syn: reject combinational loops in synthesize

src/syn/BUILD

@@ -28,6 +28,7 @@ cc_library(
         "src/flow/export.h",
         "src/flow/import.cc",
         "src/flow/liveness.cc",
+        "src/flow/loop_check.cc",
         "src/flow/opt_gatefusion.cc",
         "src/flow/opt_gatefusion.h",
         "src/flow/sequential_mapper.cc",

src/syn/README.md

@@ -33,6 +33,8 @@ synthesize
 
 The integrated synthesis tool does not support mapping to latches, always flattens the design, and loses the names of instantiated macros.
 
+`synthesize` fails with an error if the design contains a combinational loop. This includes latches inferred from incomplete assignments in `always` blocks (e.g. a latch-based clock gate), since combinational optimization does not preserve the state such a loop holds. A loop is only reported where each step is combinational: it is broken by a flip-flop, and by a cell output that neither has a liberty `function` using the input nor a combinational timing arc from it (such as a RAM read port).
+
 ## Authors
 
 The original `syn` was developed and contributed by Martin Povišer of Precision Innovations Inc. Design of the intermediate representation used by syn internally derives from the [prjunnamed project](https://prjunnamed.org/): `Copyright (C) Project Unnamed contributors`; distributed under the terms of the BSD-0 license.

src/syn/include/syn/synthesis.h

@@ -110,6 +110,10 @@ class Synthesis : public sta::dbStaState
   // combinational driver) whose value is provably never A with a constant.
   void livenessOpt(bool replace_combinational = false);
 
+  // Fail if the graph contains a combinational loop (e.g. an inferred
+  // latch), which the flow cannot implement faithfully.
+  void checkCombinationalLoops();
+
   // Export the mapped netlist to ODB.
   void exportToOdb();
 
@@ -149,5 +153,6 @@ void abcRoundtrip(Graph& g,
 void livenessOpt(Graph& g,
                  utl::Logger* logger,
                  bool replace_combinational = false);
+void checkCombinationalLoops(Graph& g, utl::Logger* logger);
 
 }  // namespace syn

src/syn/src/flow/CMakeLists.txt

@@ -12,6 +12,7 @@ add_library(syn_flow
   export.cc
   import.cc
   liveness.cc
+  loop_check.cc
   constant_fold.cc
   opt_gatefusion.cc
   sequential_mapper.cc