simplift arbHEADmaster

+ Had a higher fMax and smaller area compared to previous attempt,
  at least when synthesized with OpenROAD

1 files changed, 21 insertions, 78 deletions

diff --git a/src/arb.sv b/src/arb.sv
index 15d89b3..6dbd6a6 100644
--- a/src/arb.sv
+++ b/src/arb.sv
@@ -1,99 +1,42 @@
-module first_set #(
-	parameter WIDTH = 32
-)(
-	input  reg[WIDTH-1 :0] cand,
-	output reg[WIDTH-1 :0] selected
-);
-
-/* base case */
-if (WIDTH == 1) begin :base
-	assign selected  = cand;
-end
-
-/* recursive case */
-else begin :recursive
-	localparam SLICE_WIDTH = WIDTH / 2;
-	logic[SLICE_WIDTH-1:0] left, right;
-	logic pref_left, pref_right;
-	logic empty_left, empty_right;
-
-	first_set #(
-		.WIDTH(SLICE_WIDTH)
-	) first_set_left (
-		.cand(cand[SLICE_WIDTH +: SLICE_WIDTH]),
-		.selected (left)
-	);
-
-	first_set #(
-		.WIDTH(SLICE_WIDTH)
-	) first_set_right (
-		.cand(cand[0 +: SLICE_WIDTH]),
-		.selected (right)
-	);
-
-	assign selected = right != '0
-			? {{SLICE_WIDTH{1'b0}}, right}
-			: {left, {SLICE_WIDTH{1'b0}}}
-			;
-end
-
-always_comb begin
-	assert ($onehot0(selected));
-end
-
-`ifdef NOSUCHDEF
-/* assertions that verilator don't support, apparently */
-assert property (WIDTH >= 1)
-else $error("input width too narrow");
-
-assert property (1 << $clog2(WIDTH) == WIDTH)
-else $error("width not pow2");
-
-`endif
-
-endmodule // arb_recursive
-
 module arb #(
 	parameter WIDTH = 32
 )(
 	input clk,
 	input rstx,
 
-	input [WIDTH-1:0] cand,
+	input [WIDTH-1:0] valid,
 	output[WIDTH-1:0] selected
 );
 
-localparam W2 = 1 << $clog2(WIDTH);
-localparam DEPTH = W2 == 1 ? 1 : $clog2(W2);
+logic found0, found1;
+logic [WIDTH-1:0] mask, next_mask0, next_mask1, cand0, cand1;
 
-logic[W2-1:0] w2_cand, w2_selected, mask, next_mask, rot_selected;
+always_comb begin
+	found0 = 0;
+	found1 = 0;
+	for (int i = 0; i < WIDTH; ++i) begin
+		cand0[i] = !found0 & valid[i] & mask[i];
+		cand1[i] = !found1 & valid[i];
 
-/* if there are candidates within the mask of 'above preferred index', use them,
- * otherwise invert the mask and use candidates 'below' preferred index. */
-assign w2_cand = ((cand & mask) == '0) ? cand & ~mask : cand & mask;
+		next_mask0[i] = found0;
+		next_mask1[i] = found1;
 
-first_set #(
-	.WIDTH(W2)
-) first_set_i (
-	.cand    (w2_cand),
-	.selected(w2_selected)
-);
+		if (valid[i] & mask[i])
+			found0 = 1;
 
-assign selected = w2_selected;
-
-assign rot_selected = {
-	w2_selected[0 +: W2-1],
-	w2_selected[W2-1]
-};
+		if (valid[i])
+			found1 = 1;
+	end
+end
 
-assign next_mask = ~(rot_selected - 1) | rot_selected;
+assign selected = found0 ? cand0 : cand1;
 
 always_ff @(posedge clk or negedge rstx)
 if (!rstx) begin
-	mask  <= ~0;
+	mask <= '0;
 end else begin
-	if (selected != 0 && mask != next_mask)
-		mask <= next_mask;
+	if (|valid)
+		mask <= found0 ? next_mask0 : next_mask1;
 	else
 		mask <= mask;
 end