Refactor common parts of SAT-using optimizations into a helper.

This also aligns the functionality:

- in all cases, the onehot attribute is used to create appropriate
  constraints (previously, opt_dff didn't do it at all, and share
  created one-hot constraints based on $pmux presence alone, which
  is unsound)
- in all cases, shift and mul/div/pow cells are now skipped when
  importing the SAT problem (previously only memory_share did this)
  — this avoids creating clauses for hard cells that are unlikely
  to help with proving the UNSATness needed for optimization

1 files changed, 76 insertions, 0 deletions

diff --git a/kernel/qcsat.h b/kernel/qcsat.h
new file mode 100644
index 000000000..e4d3c3c5d
--- /dev/null
+++ b/kernel/qcsat.h
@@ -0,0 +1,76 @@
+/* -*- c++ -*-
+ *  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 QCSAT_H
+#define QCSAT_H
+
+#include "kernel/satgen.h"
+#include "kernel/modtools.h"
+
+YOSYS_NAMESPACE_BEGIN
+
+// This is a helper class meant for easy construction of quick SAT queries
+// to a combinatorial input cone of some set of signals, meant for SAT-based
+// optimizations.  Various knobs are provided to set just how much of the
+// cone should be included in the model — since this class is meant for
+// optimization, it should not be a correctness problem when some cells are
+// skipped and the solver spuriously returns SAT with a solution that
+// cannot exist in reality due to skipped constraints (ie. only UNSAT results
+// from this class should be considered binding).
+struct QuickConeSat {
+	ModWalker &modwalker;
+	ezSatPtr ez;
+	SatGen satgen;
+
+	// The effort level knobs.
+
+	// The maximum "complexity level" of cells that will be imported.
+	// - 1: bitwise operations, muxes, equality comparisons, lut, sop, fa
+	// - 2: addition, subtraction, greater/less than comparisons, lcu
+	// - 3: shifts
+	// - 4: multiplication, division, power
+	int max_cell_complexity = 2;
+	// The maximum number of cells to import, or 0 for no limit.
+	int max_cell_count = 0;
+	// If non-0, skip importing cells with more than this number of output bits.
+	int max_cell_outs = 0;
+
+	// Internal state.
+	pool<RTLIL::Cell*> imported_cells;
+	pool<RTLIL::Wire*> imported_onehot;
+	pool<RTLIL::SigBit> bits_queue;
+
+	QuickConeSat(ModWalker &modwalker) : modwalker(modwalker), ez(), satgen(ez.get(), &modwalker.sigmap) {}
+
+	// Imports a signal into the SAT solver, queues its input cone to be
+	// imported in the next prepare() call.
+	std::vector<int> importSig(SigSpec sig);
+	int importSigBit(SigBit bit);
+
+	// Imports the input cones of all previously importSig'd signals into
+	// the SAT solver.
+	void prepare();
+
+	// Returns the "complexity level" of a given cell.
+	static int cell_complexity(RTLIL::Cell *cell);
+};
+
+YOSYS_NAMESPACE_END
+
+#endif