Multiply using a carry-save accumulator

diff --git a/techlibs/common/techmap.v b/techlibs/common/techmap.v
index 9b733c6f012f2f18949f69bdea607b257129d94d..9bdff4f336d274bd71e40b40538b14d27ff0ec77 100644 (file)
--- a/techlibs/common/techmap.v
+++ b/techlibs/common/techmap.v
@@ -502,21 +502,61 @@ module \$__arraymul (A, B, Y);
        input [WIDTH-1:0] A, B;
        output [WIDTH-1:0] Y;
 
-       wire [1023:0] _TECHMAP_DO_ = "proc;;";
+       wire [1023:0] _TECHMAP_DO_ = "proc;; opt";
 
        integer i;
-       reg [WIDTH-1:0] x, y;
+       reg [WIDTH-1:0] x;
+       reg [2*WIDTH-1:0] y;
+
+       function [2*WIDTH-1:0] acc_set;
+               input [WIDTH-1:0] value;
+               integer k;
+               begin
+                       for (k = 0; k < WIDTH; k = k+1) begin
+                               acc_set[2*k +: 2] = value[k];
+                       end
+               end
+       endfunction
+
+       function [2*WIDTH-1:0] acc_add;
+               input [2*WIDTH-1:0] old_acc;
+               input [WIDTH-1:0] value;
+               integer k;
+               reg a, b, c;
+               begin
+                       for (k = 0; k < WIDTH; k = k+1) begin
+                               a = old_acc[2*k];
+                               b = k ? old_acc[2*k-1] : 1'b0;
+                               c = value[k];
+                               acc_add[2*k] = (a ^ b) ^ c;
+                               acc_add[2*k+1] = (a & b) | ((a ^ b) & c);
+                       end
+               end
+       endfunction
+
+       function [WIDTH-1:0] acc_get;
+               input [2*WIDTH-1:0] acc;
+               integer k;
+               begin
+                       // at the end of the multiplier chain the carry-save accumulator
+                       // should also have propagated all carries. thus we just need to
+                       // copy the even bits from the carry accumulator to the output.
+                       for (k = 0; k < WIDTH; k = k+1) begin
+                               acc_get[k] = acc[2*k];
+                       end
+               end
+       endfunction
 
        always @* begin
                x = B;
-               y = A[0] ? x : 0;
+               y = acc_set(A[0] ? x : 0);
                for (i = 1; i < WIDTH; i = i+1) begin
                        x = {x[WIDTH-2:0], 1'b0};
-                       y = y + (A[i] ? x : 0);
+                       y = acc_add(y, A[i] ? x : 0);
                end
        end
 
-       assign Y = y;
+       assign Y = acc_get(y);
 endmodule
 
 module \$mul (A, B, Y);