fhdl: inline synthesis directive support

doc/fhdl.rst

@@ -199,6 +199,13 @@ Options to ``get_port`` are:
 
 Migen generates behavioural V*HDL code that should be compatible with all simulators and, if the number of ports is <= 2, most FPGA synthesizers. If a specific code is needed, the memory handler can be overriden using the appropriate parameter of the V*HDL conversion function.
 
+Inline synthesis directives
+===========================
+
+Inline synthesis directives (pseudo-comments such as ``// synthesis attribute keep of clock_signal_name is true``) are supported using the ``SynthesisDirective`` object. Its constructor takes as parameters a string containing the body of the directive, and optional keyword parameters that are used to replace signal names similarly to the Python string method ``format``. The above example could be represented as follows: ::
+
+  SynthesisDirective("attribute keep of {clksig} is true", clksig=clock_domain.clk)
+
 Fragments
 *********
 A "fragment" is a unit of logic, which is composed of:

examples/basic/psync.py

@@ -1,4 +1,5 @@
 from migen.fhdl.structure import *
+from migen.fhdl.specials import SynthesisDirective
 from migen.fhdl import verilog
 
 # convert pulse into level change
@@ -14,10 +15,16 @@ osync = [
 	slevel[2].eq(slevel[1])
 ]
 
+# disable shift register extraction
+disable_srl = {
+	SynthesisDirective("attribute shreg_extract of {signal} is no", signal=slevel[0]),
+	SynthesisDirective("attribute shreg_extract of {signal} is no", signal=slevel[1])
+}
+
 # regenerate pulse
 o = Signal()
 comb = [o.eq(slevel[1] ^ slevel[2])]
 
-f = Fragment(comb, {"i": isync, "o": osync})
-v = verilog.convert(f, ios={i, o})
+f = Fragment(comb, {"i": isync, "o": osync}, specials=disable_srl)
+v = verilog.convert(f, {i, o})
 print(v)

migen/fhdl/specials.py

@@ -309,3 +309,18 @@ class Memory(Special):
 			r += "end\n\n"
 		
 		return r
+
+class SynthesisDirective(Special):
+	def __init__(self, template, **signals):
+		Special.__init__(self)
+		self.template = template
+		self.signals = signals
+
+	def list_ios(self, ins, outs, inouts):
+		return set()
+
+	@staticmethod
+	def emit_verilog(directive, ns, clock_domains):
+		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"