Pipeline ML-KEM datapath bottlenecks

sync_rtl/ntt/barrett_mul_pipe.v

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+// barrett_mul_pipe.v - DSP-friendly pipelined Barrett modular multiply
+//
+// Computes product = a * b mod Q with fixed latency. This module is used on
+// timing-critical NTT paths; the legacy barrett_mul remains combinational for
+// older users.
+
+module barrett_mul_pipe (
+    input         clk,
+    input         rst_n,
+    input         valid_i,
+    input  [11:0] a,
+    input  [11:0] b,
+    output [11:0] product,
+    output        valid_o
+);
+    localparam [24:0] Q25 = 25'd3329;
+    localparam [12:0] K13 = 13'd5039;
+
+    reg [23:0] p_s1;
+
+    reg [23:0] p_s2;
+    reg [36:0] tk_s2;
+
+    reg [23:0] p_s3;
+    reg [12:0] q_s3;
+
+    reg [24:0] p_s4;
+    reg [24:0] qprod_s4;
+
+    reg [24:0] r0_s5;
+    reg [24:0] r1_s6;
+    reg [11:0] product_r;
+    reg [6:0]  valid_sr;
+
+    assign product = product_r;
+    assign valid_o = valid_sr[6];
+
+    always @(posedge clk or negedge rst_n) begin
+        if (!rst_n) begin
+            p_s1 <= 24'd0;
+            p_s2 <= 24'd0; tk_s2 <= 37'd0;
+            p_s3 <= 24'd0; q_s3 <= 13'd0;
+            p_s4 <= 25'd0; qprod_s4 <= 25'd0;
+            r0_s5 <= 25'd0; r1_s6 <= 25'd0;
+            product_r <= 12'd0;
+            valid_sr <= 7'd0;
+        end else begin
+            valid_sr <= {valid_sr[5:0], valid_i};
+
+            // S1: full product.
+            p_s1 <= {12'd0, a} * b;
+
+            // S2: quotient-estimate product.
+            p_s2  <= p_s1;
+            tk_s2 <= {13'd0, p_s1} * K13;
+
+            // S3: quotient estimate.
+            p_s3 <= p_s2;
+            q_s3 <= tk_s2[36:24];
+
+            // S4: q * Q.
+            p_s4     <= {1'b0, p_s3};
+            qprod_s4 <= q_s3 * Q25;
+
+            // S5/S6/S7: subtract q*Q and conditionally subtract Q twice.
+            r0_s5 <= p_s4 - qprod_s4;
+            r1_s6 <= (r0_s5 >= Q25) ? (r0_s5 - Q25) : r0_s5;
+            product_r <= (r1_s6 >= Q25) ? (r1_s6 - Q25) : r1_s6[11:0];
+        end
+    end
+
+endmodule