yosys/techlibs/microchip/microchip_dffopt.cc

/*
ISC License

Copyright (C) 2024 Microchip Technology Inc. and its subsidiaries

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.
*/

#include "kernel/sigtools.h"
#include "kernel/yosys.h"

USING_YOSYS_NAMESPACE
PRIVATE_NAMESPACE_BEGIN

typedef std::pair<Const, std::vector<SigBit>> LutData;

// Compute a LUT implementing (select ^ select_inv) ? alt_data : data.  Returns true if successful.
bool merge_lut(LutData &result, const LutData &data, const LutData select, bool select_inv, SigBit alt_data, int max_lut_size)
{
	// First, gather input signals -- insert new signals at the beginning
	// of the vector, so they don't disturb the likely-critical D LUT input
	// timings.
	result.second = data.second;
	// D lut inputs initially start at 0.
	int idx_data = 0;
	// Now add the control input LUT inputs.
	std::vector<int> idx_sel;
	for (auto bit : select.second) {
		int idx = -1;
		for (int i = 0; i < GetSize(result.second); i++)
			if (result.second[i] == bit)
				idx = i;
		if (idx == -1) {
			idx = 0;
			// Insert new signal at the beginning and bump all indices.
			result.second.insert(result.second.begin(), bit);
			idx_data++;
			for (int &sidx : idx_sel)
				sidx++;
		}
		idx_sel.push_back(idx);
	}
	// Insert the Q signal, if any, to the slowest input -- it will have
	// no problem meeting timing.
	// This is to emulate CLK_EN, where output data is retained
	int idx_alt = -1;
	if (alt_data.wire) {
		// Check if we already have it.
		for (int i = 0; i < GetSize(result.second); i++)
			if (result.second[i] == alt_data)
				idx_alt = i;
		// If not, add it.
		if (idx_alt == -1) {
			idx_alt = 0;
			result.second.insert(result.second.begin(), alt_data);
			idx_data++;
			for (int &sidx : idx_sel)
				sidx++;
		}
	}

	// If LUT would be too large, bail.
	if (GetSize(result.second) > max_lut_size)
		return false;

	// Okay, we're doing it — compute the LUT mask.
	result.first = Const(0, 1 << GetSize(result.second));
	for (int i = 0; i < GetSize(result.first); i++) {
		int sel_lut_idx = 0;
		for (int j = 0; j < GetSize(select.second); j++)
			if (i & 1 << idx_sel[j])
				sel_lut_idx |= 1 << j;
		bool select_val = (select.first.bits[sel_lut_idx] == State::S1);
		bool new_bit;
		if (select_val ^ select_inv) {
			// Use alt_data.
			if (alt_data.wire)
				new_bit = (i & 1 << idx_alt) != 0;
			else
				new_bit = alt_data.data == State::S1;
		} else {
			// Use original LUT.
			int lut_idx = i >> idx_data & ((1 << GetSize(data.second)) - 1);
			new_bit = data.first.bits[lut_idx] == State::S1;
		}
		result.first.bits[i] = new_bit ? State::S1 : State::S0;
	}
	return true;
}

struct MicrochipDffOptPass : public Pass {
	MicrochipDffOptPass() : Pass("microchip_dffopt", "MICROCHIP: optimize FF control signal usage") {}
	void help() override
	{
		//   |---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|---v---|
		log("\n");
		log("    microchip_dffopt [options] [selection]\n");
		log("\n");
		log("Converts hardware clock enable and set/reset signals on FFs to emulation\n");
		log("using LUTs, if doing so would improve area. Operates on post-techmap LUT, DFF\n");
		log("cells. \n");
		log("\n");
	}
	void execute(std::vector<std::string> args, RTLIL::Design *design) override
	{
		log_header(design, "Executing MICROCHIP_DFFOPT pass (optimize FF control signal usage).\n");

		size_t argidx = 1;
		int max_lut_size = 4;

		extra_args(args, argidx, design);

		for (auto module : design->selected_modules()) {
			log("Optimizing FFs in %s.\n", log_id(module));

			SigMap sigmap(module);
			dict<SigBit, pair<LutData, Cell *>> bit_to_lut;
			dict<SigBit, int> bit_uses;

			// Gather LUTs.
			for (auto cell : module->selected_cells()) {
				for (auto port : cell->connections())
					for (auto bit : port.second)
						bit_uses[sigmap(bit)]++;
				if (cell->get_bool_attribute(ID::keep))
					continue;
				if (cell->type == ID(INV)) {
					SigBit sigout = sigmap(cell->getPort(ID::Y));
					SigBit sigin = sigmap(cell->getPort(ID::A));
					bit_to_lut[sigout] = make_pair(LutData(Const(1, 2), {sigin}), cell); // INIT = 01
				} else if (cell->type.in(ID(CFG1), ID(CFG2), ID(CFG3), ID(CFG4))) {
					SigBit sigout = sigmap(cell->getPort(ID::Y));
					const Const &init = cell->getParam(ID::INIT);
					std::vector<SigBit> sigin;
					sigin.push_back(sigmap(cell->getPort(ID(A))));
					if (cell->type == ID(CFG1))
						goto lut_sigin_done;
					sigin.push_back(sigmap(cell->getPort(ID(B))));
					if (cell->type == ID(CFG2))
						goto lut_sigin_done;
					sigin.push_back(sigmap(cell->getPort(ID(C))));
					if (cell->type == ID(CFG3))
						goto lut_sigin_done;
					sigin.push_back(sigmap(cell->getPort(ID(D))));

				lut_sigin_done:
					bit_to_lut[sigout] = make_pair(LutData(init, sigin), cell);
				}
			}
			for (auto wire : module->wires())
				if (wire->port_output || wire->port_input)
					for (int i = 0; i < GetSize(wire); i++)
						bit_uses[sigmap(SigBit(wire, i))]++;

			// Iterate through FFs.
			for (auto cell : module->selected_cells()) {

				if (!cell->type.in(ID(SLE))) // not a SLE
					continue;
				if (cell->getPort(ID(LAT)).is_fully_ones()) // skip latch
					continue;
				if (cell->get_bool_attribute(ID::keep)) // keep attribute
					continue;
				if (!cell->getPort(ID(ALn)).is_fully_ones()) // async FF
					continue;

				const bool hasSyncLoad = cell->getPort(ID(SLn)).is_wire();
				const bool has_s = hasSyncLoad && cell->getPort(ID(SD)).is_fully_ones();
				const bool has_r = hasSyncLoad && cell->getPort(ID(SD)).is_fully_zero();

				// SLE cannot have both synchronous set and reset implemented at the same time
				log_assert(!(has_s && has_r));

				// Don't bother if D has more than one use.
				SigBit sig_D = sigmap(cell->getPort(ID::D));
				if (bit_uses[sig_D] > 2)
					continue;

				// Find the D LUT.
				auto it_D = bit_to_lut.find(sig_D);
				if (it_D == bit_to_lut.end())
					continue;
				LutData lut_d = it_D->second.first;
				Cell *cell_d = it_D->second.second;

				LutData lut_d_post_ce;
				LutData lut_d_post_s;
				LutData lut_d_post_r;
				bool worthy_post_ce = false;
				bool worthy_post_s = false;
				bool worthy_post_r = false;

				// First, unmap CE.
				SigBit sig_Q = sigmap(cell->getPort(ID::Q));
				SigBit sig_CE = sigmap(cell->getPort(ID(EN)));
				LutData lut_ce = LutData(Const(2, 2), {sig_CE}); // INIT = 10
				auto it_CE = bit_to_lut.find(sig_CE);
				if (it_CE != bit_to_lut.end())
					lut_ce = it_CE->second.first;
				if (sig_CE.wire) {
					// Merge CE LUT and D LUT into one.  If it cannot be done, nothing to do about this FF.
					if (!merge_lut(lut_d_post_ce, lut_d, lut_ce, true, sig_Q, max_lut_size))
						continue;

					// If this gets rid of a CE LUT, it's worth it.  If not, it still may be worth it, if we can remove set/reset
					// as well.
					if (it_CE != bit_to_lut.end())
						worthy_post_ce = true;
				} else if (sig_CE.data != State::S1) {
					// Strange.  Should not happen in a reasonable flow, so bail.
					log_assert(false); // This DFF is always off
					continue;
				} else {
					lut_d_post_ce = lut_d;
				}

				// Second, unmap S, if any.
				lut_d_post_s = lut_d_post_ce;
				if (has_s) {
					SigBit sig_S = sigmap(cell->getPort(ID(SLn)));
					LutData lut_s = LutData(Const(2, 2), {sig_S}); // INIT = 10
					bool inv_s = true;			       // active low
					auto it_S = bit_to_lut.find(sig_S);
					if (it_S != bit_to_lut.end())
						lut_s = it_S->second.first;
					if (sig_S.wire) {
						// Merge S LUT and D LUT into one.  If it cannot be done, try to at least merge CE.
						if (!merge_lut(lut_d_post_s, lut_d_post_ce, lut_s, inv_s, SigBit(State::S1), max_lut_size))
							goto unmap;
						// If this gets rid of an S LUT, it's worth it.
						if (it_S != bit_to_lut.end())
							worthy_post_s = true;
					} else if (sig_S.data != (inv_s ? State::S1 : State::S0)) {
						// Strange.  Should not happen in a reasonable flow, so bail.
						log_assert(false); // DFF is always in set mode
						continue;
					}
				}

				// Third, unmap R, if any.
				lut_d_post_r = lut_d_post_s;
				if (has_r) {
					SigBit sig_R = sigmap(cell->getPort(ID(SLn)));
					LutData lut_r = LutData(Const(2, 2), {sig_R}); // INIT = 10
					bool inv_r = true;			       // active low
					auto it_R = bit_to_lut.find(sig_R);
					if (it_R != bit_to_lut.end())
						lut_r = it_R->second.first;
					if (sig_R.wire) {
						// Merge R LUT and D LUT into one.  If it cannot be done, try to at least merge CE/S.
						if (!merge_lut(lut_d_post_r, lut_d_post_s, lut_r, inv_r, SigBit(State::S0), max_lut_size))
							goto unmap;
						// If this gets rid of an S LUT, it's worth it.
						if (it_R != bit_to_lut.end())
							worthy_post_r = true;
					} else if (sig_R.data != (inv_r ? State::S1 : State::S0)) {
						// Strange.  Should not happen in a reasonable flow, so bail.
						log_assert(false); // DFF is always in reset mode
						continue;
					}
				}

			unmap:

				// SLE cannot have both synchronous set and reset implemented at the same time
				log_assert(!(worthy_post_r && worthy_post_s));

				LutData final_lut;
				if (worthy_post_r) {
					final_lut = lut_d_post_r;
				} else if (worthy_post_s) {
					final_lut = lut_d_post_s;
				} else if (worthy_post_ce) {
					final_lut = lut_d_post_ce;
				} else {
					// Nothing to do here.
					continue;
				}

				std::string ports;
				if (worthy_post_r)
					ports += " + R";
				if (worthy_post_s)
					ports += " + S";
				if (worthy_post_ce)
					ports += " + CE";
				log("  Merging D%s LUTs for %s/%s (%d -> %d)\n", ports.c_str(), log_id(cell), log_id(sig_Q.wire),
				    GetSize(lut_d.second), GetSize(final_lut.second));

				// Okay, we're doing it.  Unmap ports.
				if ((has_s && worthy_post_s) || worthy_post_r) {
					cell->setPort(ID(SLn), Const(1, 1));
				}

				// if we made it this far, clk enable is always merged into D
				cell->setPort(ID(EN), Const(1, 1));

				// Create the new LUT.
				Cell *lut_cell = nullptr;
				switch (GetSize(final_lut.second)) {
				case 1:
					lut_cell = module->addCell(NEW_ID, ID(CFG1));
					break;
				case 2:
					lut_cell = module->addCell(NEW_ID, ID(CFG2));
					break;
				case 3:
					lut_cell = module->addCell(NEW_ID, ID(CFG3));
					break;
				case 4:
					lut_cell = module->addCell(NEW_ID, ID(CFG4));
					break;
				default:
					log_assert(!"unknown lut size");
				}
				lut_cell->attributes = cell_d->attributes;
				Wire *lut_out = module->addWire(NEW_ID);
				lut_cell->setParam(ID::INIT, final_lut.first);
				cell->setPort(ID::D, lut_out);
				lut_cell->setPort(ID::Y, lut_out);
				lut_cell->setPort(ID(A), final_lut.second[0]);
				if (GetSize(final_lut.second) >= 2)
					lut_cell->setPort(ID(B), final_lut.second[1]);
				if (GetSize(final_lut.second) >= 3)
					lut_cell->setPort(ID(C), final_lut.second[2]);
				if (GetSize(final_lut.second) >= 4)
					lut_cell->setPort(ID(D), final_lut.second[3]);
			}
		}
	}
} MicrochipDffOptPass;

PRIVATE_NAMESPACE_END