Added netlist simplification passes (not yet quite complete !)

1 files changed, 214 insertions, 0 deletions

diff --git a/sched/simplify.ml b/sched/simplify.ml
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+++ b/sched/simplify.ml
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+(* SIMPLIFICATION PASSES *)
+
+(*
+	Order of simplifications :
+	- cascade slices and selects
+	- simplify stupid things (a xor 0 = a, a and 0 = 0, etc.)
+	  transform k = SLICE i i var into k = SELECT i var
+	- transform k = SELECT 0 var into k = var when var is also one bit
+	- look for variables with same equation, put the second to identity
+	- eliminate k' for each equation k' = k
+	- eliminate dead equations
+
+	These simplifications are run on a topologically sorted list of equations (see main.ml)
+*)
+
+open Netlist_ast
+
+module Sset = Set.Make(String)
+
+(* Simplify cascade slicing/selecting *)
+let cascade_slices p =
+	let slices = Hashtbl.create 42 in
+	let eqs_new = List.map
+		(fun (n, eq) -> (n, match eq with
+			| Eslice(u, v, Avar(x)) ->
+				let nu, nx =
+					if Hashtbl.mem slices x then begin
+						let ku, kx = Hashtbl.find slices x in
+						(ku + u, kx)
+					end else 
+						 (u, x)
+				in
+				Hashtbl.add slices n (nu, nx);
+				Eslice(nu, v, Avar(nx))
+			| Eselect(u, Avar(x)) ->
+				begin try
+					let ku, kx = Hashtbl.find slices x in
+					Eselect(ku + u, Avar(kx))
+				with
+					Not_found -> Eselect(u, Avar(x))
+				end
+			| _ -> eq))
+		p.p_eqs in
+	{
+		p_eqs = eqs_new;
+		p_inputs = p.p_inputs;
+		p_outputs = p.p_outputs;
+		p_vars = p.p_vars;
+	}
+
+(* Simplifies some trivial arithmetic possibilites :
+	a and 1 = a
+	a and 0 = 0
+	a or 1 = 1
+	a or 0 = a
+	a xor 0 = a
+	slice i i x = select i x
+*)
+let arith_simplify p =
+	{
+		p_eqs = List.map
+			(fun (n, eq) -> match eq with
+			| Ebinop(Or, Aconst(VBit(false)), x) -> (n, Earg(x))
+			| Ebinop(Or, Aconst(VBit(true)), x) -> (n, Earg(Aconst(VBit(true))))
+			| Ebinop(Or, x, Aconst(VBit(false))) -> (n, Earg(x))
+			| Ebinop(Or, x, Aconst(VBit(true))) -> (n, Earg(Aconst(VBit(true))))
+
+			| Ebinop(And, Aconst(VBit(false)), x) -> (n, Earg(Aconst(VBit(false))))
+			| Ebinop(And, Aconst(VBit(true)), x) -> (n, Earg(x))
+			| Ebinop(And, x, Aconst(VBit(false))) -> (n, Earg(Aconst(VBit(false))))
+			| Ebinop(And, x, Aconst(VBit(true))) -> (n, Earg(x))
+
+			| Ebinop(Xor, Aconst(VBit(false)), x) -> (n, Earg(x))
+			| Ebinop(Xor, x, Aconst(VBit(false))) -> (n, Earg(x))
+
+			| Eslice(i, j, k) when i = j ->
+				(n, Eselect(i, k))
+			
+			| _ -> (n, eq))
+			p.p_eqs;
+		p_inputs = p.p_inputs;
+		p_outputs = p.p_outputs;
+		p_vars = p.p_vars;
+	}
+
+(* if x is one bit, then :
+	select 0 x = x
+*)
+let select_to_id p =
+	{
+		p_eqs = List.map
+			(fun (n, eq) -> match eq with
+			| Eselect(0, Avar(id)) when
+				Env.find id p.p_vars = TBit || Env.find id p.p_vars = TBitArray(1) ->
+					(n, Earg(Avar(id)))
+			| _ -> (n, eq))
+			p.p_eqs;
+		p_inputs = p.p_inputs;
+		p_outputs = p.p_outputs;
+		p_vars = p.p_vars;
+	}
+
+(*
+	If a = eqn(v1, v2, ...) and b = eqn(v1, v2, ...)   <- the same equation
+	then say b = a
+*)
+let same_eq_simplify p =
+	let id_outputs =
+		(List.fold_left (fun x k -> Sset.add k x) Sset.empty p.p_outputs) in
+	let eq_map = Hashtbl.create 42 in
+	List.iter
+		(fun (n, eq) -> if Sset.mem n id_outputs then
+			Hashtbl.add eq_map eq n)
+		p.p_eqs;
+	let simplify_eq (n, eq) =
+		if Sset.mem n id_outputs then
+			(n, eq)
+		else if Hashtbl.mem eq_map eq then
+			(n, Earg(Avar(Hashtbl.find eq_map eq)))
+		else begin
+			Hashtbl.add eq_map eq n;
+			(n, eq)
+		end
+	in
+	let eq2 = List.map simplify_eq p.p_eqs in
+	{
+		p_eqs = eq2;
+		p_inputs = p.p_inputs;
+		p_outputs = p.p_outputs;
+		p_vars = p.p_vars;
+	}
+
+
+(*	Replace one specific variable by another argument in the arguments of all equations
+	(possibly a constant, possibly another variable)
+*)
+let eliminate_var var rep p =
+	let rep_arg = function
+		| Avar(i) when i = var -> rep
+		| k -> k
+	in
+	let rep_eqs = List.map
+		(fun (n, eq) -> (n, match eq with
+			| Earg(a) -> Earg(rep_arg a)
+			| Ereg(i) when i = var ->
+				begin match rep with
+				| Avar(j) -> Ereg(j)
+				| Aconst(k) -> Earg(Aconst(k))
+				end
+			| Ereg(j) -> Ereg(j)
+			| Enot(a) -> Enot(rep_arg a)
+			| Ebinop(o, a, b) -> Ebinop(o, rep_arg a, rep_arg b)
+			| Emux(a, b, c) -> Emux(rep_arg a, rep_arg b, rep_arg c)
+			| Erom(u, v, a) -> Erom(u, v, rep_arg a)
+			| Eram(u, v, a, b, c, d) -> Eram(u, v, rep_arg a, rep_arg b, rep_arg c, rep_arg d)
+			| Econcat(a, b) -> Econcat(rep_arg a, rep_arg b)
+			| Eslice(u, v, a) -> Eslice(u, v, rep_arg a)
+			| Eselect(u, a) -> Eselect(u, rep_arg a)
+			))
+		p.p_eqs in
+	{
+		p_eqs = List.fold_left
+			(fun x (n, eq) ->
+				if n = var then x else (n, eq)::x)
+			[] rep_eqs;
+		p_inputs = p.p_inputs;
+		p_outputs = p.p_outputs;
+		p_vars = Env.remove var p.p_vars;
+	}
+
+(* Remove all equations of type :
+	a = b
+	a = const
+	(except if a is an output variable)
+*)
+let rec eliminate_id p =
+	let id_outputs =
+		(List.fold_left (fun x k -> Sset.add k x) Sset.empty p.p_outputs) in
+
+	let rep =
+		List.fold_left
+			(fun x (n, eq) ->
+				if x = None && (not (Sset.mem n id_outputs)) then
+					match eq with
+					| Earg(rarg) -> 
+						Some(n, rarg)
+					| _ -> None
+				else
+					x)
+			None p.p_eqs in
+	match rep with
+	| None -> p, false
+	| Some(n, rep) -> fst (eliminate_id (eliminate_var n rep p)), true
+
+
+(* Eliminate dead equations *)
+let eliminate_dead p =
+	p, false (* TODO *)
+	(* a bit like a topological sort... *)
+
+
+(* Apply all the simplification passes,
+	in the order given in the header of this file
+*)
+let rec simplify p =
+	let p1 = cascade_slices p in
+	let p2 = arith_simplify p1 in
+	let p3 = select_to_id p2 in
+	let p4 = same_eq_simplify p3 in
+	let p5, use5 = eliminate_id p4 in
+	let p6, use6 = eliminate_dead p5 in
+	let pp = p6 in
+	if use5 || use6 then simplify pp else pp
+