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authorClifford Wolf <clifford@clifford.at>2014-08-16 21:07:29 +0200
committerClifford Wolf <clifford@clifford.at>2014-08-16 21:07:29 +0200
commit976bda71029b9d25ffad9ec8fbf3ceed2d1287e1 (patch)
treeb13268a7e2f75e3b681cbef5de42755ba93fc1dd /techlibs
parent3b9157f9a605b5ae2f535c4da932891e504dc57e (diff)
Multiply using a carry-save accumulator
Diffstat (limited to 'techlibs')
-rw-r--r--techlibs/common/techmap.v50
1 files changed, 45 insertions, 5 deletions
diff --git a/techlibs/common/techmap.v b/techlibs/common/techmap.v
index 9b733c6f..9bdff4f3 100644
--- 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);