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Diffstat (limited to 'kernel/ffmerge.h')
| -rw-r--r-- | kernel/ffmerge.h | 141 |
1 files changed, 141 insertions, 0 deletions
diff --git a/kernel/ffmerge.h b/kernel/ffmerge.h new file mode 100644 index 000000000..5428da324 --- /dev/null +++ b/kernel/ffmerge.h @@ -0,0 +1,141 @@ +/* + * yosys -- Yosys Open SYnthesis Suite + * + * Copyright (C) 2021 Marcelina Kościelnicka <mwk@0x04.net> + * + * Permission to use, copy, modify, and/or distribute this software for any + * purpose with or without fee is hereby granted, provided that the above + * copyright notice and this permission notice appear in all copies. + * + * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES + * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF + * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR + * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES + * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN + * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF + * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. + * + */ + +#ifndef FFMERGE_H +#define FFMERGE_H + +#include "kernel/ffinit.h" +#include "kernel/ff.h" + +YOSYS_NAMESPACE_BEGIN + +// A helper class for passes that want to merge FFs on the input or output +// of a cell into the cell itself. +// +// The procedure is: +// +// 1. Construct this class (at beginning of processing for a given module). +// 2. For every considered cell: +// +// a. Call find_output_ff for every considered output. +// b. Call find_input_ff for every considered input. +// c. Look at the FF description returned (if any) from each call, reject +// results that cannot be merged into given cell for any reason. +// If both inputs and outputs are being merged, take care of FF bits that +// are returned in both input and output results (a FF bit cannot be +// merged to both). Decide on the final set of FF bits to merge. +// d. Call remove_output_ff for every find_output_ff result that will be used +// for merging. This removes the actual FF bits from design and from index. +// e. Call mark_input_ff for every find_input_ff result that will be used +// for merging. This updates the index disallowing further usage of these +// FF bits for output FF merging, if they were eligible before. The actual +// FF bits are still left in the design and can be merged into other inputs. +// If the FF bits are not otherwise used, they will be removed by later +// opt passes. +// f. Merge the FFs into the cell. +// +// Note that, if both inputs and outputs are being considered for merging in +// a single pass, the result may be nondeterministic (depending on cell iteration +// order) because a given FF bit could be eligible for both input and output merge, +// perhaps in different cells. For this reason, it may be a good idea to separate +// input and output merging. + +struct FfMergeHelper +{ + const SigMap *sigmap; + RTLIL::Module *module; + FfInitVals *initvals; + + dict<SigBit, std::pair<Cell*, int>> dff_driver; + dict<SigBit, pool<std::pair<Cell*, int>>> dff_sink; + dict<SigBit, int> sigbit_users_count; + + // Returns true if all bits in sig are completely unused. + bool is_output_unused(RTLIL::SigSpec sig); + + // Finds the FF to merge into a given cell output. Takes sig, which + // is the current cell output — it will be the sig_d of the found FF. + // If found, returns true, and fills the two output arguments. + // + // For every bit of sig, this function finds a FF bit that has + // the same sig_d, and fills the output FfData according to the FF + // bits found. This function will only consider FF bits that are + // the only user of the given sig bits — if any bit in sig is used + // by anything other than a single FF, this function will return false. + // + // The returned FfData structure does not correspond to any actual FF + // cell in the design — it is the amalgamation of extracted FF bits, + // possibly coming from several FF cells. + // + // If some of the bits in sig have no users at all, this function + // will accept them as well (and fill returned FfData with dummy values + // for the given bit, effectively synthesizing an unused FF bit of the + // appropriate type). However, if all bits in sig are completely + // unused, this function will fail and return false (having no idea + // what kind of FF to produce) — use the above helper if that case + // is important to handle. + // + // Note that this function does not remove the FF bits returned from + // the design — this is so that the caller can decide whether to accept + // this FF for merging or not. If the result is accepted, + // remove_output_ff should be called on the second output argument. + bool find_output_ff(RTLIL::SigSpec sig, FfData &ff, pool<std::pair<Cell *, int>> &bits); + + // Like above, but returns a FF to merge into a given cell input. Takes + // sig_q, which is the current cell input — it will search for FFs with + // matching sig_q. + // + // As opposed to find_output_ff, this function doesn't care about usage + // counts, and may return FF bits that also have other fanout. This + // should not be a problem for input FF merging. + // + // As a special case, if some of the bits in sig_q are constant, this + // function will accept them as well, by synthesizing in-place + // a constant-input FF bit (with matching initial value and reset value). + // However, this will not work if the input is all-constant — if the caller + // cares about this case, it needs to check for it explicitely. + bool find_input_ff(RTLIL::SigSpec sig, FfData &ff, pool<std::pair<Cell *, int>> &bits); + + // To be called on find_output_ff result that will be merged. This + // marks the given FF bits as used up (and not to be considered for + // further merging as inputs), and reconnects their Q ports to a dummy + // wire (since the wire previously connected there will now be driven + // by the merged-to cell instead). + void remove_output_ff(const pool<std::pair<Cell *, int>> &bits); + + // To be called on find_input_ff result that will be merged. This + // marks the given FF bits as used, and disallows merging them as + // outputs. They can, however, still be merged as inputs again + // (perhaps for another cell). + void mark_input_ff(const pool<std::pair<Cell *, int>> &bits); + + void set(FfInitVals *initvals_, RTLIL::Module *module_); + + void clear(); + + FfMergeHelper(FfInitVals *initvals, RTLIL::Module *module) { + set(initvals, module); + } + + FfMergeHelper() {} +}; + +YOSYS_NAMESPACE_END + +#endif |