ; MASKS is a list of opcode masks.
187 ; MASK-LENS is a list of lengths of each value in MASKS.
188 ; BITNUM is the number of the bit to test. It's value depends on LSB0?.
189 ; It can be no larger than the smallest element in MASKS.
190 ; E.g. If MASK-LENS consists of 16 and 32 and LSB0? is #f, BITNUM must
192 ; FIXME: This isn't quite right. What if LSB0? = #t? Need decode-bitsize.
193 ; LSB0? is non-#f if bit number 0 is the least significant bit.
195 ; FIXME: This is just a first cut, but the governing intent is to not require
196 ; targets to specify decode tables, hints, or algorithms.
197 ; Certainly as it becomes useful they can supply such information.
198 ; The point is to avoid having to as much as possible.
200 ; FIXME: Bit numbers shouldn't be considered in isolation.
201 ; It would be better to compute use counts of all of them and then see
202 ; if there's a cluster of high use counts.
204 (define (-usable-decode-bit? masks mask-lens bitnum lsb0?)
205 (let* ((has-bit (map (lambda (msk len)
206 (bit-set? msk (if lsb0? bitnum (- len bitnum 1))))
208 (or (all-true? has-bit)
209 ; If half or more insns use the bit, it's a good one.
210 ; FIXME: An empirical guess at best.
211 (>= (count-true has-bit) (quotient (length has-bit) 2))
216 ; Compute population counts for each bit. Return it as a vector indexed by bit number.
217 ; Rather than computing raw popularity, attempt to compute "disinguishing value" or
218 ; inverse-entropy for each bit. The idea is that the larger the number for any particular
219 ; bit slot, the more instructions it can be used to distinguish. Raw mask popularity
220 ; is not enough -- popular masks may include useless "reserved" fields whose values
221 ; don't change, and thus are useless in distinguishing.
222 (define (-distinguishing-bit-population masks mask-lens values lsb0?)
223 (let* ((max-length (apply max mask-lens))
224 (0-population (make-vector max-length 0))
225 (1-population (make-vector max-length 0))
226 (num-insns (length masks)))
227 ; Compute the 1- and 0-population vectors
228 (for-each (lambda (mask len value)
229 (logit 5 " population count mask=" (number->hex mask) " len=" len "\n")
230 (for-each (lambda (bitno)
231 (let ((lsb-bitno (if lsb0? bitno (- len bitno 1))))
232 ; ignore this bit if it's not set in the mask
233 (if (bit-set? mask lsb-bitno)
234 (let ((chosen-pop-vector (if (bit-set? value lsb-bitno)
235 1-population 0-population)))
236 (vector-set! chosen-pop-vector bitno
237 (+ 1 (vector-ref chosen-pop-vector bitno)))))))
239 masks mask-lens values)
240 ; Compute an aggregate "distinguishing value" for each bit.
243 (logit 4 p0 "/" p1 " ")
244 ; The most useful bits for decoding are those with counts in both
245 ; p0 and p1. These are the bits which distinguish one insn from
246 ; another. Assign these bits a high value (greater than num-insns).
248 ; The next most useful bits are those with counts in either p0
249 ; or p1. These bits represent specializations of other insns.
250 ; Assign these bits a value between 0 and (num-insns - 1). Note that
251 ; p0 + p1 is guaranteed to be <= num-insns. The value 0 is assigned
252 ; to bits for which p0 or p1 is equal to num_insns. These are bits
253 ; which are always 1 or always 0 in the ISA and are useless for
256 ; Bits with no count in either p0 or p1 are useless for decoding
257 ; and should never be considered. Assigning these bits a value of
261 ((= (* p0 p1) 0) (- num-insns (+ p0 p1)))
262 (else (+ num-insns (sqrt (* p0 p1))))))
263 (vector->list 0-population) (vector->list 1-population))))
267 ; Return a list (0 ... limit-1)
268 (define (-range limit)
271 (if (= i limit) (reverse indices) (loop (+ i 1) (cons i indices))))
274 ; Return a list (base ... base+size-1)
275 (define (-range2 base size)
278 (if (= i (+ base size)) (reverse indices) (loop (+ i 1) (cons i indices))))
282 ; Return a copy of given vector, with all entries with given indices set to `value'
283 (define (-vector-copy-set-all vector indices value)
284 (let ((new-vector (make-vector (vector-length vector))))
285 (for-each (lambda (index)
286 (vector-set! new-vector index (if (memq index indices)
288 (vector-ref vector index))))
289 (-range (vector-length vector)))
294 ; Return a list of indices whose counts in the given vector exceed the given threshold.
295 ; Sort them in decreasing order of populatority.
296 (define (-population-above-threshold population threshold)
298 (find (lambda (index) (if (vector-ref population index)
299 (>= (vector-ref population index) threshold)
301 (-range (vector-length population))))
303 (sort unsorted (lambda (i1 i2) (> (vector-ref population i1)
304 (vector-ref population i2))))))
309 ; Return the top few most popular indices in the population vector,
310 ; ignoring any that are already used (marked by #f). Don't exceed
311 ; `size' unless the clustering is just too good to pass up.
312 (define (-population-top-few population size)
313 (let loop ((old-picks (list))
314 (remaining-population population)
315 (count-threshold (apply max (map (lambda (value) (if value value 0))
316 (vector->list population)))))
317 (let* ((new-picks (-population-above-threshold remaining-population count-threshold)))
318 (logit 4 "-population-top-few"
320 " picks=(" old-picks ") pop=(" remaining-population ")"
321 " threshold=" count-threshold " new-picks=(" new-picks ")\n")
323 ; No point picking bits with population count of zero. This leads to
324 ; the generation of layers of subtables which resolve nothing. Generating
325 ; these tables can slow the build by several orders of magnitude.
326 ((= 0 count-threshold)
327 (logit 2 "-population-top-few: count-threshold is zero!\n")
331 (if (null? old-picks)
332 (logit 2 "-population-top-few: No bits left to pick from!\n"))
334 ; Way too many matches?
335 ((> (+ (length new-picks) (length old-picks)) (+ size 3))
336 (list-take (+ 3 size) (append old-picks new-picks))) ; prefer old-picks
337 ; About right number of matches?
338 ((> (+ (length new-picks) (length old-picks)) (- size 1))
339 (append old-picks new-picks))
340 ; Not enough? Lower the threshold a bit and try to add some more.
342 (loop (append old-picks new-picks)
343 (-vector-copy-set-all remaining-population new-picks #f)
344 ; Notice magic clustering decay parameter
346 (* 0.75 count-threshold))))))
351 ; Given list of insns, return list of bit numbers of constant bits in opcode
352 ; that they all share (or mostly share), up to MAX elements.
353 ; ALREADY-USED is a list of bitnums we can't use.
354 ; STARTBIT is the bit offset of the instruction value that C variable `insn'
355 ; holds (note that this is independent of LSB0?).
356 ; DECODE-BITSIZE is the number of bits of the insn that `insn' holds.
357 ; LSB0? is non-#f if bit number 0 is the least significant bit.
359 ; Nil is returned if there are none, meaning that there is an ambiguity in
360 ; the specification up to the current word.
362 ; We assume INSN-LIST matches all opcode bits before STARTBIT.
363 ; FIXME: Revisit, as a more optimal decoder is sometimes achieved by doing
364 ; a cluster of opcode bits that appear later in the insn, and then coming
365 ; back to earlier ones.
367 ; All insns are assumed to start at the same address so we handle insns of
368 ; varying lengths - we only analyze the common bits in all of them.
370 ; Note that if we get called again to compute further opcode bits, we
371 ; start looking at STARTBIT again (rather than keeping track of how far in
372 ; the insn word we've progressed). We could do this as an optimization, but
373 ; we also have to handle the case where the initial set of decode bits misses
374 ; some and thus we have to go back and look at them. It may also turn out
375 ; that an opcode bit is skipped over because it doesn't contribute much
376 ; information to the decoding process (see -usable-decode-bit?). As the
377 ; possible insn list gets wittled down, the bit will become significant. Thus
378 ; the optimization is left for later. Also, see preceding FIXME.
380 (define (decode-get-best-bits insn-list already-used startbit max decode-bitsize lsb0?)
381 (let* ((raw-population (-distinguishing-bit-population (map insn-base-mask insn-list)
382 (map insn-base-mask-length insn-list)
383 (map insn-value insn-list)
385 ; (undecoded (if lsb0?
386 ; (-range2 startbit (+ startbit decode-bitsize))
387 ; (-range2 (- startbit decode-bitsize) startbit)))
388 (used+undecoded already-used) ; (append already-used undecoded))
389 (filtered-population (-vector-copy-set-all raw-population used+undecoded #f))
390 (favorite-indices (-population-top-few filtered-population max))
391 (sorted-indices (sort favorite-indices (lambda (a b)
392 (if lsb0? (> a b) (< a b))))))
394 "Best decode bits (prev=" already-used " start=" startbit " decode=" decode-bitsize ")"
396 "(" sorted-indices ")\n")
401 (define (OLDdecode-get-best-bits insn-list already-used startbit max decode-bitsize lsb0?)
402 (let ((masks (map insn-base-mask insn-list))
403 ; ??? We assume mask lengths are repeatedly used for insns longer
404 ; than the base insn size.
405 (mask-lens (map insn-base-mask-length insn-list))
407 -1 ; FIXME: for now (gets sparc port going)
408 (+ startbit decode-bitsize)))
409 (incr (if lsb0? -1 1)))
410 (let loop ((result nil)
412 (+ startbit (- decode-bitsize 1))
414 (if (or (= (length result) max) (= bitnum endbit))
416 (if (and (not (memq bitnum already-used))
417 (-usable-decode-bit? masks mask-lens bitnum lsb0?))
418 (loop (cons bitnum result) (+ bitnum incr))
419 (loop result (+ bitnum incr))))
423 ; Return list of decode table entry numbers for INSN's opcode bits BITNUMS.
424 ; This is the indices into the decode table that match the instruction.
425 ; LSB0? is non-#f if bit number 0 is the least significant bit.
427 ; Example: If BITNUMS is (0 1 2 3 4 5), and the constant (i.e. opcode) part of
428 ; the those bits of INSN is #b1100xx (where 'x' indicates a non-constant
429 ; part), then the result is (#b110000 #b110001 #b110010 #b110011).
431 (define (-opcode-slots insn bitnums lsb0?)
432 (letrec ((opcode (insn-value insn))
433 (insn-len (insn-base-mask-length insn))
434 (decode-len (length bitnums))
435 (compute (lambda (val insn-len decode-len bl)
436 ;(display (list val insn-len decode-len bl)) (newline)
437 ; Oh My God. This isn't tail recursive.
443 (- insn-len (car bl) 1)))
444 (integer-expt 2 (- (length bl) 1))
446 (compute val insn-len decode-len (cdr bl)))))))
447 (let* ((opcode (compute (insn-value insn) insn-len decode-len bitnums))
448 (opcode-mask (compute (insn-base-mask insn) insn-len decode-len bitnums))
449 (indices (missing-bit-indices opcode-mask (- (integer-expt 2 decode-len) 1))))
450 (logit 3 "insn =" (obj:name insn) " opcode=" opcode " indices=" indices "\n")
451 (map (lambda (index) (+ opcode index)) indices)))
454 ; Subroutine of -build-slots.
455 ; Fill slot in INSN-VEC that INSN goes into.
456 ; BITNUMS is the list of opcode bits.
457 ; LSB0? is non-#f if bit number 0 is the least significant bit.
459 ; Example: If BITNUMS is (0 1 2 3 4 5) and the constant (i.e. opcode) part of
460 ; the first six bits of INSN is #b1100xx (where 'x' indicates a non-constant
461 ; part), then elements 48 49 50 51 of INSN-VEC are cons'd with INSN.
462 ; Each "slot" is a list of matching instructions.
464 (define (-fill-slot! insn-vec insn bitnums lsb0?)
465 ;(display (string-append "fill-slot!: " (obj:name insn) " ")) (display bitnums) (newline)
466 (let ((slot-nums (-opcode-slots insn bitnums lsb0?)))
467 ;(display (list "Filling slot(s)" slot-nums "...")) (newline)
468 (for-each (lambda (slot-num)
469 (vector-set! insn-vec slot-num
470 (cons insn (vector-ref insn-vec slot-num))))
476 ; Given a list of constant bitnums (ones that are predominantly, though perhaps
477 ; not always, in the opcode), record each insn in INSN-LIST in the proper slot.
478 ; LSB0? is non-#f if bit number 0 is the least significant bit.
479 ; The result is a vector of insn lists. Each slot is a list of insns
480 ; that go in that slot.
482 (define (-build-slots insn-list bitnums lsb0?)
483 (let ((result (make-vector (integer-expt 2 (length bitnums)) nil)))
484 ; Loop over each element, filling RESULT.
485 (for-each (lambda (insn)
486 (-fill-slot! result insn bitnums lsb0?))
491 ; Compute the name of a decode table, prefixed with PREFIX.
492 ; INDEX-LIST is a list of pairs: list of bitnums, table entry number,
493 ; in reverse order of traversal (since they're built with cons).
494 ; INDEX-LIST may be empty.
496 (define (-gen-decode-table-name prefix index-list)
497 (set! index-list (reverse index-list))
501 (string-map (lambda (elm) (string-append "_" (number->string elm)))
502 ; CDR of each element is the table index.
503 (map cdr index-list)))
506 ; Generate one decode table entry for INSN-VEC at INDEX.
507 ; INSN-VEC is a vector of slots where each slot is a list of instructions that
508 ; map to that slot (opcode value). If a slot is nil, no insn has that opcode
509 ; value so the decoder marks it as being invalid.
510 ; STARTBIT is the bit offset of the instruction value that C variable `insn'
511 ; holds (note that this is independent of LSB0?).
512 ; DECODE-BITSIZE is the number of bits of the insn that `insn' holds.
513 ; INDEX-LIST is a list of pairs: list of bitnums, table entry number.
514 ; LSB0? is non-#f if bit number 0 is the least significant bit.
515 ; INVALID-INSN is an <insn> object to use for invalid insns.
516 ; The result is a dtable-entry element (or "slot").
519 (define -build-decode-table-entry-args #f)
521 (define (-build-decode-table-entry insn-vec startbit decode-bitsize index index-list lsb0? invalid-insn)
522 (let ((slot (vector-ref insn-vec index)))
523 (logit 2 "Processing decode entry "
524 (number->string index)
526 (-gen-decode-table-name "decode_" index-list)
528 (cond ((null? slot) "invalid")
529 ((= 1 (length slot)) (insn-syntax (car slot)))
534 ; If no insns map to this value, mark it as invalid.
535 ((null? slot) (dtable-entry-make index 'insn invalid-insn))
537 ; If only one insn maps to this value, that's it for this insn.
539 ; FIXME: Incomplete: need to check further opcode bits.
540 (dtable-entry-make index 'insn (car slot)))
542 ; Otherwise more than one insn maps to this value and we need to look at
543 ; further opcode bits.
545 (logit 3 "Building subtable at index " (number->string index)
546 ", decode-bitsize = " (number->string decode-bitsize)
547 ", indices used thus far:"
548 (string-map (lambda (i) (string-append " " (number->string i)))
549 (apply append (map car index-list)))
552 (let ((bitnums (decode-get-best-bits slot
553 (apply append (map car index-list))
555 decode-bitsize lsb0?)))
557 ; If bitnums is nil, either there is an ambiguity or we need to read
558 ; more of the instruction in order to distinguish insns in SLOT.
559 (if (and (null? bitnums)
560 (< startbit (apply min (map insn-length slot))))
562 ; We might be able to resolve the ambiguity by reading more bits.
563 ; We know from the < test that there are, indeed, more bits to
565 (set! startbit (+ startbit decode-bitsize))
566 ; FIXME: The calculation of the new decode-bitsize will
567 ; undoubtedly need refinement.
570 (- (apply min (map insn-length slot))
572 (set! bitnums (decode-get-best-bits slot
573 ;nil ; FIXME: what to put here?
574 (apply append (map car index-list))
576 decode-bitsize lsb0?))))
578 ; If bitnums is still nil there is an ambiguity.
581 ; Try filtering out insns which are more general cases of
582 ; other insns in the slot. The filtered insns will appear
583 ; in other slots as appropriate.
584 (set! slot (filter-non-specialized-ambiguous-insns slot))
586 (if (= 1 (length slot))
587 ; Only 1 insn left in the slot, so take it.
588 (dtable-entry-make index 'insn (car slot))
589 ; There is still more than one insn in 'slot', so there is still an ambiguity.
591 ; If all insns are marked as DECODE-SPLIT, don't warn.
592 (if (not (all-true? (map (lambda (insn)
593 (obj-has-attr? insn 'DECODE-SPLIT))
595 (message "WARNING: Decoder ambiguity detected: "
596 (string-drop1 ; drop leading comma
597 (string-map (lambda (insn)
598 (string-append ", " (obj:name insn)))
601 ; Things aren't entirely hopeless. We've warned about the ambiguity.
602 ; Now, if there are any identical insns, filter them out. If only one
603 ; remains, then use it.
604 (set! slot (filter-identical-ambiguous-insns slot))
605 (if (= 1 (length slot))
606 ; Only 1 insn left in the slot, so take it.
607 (dtable-entry-make index 'insn (car slot))
608 ; Otherwise, see if any ifield-assertion
610 ; FIXME: For now we assume that if they all have an
611 ; ifield-assertion spec, then there is no ambiguity (it's left
612 ; to the programmer to get it right). This can be made more
614 ; FIXME: May need to back up startbit if we've tried to read
615 ; more of the instruction.
616 (let ((assertions (map insn-ifield-assertion slot)))
617 (if (not (all-true? assertions))
619 ; Save arguments for debugging purposes.
620 (set! -build-decode-table-entry-args
621 (list insn-vec startbit decode-bitsize index index-list lsb0? invalid-insn))
622 (error "Unable to resolve ambiguity (maybe need some ifield-assertion specs?)")))
623 ; FIXME: Punt on even simple cleverness for now.
624 (let ((exprtable-entries
625 (exprtable-sort (map exprtable-entry-make
628 (dtable-entry-make index 'expr
630 (-gen-exprtable-name exprtable-entries)
631 exprtable-entries))))))))
633 ; There is no ambiguity so generate the subtable.
634 ; Need to build `subtable' separately because we
635 ; may be appending to -decode-subtables recursively.
636 (let* ((insn-vec (-build-slots slot bitnums lsb0?))
638 (-build-decode-table-guts insn-vec bitnums startbit
639 decode-bitsize index-list lsb0?
641 (dtable-entry-make index 'table
642 (subdtable-add subtable
643 (-gen-decode-table-name "" index-list)))))))
648 ; Given vector of insn slots, generate the guts of the decode table, recorded
649 ; as a list of 3 elements: bitnums, decode-bitsize, and list of entries.
650 ; Bitnums is recorded with the guts so that tables whose contents are
651 ; identical but are accessed by different bitnums are treated as separate in
652 ; -decode-subtables. Not sure this will ever happen, but play it safe.
654 ; BITNUMS is the list of bit numbers used to build the slot table.
655 ; STARTBIT is the bit offset of the instruction value that C variable `insn'
656 ; holds (note that this is independent of LSB0?).
657 ; For example, it is initially zero. If DECODE-BITSIZE is 16 and after
658 ; scanning the first fetched piece of the instruction, more decoding is
659 ; needed, another piece will be fetched and STARTBIT will then be 16.
660 ; DECODE-BITSIZE is the number of bits of the insn that `insn' holds.
661 ; INDEX-LIST is a list of pairs: list of bitnums, table entry number.
662 ; Decode tables consist of entries of two types: actual insns and
663 ; pointers to other tables.
664 ; LSB0? is non-#f if bit number 0 is the least significant bit.
665 ; INVALID-INSN is an <insn> object representing invalid insns.
667 (define (-build-decode-table-guts insn-vec bitnums startbit decode-bitsize index-list lsb0? invalid-insn)
668 (logit 2 "Processing decoder for bits"
669 (numbers->string bitnums " ")
673 bitnums startbit decode-bitsize
675 (-build-decode-table-entry insn-vec startbit decode-bitsize index
676 (cons (cons bitnums index)
679 (iota (vector-length insn-vec))))
683 ; Return a table that efficiently decodes INSN-LIST.
684 ; BITNUMS is the set of bits to initially key off of.
685 ; DECODE-BITSIZE is the number of bits of the instruction that `insn' holds.
686 ; LSB0? is non-#f if bit number 0 is the least significant bit.
687 ; INVALID-INSN is an <insn> object representing the `invalid' insn (for
688 ; instructions values that don't decode to any entry in INSN-LIST).
690 (define (decode-build-table insn-list bitnums decode-bitsize lsb0? invalid-insn)
691 ; Initialize the list of subtables computed.
692 (set! -decode-subtables nil)
694 ; ??? Another way to handle simple forms of ifield-assertions (like those
695 ; created by insn specialization) is to record a copy of the insn for each
696 ; possible value of the ifield and modify its ifield list with the ifield's
697 ; value. This would then let the decoder table builder handle it normally.
698 ; I wouldn't create N insns, but would rather create an intermediary record
699 ; that recorded the necessary bits (insn, ifield-list, remaining
700 ; ifield-assertions).
702 (let ((insn-vec (-build-slots insn-list bitnums lsb0?)))
703 (let ((table-guts (-build-decode-table-guts insn-vec bitnums