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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
|
