fhdl/specials: add Keep SynthesisDirective

migen/fhdl/specials.py

@@ -337,3 +337,8 @@ class SynthesisDirective(Special):
         name_dict = dict((k, ns.get_name(sig)) for k, sig in directive.signals.items())
         formatted = directive.template.format(**name_dict)
         return "// synthesis " + formatted + "\n"
+
+
+class Keep(SynthesisDirective):
+    def __init__(self, signal):
+        SynthesisDirective.__init__(self, "attribute keep of {s} is true", s=signal)