verilog: refactored constant function evaluation

Elaboration now attempts constant evaluation of any function call with
only constant arguments, regardless of the context or contents of the
function. This removes the concept of "recommended constant evaluation"
which previously applied to functions with `for` loops or which were
(sometimes erroneously) identified as recursive. Any function call in a
constant context (e.g., `localparam`) or which contains a constant-only
procedural construct (`while` or `repeat`) in its body will fail as
before if constant evaluation does not succeed.

2 files changed, 94 insertions, 0 deletions

diff --git a/tests/simple/const_fold_func.v b/tests/simple/const_fold_func.v
new file mode 100644
index 000000000..ee2f12e06
--- /dev/null
+++ b/tests/simple/const_fold_func.v
@@ -0,0 +1,61 @@
+module top(
+	input wire [3:0] inp,
+	output wire [3:0] out1, out2, out3, out4, out5,
+	output reg [3:0] out6
+);
+	function automatic [3:0] flip;
+		input [3:0] inp;
+		flip = ~inp;
+	endfunction
+
+	function automatic [3:0] help;
+		input [3:0] inp;
+		help = flip(inp);
+	endfunction
+
+	// while loops are const-eval-only
+	function automatic [3:0] loop;
+		input [3:0] inp;
+		reg [3:0] val;
+		begin
+			val = inp;
+			loop = 1;
+			while (val != inp) begin
+				loop = loop * 2;
+				val = val + 1;
+			end
+		end
+	endfunction
+
+	// not const-eval-only, despite calling a const-eval-only function
+	function automatic [3:0] help_mul;
+		input [3:0] inp;
+		help_mul = inp * loop(2);
+	endfunction
+
+	// can be elaborated so long as exp is a constant
+	function automatic [3:0] pow_flip_a;
+		input [3:0] base, exp;
+		begin
+			pow_flip_a = 1;
+			if (exp > 0)
+				pow_flip_a = base * pow_flip_a(flip(base), exp - 1);
+		end
+	endfunction
+
+	function automatic [3:0] pow_flip_b;
+		input [3:0] base, exp;
+		begin
+			out6[exp] = base & 1;
+			pow_flip_b = 1;
+			if (exp > 0)
+				pow_flip_b = base * pow_flip_b(flip(base), exp - 1);
+		end
+	endfunction
+
+	assign out1 = flip(flip(inp));
+	assign out2 = help(flip(inp));
+	assign out3 = help_mul(inp);
+	assign out4 = pow_flip_a(flip(inp), 3);
+	assign out5 = pow_flip_b(2, 2);
+endmodule
diff --git a/tests/simple/const_func_shadow.v b/tests/simple/const_func_shadow.v
new file mode 100644
index 000000000..ca63606d9
--- /dev/null
+++ b/tests/simple/const_func_shadow.v
@@ -0,0 +1,33 @@
+module top(w, x, y, z);
+	function [11:0] func;
+		input reg [2:0] x;
+		input reg [2:0] y;
+		begin
+			x = x * (y + 1);
+			begin : foo
+				reg [2:0] y;
+				y = x + 1;
+				begin : bar
+					reg [2:0] x;
+					x = y + 1;
+					begin : blah
+						reg [2:0] y;
+						y = x + 1;
+						func[2:0] = y;
+					end
+					func[5:3] = x;
+				end
+				func[8:6] = y;
+			end
+			func[11:9] = x;
+		end
+	endfunction
+	output wire [func(2, 3) - 1:0] w;
+	output wire [func(1, 3) - 1:0] x;
+	output wire [func(3, 1) - 1:0] y;
+	output wire [func(5, 2) - 1:0] z;
+	assign w = 1'sb1;
+	assign x = 1'sb1;
+	assign y = 1'sb1;
+	assign z = 1'sb1;
+endmodule