From 2b7b2185d381f2c5fd4aee19bd3a3508b4c9557f Mon Sep 17 00:00:00 2001
From: Alejandro Soto <alejandro@34project.org>
Date: Sun, 3 Mar 2024 20:39:39 -0600
Subject: platform/wavelet3d: implement rounded fmul

---
 platform/wavelet3d/gfx_fmul_lane.sv | 82 +++++++++++++++++++++++++++++++++++++
 1 file changed, 82 insertions(+)
 create mode 100644 platform/wavelet3d/gfx_fmul_lane.sv

(limited to 'platform/wavelet3d/gfx_fmul_lane.sv')

diff --git a/platform/wavelet3d/gfx_fmul_lane.sv b/platform/wavelet3d/gfx_fmul_lane.sv
new file mode 100644
index 0000000..17a988e
--- /dev/null
+++ b/platform/wavelet3d/gfx_fmul_lane.sv
@@ -0,0 +1,82 @@
+module gfx_fmul_lane
+(
+	input  logic              clk,
+
+	input  gfx::float_special a,
+	                          b,
+	input  logic              slow_in,
+
+	output gfx::float_round   q
+);
+
+	import gfx::*;
+
+	/* Queremos calcular q = a * b.
+	 *
+	 * Donde a = (-1)^s * 1.m * 2^f,
+	 *       b = (-1)^t * 1.n * 2^g
+	 *
+	 * Entonces q = (-1)^(s + t) (1.m * 1.n) 2^(f + g)
+	 *
+	 * El producto es entre números >= 1.0 y < 2.0. En el peor caso:
+	 *   Mejor caso: 1.000... * 1.000... ~ 1.000...
+	 *   Peor caso:  1.999... * 1.999... ~ 3.999... = 2^1 * 1.999
+	 *
+	 * Así que, si el producto es >= 2, hay que hacerle >> 1 a la mantisa
+	 * y sumarle 1 al exponente para normalizar.
+	 */
+
+	logic guard, lo_msb, lo_reduce, overflow_0, overflow_1,
+	      round, sign, slow_0, slow_1, zero;
+
+	float_exp exp;
+	float_round out;
+	float_mant_full hi;
+	logic[$bits(float_mant_full) - 3:0] lo;
+
+	assign lo_msb = lo[$bits(lo) - 1];
+	assign lo_reduce = |lo[$bits(lo) - 2:0];
+
+	always_comb begin
+		q = out;
+		q.slow = slow_1 | overflow_1;
+	end
+
+	always_ff @(posedge clk) begin
+		// Stage 0: producto
+
+		sign <= a.val.sign ^ b.val.sign;
+		zero <= a.exp_min | b.exp_min;
+		slow_0 <= slow_in;
+
+		{overflow_0, exp} <= {1'b0, a.val.exp} + {1'b0, b.val.exp} - {1'b0, FLOAT_EXP_BIAS};
+		{hi, guard, round, lo} <= full_mant(a.val.mant) * full_mant(b.val.mant);
+
+		// Stage 1: normalización
+
+		slow_1 <= slow_0 | overflow_0;
+		overflow_1 <= 0;
+
+		out.slow <= 1'bx; // Ver 'q'
+		out.zero <= zero;
+		out.normal.sign <= sign;
+
+		if (hi[$bits(hi) - 1]) begin
+			out.guard <= guard;
+			out.round <= round;
+			out.sticky <= lo_msb | lo_reduce;
+			out.normal.mant <= implicit_mant(hi);
+			{overflow_1, out.normal.exp} <= {1'b0, exp} + 1;
+		end else begin
+			/* Bit antes de msb es necesariamente 1, ya que los msb de
+			 * ambos multiplicandos son 1. Ver assert en implicit_mant().
+			 */
+			out.guard <= round;
+			out.round <= lo[$bits(lo) - 1];
+			out.sticky <= lo_reduce;
+			out.normal.exp <= exp;
+			out.normal.mant <= implicit_mant({hi[$bits(hi) - 2:0], guard});
+		end
+	end
+
+endmodule
-- 
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