techmap +/cmp2lcu.v for decomposing arithmetic compares to $lcu

2 files changed, 84 insertions, 0 deletions

diff --git a/techlibs/common/Makefile.inc b/techlibs/common/Makefile.inc
index d5e69a241..7b1e4b430 100644
--- a/techlibs/common/Makefile.inc
+++ b/techlibs/common/Makefile.inc
@@ -30,3 +30,4 @@ $(eval $(call add_share_file,share,techlibs/common/cmp2lut.v))
 $(eval $(call add_share_file,share,techlibs/common/cells.lib))
 $(eval $(call add_share_file,share,techlibs/common/mul2dsp.v))
 $(eval $(call add_share_file,share,techlibs/common/abc9_model.v))
+$(eval $(call add_share_file,share,techlibs/common/cmp2lcu.v))
diff --git a/techlibs/common/cmp2lcu.v b/techlibs/common/cmp2lcu.v
new file mode 100644
index 000000000..433f88fae
--- /dev/null
+++ b/techlibs/common/cmp2lcu.v
@@ -0,0 +1,83 @@
+// This pass performs an optimisation that decomposes wide arithmetic
+//   comparisons into LUT-size chunks (as guided by the `LUT_WIDTH
+//   macro) connected to a single lookahead-carry-unit $lcu cell,
+//   which is typically mapped to dedicated (and fast) FPGA
+//   carry-chains.
+(* techmap_celltype = "$lt $le $gt $ge" *)
+module _90_lcu_cmp_ (A, B, Y);
+
+parameter A_SIGNED = 0;
+parameter B_SIGNED = 0;
+parameter A_WIDTH = 0;
+parameter B_WIDTH = 0;
+parameter Y_WIDTH = 0;
+
+input [A_WIDTH-1:0] A;
+input [B_WIDTH-1:0] B;
+output [Y_WIDTH-1:0] Y;
+
+parameter _TECHMAP_CELLTYPE_ = "";
+localparam gt_width = `LUT_WIDTH/2;
+
+generate
+    if (_TECHMAP_CELLTYPE_ == "" || (A_WIDTH <= gt_width || B_WIDTH <= gt_width))
+        wire _TECHMAP_FAIL_ = 1;
+    else if (_TECHMAP_CELLTYPE_ == "$lt") begin
+        // Transform $lt into $gt by swapping A and B
+		$gt #(.A_SIGNED(B_SIGNED), .B_SIGNED(A_SIGNED), .A_WIDTH(B_WIDTH), .B_WIDTH(A_WIDTH), .Y_WIDTH(Y_WIDTH)) _TECHMAP_REPLACE_ (.A(B), .B(A), .Y(Y));
+    end
+    else if (_TECHMAP_CELLTYPE_ == "$le") begin
+        // Transform $le into $ge by swapping A and B
+		$ge #(.A_SIGNED(B_SIGNED), .B_SIGNED(A_SIGNED), .A_WIDTH(B_WIDTH), .B_WIDTH(A_WIDTH), .Y_WIDTH(Y_WIDTH)) _TECHMAP_REPLACE_ (.A(B), .B(A), .Y(Y));
+    end
+    else begin
+        // Perform sign extension on A and B
+        localparam WIDTH = A_WIDTH > B_WIDTH ? A_WIDTH : B_WIDTH;
+        wire [WIDTH-1:0] AA = {{(WIDTH-A_WIDTH){A_SIGNED ? A[A_WIDTH-1] : 1'b0}}, A};
+        wire [WIDTH-1:0] BB = {{(WIDTH-B_WIDTH){B_SIGNED ? B[B_WIDTH-1] : 1'b0}}, B};
+
+        // Compute width of $lcu/carry-chain cell
+        localparam lcu_width = (WIDTH+gt_width-1)/gt_width;
+        wire [lcu_width-1:0] P, G, CO;
+        genvar i, j;
+        integer j;
+        for (i = 0; i < WIDTH; i=i+gt_width) begin
+            wire [gt_width-1:0] PP, GG;
+            if (i < WIDTH-gt_width) begin
+                // Bit-wise equality (xnor) of sign-extended A and B
+                assign PP = AA[i +: gt_width] ^~ BB[i +: gt_width];
+                // Priority "encoder" that checks A[i] == 1'b1 && B[i] == 1'b0
+                //   from MSB down, deferring to less significant bits if the
+                //   MSBs are equal
+                assign GG[gt_width-1] = AA[i+gt_width-1] & ~BB[i+gt_width-1];
+                for (j = gt_width-2; j >= 0; j=j-1)
+                    assign GG[j] = &PP[gt_width-1:j+1] & (AA[i+j] & ~BB[i+j]);
+                // Propagate only if all bits are equal
+                //   (inconclusive evidence to say A >= B)
+                assign P[i/gt_width] = &PP;
+                // Generate if any pairs call for it
+                assign G[i/gt_width] = |GG;
+            end
+            else begin
+                assign PP = AA[WIDTH-1:i] ^~ BB[WIDTH-1:i];
+                if (A_SIGNED && B_SIGNED)
+                    assign GG[WIDTH-i-1] = ~AA[WIDTH-1] & BB[WIDTH-1];
+                else
+                    assign GG[WIDTH-i-1] = AA[WIDTH-1] & ~BB[WIDTH-1];
+                for (j = WIDTH-i-2; j >= 0; j=j-1)
+                    assign GG[j] = &PP[WIDTH-i-1:j+1] & (AA[i+j] & ~BB[i+j]);
+                assign P[i/gt_width] = &PP[WIDTH-i-1:0];
+                assign G[i/gt_width] = |GG[WIDTH-i-1:0];
+            end
+        end
+        // For $ge operation, start with the assumption that A and B are
+        //   equal (propagating this equality if A and B turn out to be so)
+        if (_TECHMAP_CELLTYPE_ == "$ge")
+            wire CI = 1'b1;
+        else
+            wire CI = 1'b0;
+        $lcu #(.WIDTH(lcu_width)) lcu (.P(P), .G(G), .CI(CI), .CO(CO));
+        assign Y = CO[lcu_width-1];
+    end
+endgenerate
+endmodule