feat: add pipelined scalar tmem softmax unit

hw/rtl/core/VX_tmem_softmax_unit.sv

@@ -161,50 +161,29 @@ module VX_tmem_softmax_unit #(
         end
     endfunction
 
-    // Convert a positive unsigned Q15 probability to FP8 E4M3 using
-    // round-to-nearest-even. Softmax probabilities are in [0, 1], so the
-    // positive subnormal and exponent ranges through 1.0 are sufficient.
-    function automatic [7:0] q15_to_fp8e4m3;
+    function automatic [15:0] q15_to_fp16;
         input [15:0] value;
         integer msb;
         integer i;
-        integer shift;
-        reg [3:0] exponent;
-        reg [4:0] rounded;
-        reg [15:0] remainder;
-        reg [15:0] halfway;
+        reg [15:0] normalized;
         begin
-            if (value < 16'd512) begin
-                // E4M3 subnormals and the minimum normal are spaced by 2^-9,
-                // which is 64 in unsigned Q15.
-                rounded = value[10:6];
-                remainder = value & 16'h003f;
-                if ((remainder > 16'd32)
-                 || ((remainder == 16'd32) && rounded[0]))
-                    rounded = rounded + 1'b1;
-                q15_to_fp8e4m3 = {3'b000, rounded};
+            msb = -1;
+            for (i = 0; i < 16; ++i) begin
+                if (value[i]) msb = i;
+            end
+            if (msb < 0) begin
+                q15_to_fp16 = 16'd0;
             end else begin
-                msb = 0;
-                for (i = 0; i < 16; ++i) begin
-                    if (value[i]) msb = i;
-                end
-                exponent = 4'(msb - 8);
-                shift = integer'(exponent) + 5;
-                rounded = 5'(value >> shift);
-                remainder = value & ((16'd1 << shift) - 1'b1);
-                halfway = 16'd1 << (shift - 1);
-                if ((remainder > halfway)
-                 || ((remainder == halfway) && rounded[0]))
-                    rounded = rounded + 1'b1;
-                if (rounded == 5'd16)
-                    q15_to_fp8e4m3 = {1'b0, exponent + 1'b1, 3'b000};
+                if (msb >= 10)
+                    normalized = value >> (msb - 10);
                 else
-                    q15_to_fp8e4m3 = {1'b0, exponent, rounded[2:0]};
+                    normalized = value << (10 - msb);
+                q15_to_fp16 = {1'b0, msb[4:0], normalized[9:0]};
             end
         end
     endfunction
 
-    function automatic [7:0] normalize_to_fp8e4m3;
+    function automatic [15:0] normalize_to_fp16;
         input [23:0] exp_value;
         input [31:0] reciprocal;
         reg [55:0] product;
@@ -212,10 +191,10 @@ module VX_tmem_softmax_unit #(
         begin
             product = exp_value * reciprocal;
             probability = (product + 56'd524288) >> 20;
-            if (probability > 56'd32768)
-                normalize_to_fp8e4m3 = 8'h38;
+            if (probability > 36'd32768)
+                normalize_to_fp16 = 16'h3c00;
             else
-                normalize_to_fp8e4m3 = q15_to_fp8e4m3(probability[15:0]);
+                normalize_to_fp16 = q15_to_fp16(probability[15:0]);
         end
     endfunction
 
@@ -368,7 +347,7 @@ module VX_tmem_softmax_unit #(
                 end else if (!out_valid || out_ready) begin
                     for (lane = 0; lane < NUM_LANES; ++lane) begin
                         out_data[lane] <= {
-                            4{normalize_to_fp8e4m3(
+                            2{normalize_to_fp16(
                                 exp_values[norm_index[4:0]][lane], reciprocals[lane])}
                         };
                     end