diff --git a/passes/pmgen/xilinx_dsp.pmg b/passes/pmgen/xilinx_dsp.pmg
index 0ba5290..5d3b9c2 100644
--- a/passes/pmgen/xilinx_dsp.pmg
+++ b/passes/pmgen/xilinx_dsp.pmg
@@ -347,9 +347,9 @@
 	index <SigBit> port(postAdd, AB)[0] === sigP[0]
 	filter GetSize(port(postAdd, AB)) >= GetSize(sigP)
 	filter port(postAdd, AB).extract(0, GetSize(sigP)) == sigP
-	// Check that remainder of AB is a sign-extension
-	define <bool> AB_SIGNED (param(postAdd, AB == \A ? \A_SIGNED : \B_SIGNED).as_bool())
-	filter port(postAdd, AB).extract_end(GetSize(sigP)) == SigSpec(AB_SIGNED ? sigP[GetSize(sigP)-1] : State::S0, GetSize(port(postAdd, AB))-GetSize(sigP))
+	// Check that remainder of AB is a sign- or zero-extension
+	filter port(postAdd, AB).extract_end(GetSize(sigP)) == SigSpec(sigP[GetSize(sigP)-1], GetSize(port(postAdd, AB))-GetSize(sigP)) || port(postAdd, AB).extract_end(GetSize(sigP)) == SigSpec(State::S0, GetSize(port(postAdd, AB))-GetSize(sigP))
+
 	set postAddAB AB
 	optional
 endmatch
diff --git a/tests/arch/xilinx/dsp_fastfir.ys b/tests/arch/xilinx/dsp_fastfir.ys
new file mode 100644
index 0000000..0067a82
--- /dev/null
+++ b/tests/arch/xilinx/dsp_fastfir.ys
@@ -0,0 +1,69 @@
+read_verilog <<EOT
+// Citation https://github.com/ZipCPU/dspfilters/blob/master/rtl/fastfir.v
+module fastfir_dynamictaps(i_clk, i_reset, i_tap_wr, i_tap, i_ce, i_sample, o_result);
+  wire [30:0] _00_;
+  wire [23:0] _01_;
+  wire [11:0] _02_;
+  wire [30:0] _03_;
+  wire [23:0] _04_;
+  wire [30:0] _05_;
+  wire [23:0] _06_;
+  wire [30:0] _07_;
+  wire [23:0] _08_;
+  wire [11:0] _09_;
+  wire [30:0] _10_;
+  wire [23:0] _11_;
+  wire [30:0] _12_;
+  wire [23:0] _13_;
+  wire [11:0] \fir.FILTER[0].tapk.delayed_sample ;
+  reg [30:0] \fir.FILTER[0].tapk.o_acc  = 31'h00000000;
+  wire [11:0] \fir.FILTER[0].tapk.o_sample ;
+  reg [23:0] \fir.FILTER[0].tapk.product ;
+  reg [11:0] \fir.FILTER[0].tapk.tap  = 12'h000;
+  wire [11:0] \fir.FILTER[1].tapk.delayed_sample ;
+  wire [30:0] \fir.FILTER[1].tapk.o_acc ;
+  wire [11:0] \fir.FILTER[1].tapk.o_sample ;
+  reg [23:0] \fir.FILTER[1].tapk.product ;
+  reg [11:0] \fir.FILTER[1].tapk.tap  = 12'h000;
+  input i_ce;
+  input i_clk;
+  input i_reset;
+  input [11:0] i_sample;
+  input [11:0] i_tap;
+  input i_tap_wr;
+  output [30:0] o_result;
+  reg [30:0] o_result;
+  assign _03_ = 31'h00000000 + { \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product [23], \fir.FILTER[0].tapk.product  };
+  assign _04_ = $signed(\fir.FILTER[0].tapk.tap ) * $signed(i_sample);
+  always @(posedge i_clk)
+      \fir.FILTER[0].tapk.tap  <= _02_;
+  always @(posedge i_clk)
+      \fir.FILTER[0].tapk.o_acc  <= _00_;
+  always @(posedge i_clk)
+      \fir.FILTER[0].tapk.product  <= _01_;
+  assign _02_ = i_tap_wr ? i_tap : \fir.FILTER[0].tapk.tap ;
+  assign _05_ = i_ce ? _03_ : \fir.FILTER[0].tapk.o_acc ;
+  assign _00_ = i_reset ? 31'h00000000 : _05_;
+  assign _06_ = i_ce ? _04_ : \fir.FILTER[0].tapk.product ;
+  assign _01_ = i_reset ? 24'h000000 : _06_;
+  assign _10_ = \fir.FILTER[0].tapk.o_acc  + { \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product [23], \fir.FILTER[1].tapk.product  };
+  assign _11_ = $signed(\fir.FILTER[1].tapk.tap ) * $signed(i_sample);
+  always @(posedge i_clk)
+      \fir.FILTER[1].tapk.tap  <= _09_;
+  always @(posedge i_clk)
+      o_result <= _07_;
+  always @(posedge i_clk)
+      \fir.FILTER[1].tapk.product  <= _08_;
+  assign _09_ = i_tap_wr ? \fir.FILTER[0].tapk.tap  : \fir.FILTER[1].tapk.tap ;
+  assign _12_ = i_ce ? _10_ : o_result;
+  assign _07_ = i_reset ? 31'h00000000 : _12_;
+  assign _13_ = i_ce ? _11_ : \fir.FILTER[1].tapk.product ;
+  assign _08_ = i_reset ? 24'h000000 : _13_;
+  assign \fir.FILTER[1].tapk.o_acc  = o_result;
+endmodule
+EOT
+
+synth_xilinx
+cd fastfir_dynamictaps
+select -assert-count 2 t:DSP48E1
+select -assert-none t:* t:DSP48E1 %d t:BUFG %d