Library Constprop
Constant propagation over RTL. This is one of the optimizations
performed at RTL level. It proceeds by a standard dataflow analysis
and the corresponding code rewriting.
Require Import Coqlib.
Require Import Maps.
Require Import AST.
Require Import Integers.
Require Import Floats.
Require Import Values.
Require Import Globalenvs.
Require Import Op.
Require Import Registers.
Require Import RTL.
Require Import Lattice.
Require Import Kildall.
Require Import ConstpropOp.
The type
approx of compile-time approximations of values is
defined in the machine-dependent part ConstpropOp.
We equip this type of approximations with a semi-lattice structure.
The ordering is inclusion between the sets of values denoted by
the approximations.
Module Approx <: SEMILATTICE_WITH_TOP.
Definition t := approx.
Definition eq (x y: t) := (x = y).
Definition eq_refl: forall x, eq x x := (@refl_equal t).
Definition eq_sym: forall x y, eq x y -> eq y x := (@sym_equal t).
Definition eq_trans: forall x y z, eq x y -> eq y z -> eq x z := (@trans_equal t).
Lemma eq_dec: forall (x y: t), {x=y} + {x<>y}.
Definition beq (x y: t) := if eq_dec x y then true else false.
Lemma beq_correct: forall x y, beq x y = true -> x = y.
Definition ge (x y: t) : Prop :=
x = Unknown \/ y = Novalue \/ x = y.
Lemma ge_refl: forall x y, eq x y -> ge x y.
Lemma ge_trans: forall x y z, ge x y -> ge y z -> ge x z.
Lemma ge_compat: forall x x' y y', eq x x' -> eq y y' -> ge x y -> ge x' y'.
Definition bot := Novalue.
Definition top := Unknown.
Lemma ge_bot: forall x, ge x bot.
Lemma ge_top: forall x, ge top x.
Definition lub (x y: t) : t :=
if eq_dec x y then x else
match x, y with
| Novalue, _ => y
| _, Novalue => x
| _, _ => Unknown
end.
Lemma lub_commut: forall x y, eq (lub x y) (lub y x).
Lemma ge_lub_left: forall x y, ge (lub x y) x.
End Approx.
Module D := LPMap Approx.
The transfer function for the dataflow analysis is straightforward:
for
Iop instructions, we set the approximation of the destination
register to the result of executing abstractly the operation;
for Iload and Icall, we set the approximation of the destination
to Unknown.
Definition approx_reg (app: D.t) (r: reg) :=
D.get r app.
Definition approx_regs (app: D.t) (rl: list reg):=
List.map (approx_reg app) rl.
Definition transfer (f: function) (pc: node) (before: D.t) :=
match f.(fn_code)!pc with
| None => before
| Some i =>
match i with
| Iop op args res s =>
let a := eval_static_operation op (approx_regs before args) in
D.set res a before
| Iload chunk addr args dst s =>
D.set dst Unknown before
| Icall sig ros args res s =>
D.set res Unknown before
| _ =>
before
end
end.
The static analysis itself is then an instantiation of Kildall's
generic solver for forward dataflow inequations.
analyze f
returns a mapping from program points to mappings of pseudo-registers
to approximations. It can fail to reach a fixpoint in a reasonable
number of iterations, in which case None is returned.
Module DS := Dataflow_Solver(D)(NodeSetForward).
Definition analyze (f: RTL.function): PMap.t D.t :=
match DS.fixpoint (successors f) (transfer f)
((f.(fn_entrypoint), D.top) :: nil) with
| None => PMap.init D.top
| Some res => res
end.
The code transformation proceeds instruction by instruction.
Operators whose arguments are all statically known are turned
into ``load integer constant'', ``load float constant'' or
``load symbol address'' operations. Operators for which some
but not all arguments are known are subject to strength reduction,
and similarly for the addressing modes of load and store instructions.
Other instructions are unchanged.
Definition transf_ros (app: D.t) (ros: reg + ident) : reg + ident :=
match ros with
| inl r =>
match D.get r app with
| S symb ofs => if Int.eq ofs Int.zero then inr _ symb else ros
| _ => ros
end
| inr s => ros
end.
Definition transf_instr (app: D.t) (instr: instruction) :=
match instr with
| Iop op args res s =>
match eval_static_operation op (approx_regs app args) with
| I n =>
Iop (Ointconst n) nil res s
| F n =>
Iop (Ofloatconst n) nil res s
| S symb ofs =>
Iop (Oaddrsymbol symb ofs) nil res s
| _ =>
let (op', args') := op_strength_reduction (approx_reg app) op args in
Iop op' args' res s
end
| Iload chunk addr args dst s =>
let (addr', args') := addr_strength_reduction (approx_reg app) addr args in
Iload chunk addr' args' dst s
| Istore chunk addr args src s =>
let (addr', args') := addr_strength_reduction (approx_reg app) addr args in
Istore chunk addr' args' src s
| Icall sig ros args res s =>
Icall sig (transf_ros app ros) args res s
| Itailcall sig ros args =>
Itailcall sig (transf_ros app ros) args
| Icond cond args s1 s2 =>
match eval_static_condition cond (approx_regs app args) with
| Some b =>
if b then Inop s1 else Inop s2
| None =>
let (cond', args') := cond_strength_reduction (approx_reg app) cond args in
Icond cond' args' s1 s2
end
| Ijumptable arg tbl =>
match intval (approx_reg app) arg with
| Some n =>
match list_nth_z tbl (Int.signed n) with
| Some s => Inop s
| None => instr
end
| None => instr
end
| _ =>
instr
end.
Definition transf_code (approxs: PMap.t D.t) (instrs: code) : code :=
PTree.map (fun pc instr => transf_instr approxs!!pc instr) instrs.
Definition transf_function (f: function) : function :=
let approxs := analyze f in
mkfunction
f.(fn_sig)
f.(fn_params)
f.(fn_stacksize)
(transf_code approxs f.(fn_code))
f.(fn_entrypoint).
Definition transf_fundef (fd: fundef) : fundef :=
AST.transf_fundef transf_function fd.
Definition transf_program (p: program) : program :=
transform_program transf_fundef p.