Merge pull request #1312 from YosysHQ/xaig_arrival

Allow arrival times of sequential outputs to be specified to abc9

README.md

@@ -355,19 +355,16 @@ Verilog Attributes and non-standard features
   blackbox or whitebox definition to a corresponding entry in a `abc9`
   box-file.
 
-- The port attribute ``abc_scc_break`` indicates a module input port that will
-  be treated as a primary output during `abc9` techmapping. Doing so eliminates
-  the possibility of a strongly-connected component (i.e. a combinatorial loop)
-  existing. Typically, this is specified for sequential inputs on otherwise
-  combinatorial boxes -- for example, applying ``abc_scc_break`` onto the `D`
-  port of a LUTRAM cell prevents `abc9` from interpreting any `Q` -> `D` paths
-  as a combinatorial loop.
-
 - The port attribute ``abc_carry`` marks the carry-in (if an input port) and
   carry-out (if output port) ports of a box. This information is necessary for
   `abc9` to preserve the integrity of carry-chains. Specifying this attribute
   onto a bus port will affect only its most significant bit.
 
+- The port attribute ``abc_arrival`` specifies an integer (for output ports
+  only) to be used as the arrival time of this sequential port. It can be used,
+  for example, to specify the clk-to-Q delay of a flip-flop for consideration
+  during techmapping.
+
 - In addition to the ``(* ... *)`` attribute syntax, Yosys supports
   the non-standard ``{* ... *}`` attribute syntax to set default attributes
   for everything that comes after the ``{* ... *}`` statement. (Reset