yosys/passes/pmgen/microchip_dsp_CREG.pmg

// 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.


// This file describes the second of three pattern matcher setups that
//   forms the `microchip_dsp` pass described in microchip_dsp.cc
// At a high level, it works as follows:
//   (1) Starting from a DSP cell that (a) doesn't have a CREG already,
//       and (b) uses the 'C' port
//   (2) Match the driver of the 'C' input to a possible $dff cell (CREG)
//       (attached to at most two $mux cells that implement clock-enable or
//        reset functionality, using a subpattern discussed below)
// Notes:
//   - Running CREG packing after microchip_dsp_pack is necessary since there is no
//     guarantee that the cell ordering corresponds to the "expected" case (i.e.
//     the order in which they appear in the source) thus the possiblity existed
//     where a register got packed as a CREG into a downstream DSP, while it should
//     have otherwise been a PREG of an upstream DSP that had not been visited.
//     yet.
//   - The reason this is separated out from the microchip_dsp.pmg file is
//     for efficiency --- each *.pmg file creates a class of the same basename,
//     which when constructed, creates a custom database tailored to the
//     pattern(s) contained within. Since the pattern in this file must be
//     executed after the pattern contained in microchip_dsp.pmg, it is necessary
//     to reconstruct this database. Separating the two patterns into
//     independent files causes two smaller, more specific, databases.

pattern microchip_dsp_packC

udata <std::function<SigSpec(const SigSpec&)>> unextend
state <SigBit> clock
state <SigSpec> sigC sigP
state <Cell*> ffC

// Variables used for subpatterns
state <SigSpec> argQ argD
state <int> ffoffset
udata <SigSpec> dffD dffQ
udata <SigBit> dffclock
udata <Cell*> dff

// (1) Starting from a DSP cell that (a) doesn't have a CREG already,
//     and (b) uses the 'C' port
match dsp
	select dsp->type.in(\MACC_PA)
	select port(dsp, \C_BYPASS, SigSpec()).is_fully_ones()
	select nusers(port(dsp, \C, SigSpec())) > 1
endmatch

code sigC sigP clock
	//helper function to remove unused bits
	unextend = [](const SigSpec &sig) {
		int i;
		for (i = GetSize(sig)-1; i > 0; i--)
			if (sig[i] != sig[i-1])
				break;
		// Do not remove non-const sign bit
		if (sig[i].wire)
			++i;
		return sig.extract(0, i);
	};
	sigC = unextend(port(dsp, \C, SigSpec()));

	SigSpec P = port(dsp, \P);

	// Only care about those bits that are used
	int i;
	for (i = GetSize(P)-1; i >= 0; i--)
		if (nusers(P[i]) > 1)
			break;
	i++;
	log_assert(nusers(P.extract_end(i)) <= 1);
	sigP = P.extract(0, i);

	clock = port(dsp, \CLK, SigBit());
endcode

// (2) Match the driver of the 'C' input to a possible $dff cell (CREG)
//     (attached to at most two $mux cells that implement clock-enable or
//      reset functionality, using the in_dffe subpattern)
code argQ ffC sigC clock
	argQ = sigC;
	subpattern(in_dffe);
	if (dff) {
		ffC = dff;
		clock = dffclock;
		sigC = dffD;
	}
endcode

code
	if (ffC)
		accept;
endcode

// #######################

// Subpattern for matching against input registers, based on knowledge of the
//   'Q' input.
subpattern in_dffe
arg argQ clock

code
	dff = nullptr;
	if (argQ.empty())
		reject;
	for (const auto &c : argQ.chunks()) {
		// Abandon matches when 'Q' is a constant
		if (!c.wire)
			reject;
		// Abandon matches when 'Q' has the keep attribute set
		if (c.wire->get_bool_attribute(\keep))
			reject;
		// Abandon matches when 'Q' has a non-zero init attribute set
		Const init = c.wire->attributes.at(\init, Const());
		if (!init.empty())
			for (auto b : init.extract(c.offset, c.width))
				if (b != State::Sx && b != State::S0)
					reject;
	}
endcode

match ff
	select ff->type.in($dff, $dffe, $sdff, $sdffe, $adff, $adffe)
	// does not support clock inversion
	select param(ff, \CLK_POLARITY).as_bool()

	slice offset GetSize(port(ff, \D))
	index <SigBit> port(ff, \Q)[offset] === argQ[0]

	// Check that the rest of argQ is present
	filter GetSize(port(ff, \Q)) >= offset + GetSize(argQ)
	filter port(ff, \Q).extract(offset, GetSize(argQ)) == argQ

	filter clock == SigBit() || port(ff, \CLK) == clock
endmatch

code argQ
	// Check that reset value, if present, is fully 0.
	bool noResetFlop = ff->type.in($dff, $dffe);
	bool srstZero = ff->type.in($sdff, $sdffe) && param(ff, \SRST_VALUE).is_fully_zero();
	bool arstZero = ff->type.in($adff, $adffe) && param(ff, \ARST_VALUE).is_fully_zero();
	bool resetLegal = noResetFlop || srstZero || arstZero;
	if (resetLegal)
	{
		SigSpec Q = port(ff, \Q);
		dff = ff;
		dffclock = port(ff, \CLK);
		dffD = argQ;
		SigSpec D = port(ff, \D);
		argQ = Q;
		dffD.replace(argQ, D);
	}
	
endcode