[FIFO] use impl_style=hls for first&last FIFOs to avoid glitch

src/finn/transformation/fpgadataflow/insert_fifo.py

@@ -209,13 +209,9 @@ class InsertFIFO(Transformation):
                         graph.value_info.append(fifo_output_tensor)
                         model.set_tensor_datatype(fifo_output_tensor.name, dtype)
 
-                        if (
-                            self.max_qsrl_depth is None
-                            or fifo_depth <= self.max_qsrl_depth
-                        ):
-                            impl_style = "rtl"
-                        else:
-                            impl_style = "vivado"
+                        # only use rtl-style FIFOs to avoid simulation bug
+                        # (top-level IOs should not have impl_style=vivado)
+                        impl_style = "rtl"
 
                         fifo_node = oh.make_node(
                             "StreamingFIFO",
@@ -271,13 +267,9 @@ class InsertFIFO(Transformation):
                         graph.value_info.append(fifo_input_tensor)
                         model.set_tensor_datatype(fifo_input_tensor.name, dtype)
 
-                        if (
-                            self.max_qsrl_depth is None
-                            or fifo_depth <= self.max_qsrl_depth
-                        ):
-                            impl_style = "rtl"
-                        else:
-                            impl_style = "vivado"
+                        # only use rtl-style FIFOs to avoid simulation bug
+                        # (top-level IOs should not have impl_style=vivado)
+                        impl_style = "rtl"
 
                         fifo_node = oh.make_node(
                             "StreamingFIFO",

src/finn/transformation/fpgadataflow/set_fifo_depths.py

@@ -412,8 +412,13 @@ class InsertAndSetFIFODepths(Transformation):
                 node_inst = getCustomOp(node)
                 node_inst.set_nodeattr("depth", depth)
                 node_inst.set_nodeattr("depth_monitor", 0)
+                # exception for top-level IO FIFOs which cause a bug in simulation
+                # (top-level IOs should not have impl_style=vivado)
+                toplevel_in = node.input[0] in [x.name for x in model.graph.input]
+                toplevel_out = node.output[0] in [x.name for x in model.graph.output]
+                toplevel_style_exception = toplevel_in or toplevel_out
                 # Set FIFO implementation/ram styles
-                if depth > self.max_qsrl_depth:
+                if (depth > self.max_qsrl_depth) and (not toplevel_style_exception):
                     node_inst.set_nodeattr("impl_style", "vivado")
                     node_inst.set_nodeattr("ram_style", self.vivado_ram_style)
                 else: