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; Application independent decoder support.
; Copyright (C) 2000, 2004, 2009 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
; Accessors.
; 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.
; Accessors.
; List of 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.
" "" "" "" "" ""bad dtable entry type:"; An instruction and predicate for final matching.
; Accessors.
; Return a pseudo-cost of processing exprentry X.
; Sort an exprtable, optimum choices first.
; Basically an optimum choice is a cheaper choice.
; Return the name of the expr table for INSN-EXPRS,
; which is a list of exprtable-entry elements.
"-"; 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.
; Accessors.
; 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
; Accessors.
; 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.
; If half or more insns use the bit, it's a good one.
; FIXME: An empirical guess at best.
; 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.
;
; NOTE: mask-lens are not necessarily all the same value.
; E.g. for the m32r it can consist of both 16 and 32.
; But all masks must exist in the window specified by STARTBIT,DECODE-BITSIZE,
; and all bits in the result must live in that window.
; If no distinguishing bit fits in the window, return an empty vector.
; Compute the 1- and 0-population vectors
" population count mask="" len=""\n"; ignore this bit if it's not set in the mask
; Compute an aggregate "distinguishing value" for each bit.
"/"" "; 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.
; Return a list (0 ... LIMIT-1).
; Return a list (BASE ... BASE+SIZE-1).
; Return a copy of VECTOR, with all entries with given INDICES set
; to VALUE.
; Return a list of indices whose counts in the given vector exceed the given
; threshold.
; Sort them in decreasing order of popularity.
; 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.
"-population-top-few"" desired="" picks=("") pop=("")"" threshold="" new-picks=("")\n"; 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.
"-population-top-few: count-threshold is zero!\n"; No new matches?
"-population-top-few: No bits left to pick from!\n"; Way too many matches?
; prefer old-picks
; About right number of matches?
; Not enough?  Lower the threshold a bit and try to add some more.
; Notice magic clustering decay parameter
;  vvvv
; 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 as defined by startbit,
; decode-bitsize, and more bytes need to be fetched.
;
; We assume INSN-LIST matches all opcode bits before STARTBIT (if any).
; 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: We can't proceed past startbit + decode-bitsize
; until we've processed all bits up to startbit + decode-bitsize.
;; (undecoded (if lsb0?
;; 		(-range2 startbit (+ startbit decode-bitsize))
;;		(-range2 (- startbit decode-bitsize) startbit)))
; (append already-used undecoded))
"Best decode bits (prev="" start="" decode="")""=>""("")\n"; ??? We assume mask lengths are repeatedly used for insns longer
; than the base insn size.
; FIXME: for now (gets sparc port going)
; 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).
;(display (list val insn-len decode-len bl)) (newline)
; Oh My God.  This isn't tail recursive.
"insn ="" insn-value="" insn-base-mask="" insn-len="" decode-len="" opcode="" opcode-mask="" indices=""\n"; 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.
;(display (string-append "fill-slot!: " (obj:str-name insn) " ")) (display bitnums) (newline)
;(display (list "Filling slot(s)" slot-nums "...")) (newline)
; 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.
; Loop over each element, filling 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.
"table""_"; CDR of each element is the table index.
; 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.
"Processing decode entry "" in ""decode_"", ""invalid""subtable"" ...\n"; If no insns map to this value, mark it as invalid.
; If only one insn maps to this value, that's it for this insn.
; FIXME: Incomplete: need to check further opcode bits.
; Otherwise more than one insn maps to this value and we need to look at
; further opcode bits.
"Building subtable at index "", decode-bitsize = "", indices used thus far:"" ""\n"; 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.
; 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.
; FIXME: It's technically possible that the next
; startbit+decode-bitsize chunk has no usable bits and we have to
; iterate, but rather unlikely.
; The calculation of the new startbit, decode-bitsize will
; undoubtedly need refinement.
;nil ; FIXME: what to put here?
; If bitnums is still nil there is an ambiguity.
; 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.
; Only 1 insn left in the slot, so take it.
; There is still more than one insn in 'slot',
; so there is still an ambiguity.
; If all insns are marked as DECODE-SPLIT, don't warn.
"WARNING: Decoder ambiguity detected: "; drop leading comma
", ""\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.
; Only 1 insn left in the slot, so take it.
; 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.  We currently require that
; all bits get used before advancing startbit, so this
; shouldn't be necessary.  Verify.
; Save arguments for debugging purposes.
"Unable to resolve ambiguity (maybe need some ifield-assertion specs?)"; FIXME: Punt on even simple cleverness for now.
; There is no ambiguity so generate the subtable.
; Need to build `subtable' separately because we
; may be appending to -decode-subtables recursively.
""; 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.
"Processing decoder for bits"" "", startbit "", decode-bitsize "", index-list "" ...\n"; 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).
; Initialize the list of subtables computed.
; ??? 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).