Module CSE

Common subexpression elimination over RTL. This optimization proceeds by value numbering over extended basic blocks.

Require Import Coqlib Maps Errors Integers Floats Lattice Kildall.
Require Import Values Memory.
Require Import Op Registers RTL.
Require Import ValueDomain ValueAnalysis CSEdomain CombineOp.

The idea behind value numbering algorithms is to associate abstract identifiers (``value numbers'') to the contents of registers at various program points, and record equations between these identifiers. For instance, consider the instruction r1 = add(r2, r3) and assume that r2 and r3 are mapped to abstract identifiers x and y respectively at the program point just before this instruction. At the program point just after, we can add the equation z = add(x, y) and associate r1 with z, where z is a fresh abstract identifier. However, if we already knew an equation u = add(x, y), we can preferably add no equation and just associate r1 with u. If there exists a register r4 mapped with u at this point, we can then replace the instruction r1 = add(r2, r3) by a move instruction r1 = r4, therefore eliminating a common subexpression and reusing the result of an earlier addition. The representation of value numbers and equations is described in module CSEdomain.

Operations on value numberings

valnum_reg n r returns the value number for the contents of register r. If none exists, a fresh value number is returned and associated with register r. The possibly updated numbering is also returned. valnum_regs is similar, but for a list of registers.

Definition valnum_reg (n: numbering) (r: reg) : numbering * valnum :=
match PTree.get r n.(num_reg) with
| Some v => (n, v)
| None =>
let v := n.(num_next) in
( {| num_next := Pos.succ v;
num_eqs := n.(num_eqs);
num_reg := PTree.set r v n.(num_reg);
num_val := PMap.set v (r :: nil) n.(num_val) |},
v)
end.

Fixpoint valnum_regs (n: numbering) (rl: list reg)
{struct rl} : numbering * list valnum :=
match rl with
| nil =>
(n, nil)
| r1 :: rs =>
let (n1, v1) := valnum_reg n r1 in
let (ns, vs) := valnum_regs n1 rs in
(ns, v1 :: vs)
end.

find_valnum_rhs rhs eqs searches the list of equations eqs for an equation of the form vn = rhs for some value number vn. If found, Some vn is returned, otherwise None is returned.

Fixpoint find_valnum_rhs (r: rhs) (eqs: list equation)
{struct eqs} : option valnum :=
match eqs with
| nil => None
| Eq v str r' :: eqs1 =>
if str && eq_rhs r r' then Some v else find_valnum_rhs r eqs1
end.

find_valnum_rhs' rhs eqs is similar, but also accepts equations of the form vn >= rhs.

Fixpoint find_valnum_rhs' (r: rhs) (eqs: list equation)
{struct eqs} : option valnum :=
match eqs with
| nil => None
| Eq v str r' :: eqs1 =>
if eq_rhs r r' then Some v else find_valnum_rhs' r eqs1
end.

find_valnum_num vn eqs searches the list of equations eqs for an equation of the form vn = rhs for some equation rhs. If found, Some rhs is returned, otherwise None is returned.

Fixpoint find_valnum_num (v: valnum) (eqs: list equation)
{struct eqs} : option rhs :=
match eqs with
| nil => None
| Eq v' str r' :: eqs1 =>
if str && peq v v' then Some r' else find_valnum_num v eqs1
end.

reg_valnum n vn returns a register that is mapped to value number vn, or None if no such register exists.

Definition reg_valnum (n: numbering) (vn: valnum) : option reg :=
match PMap.get vn n.(num_val) with
| nil => None
| r :: rs => Some r
end.

regs_valnums is similar, for a list of value numbers.

Fixpoint regs_valnums (n: numbering) (vl: list valnum) : option (list reg) :=
match vl with
| nil => Some nil
| v1 :: vs =>
match reg_valnum n v1, regs_valnums n vs with
| Some r1, Some rs => Some (r1 :: rs)
| _, _ => None
end
end.

find_rhs return a register that already holds the result of the given arithmetic operation or memory load, or a value more defined than this result, according to the given numbering. None is returned if no such register exists.

Definition find_rhs (n: numbering) (rh: rhs) : option reg :=
match find_valnum_rhs' rh n.(num_eqs) with
| None => None
| Some vres => reg_valnum n vres
end.

Update the num_val mapping prior to a redefinition of register r.

Definition forget_reg (n: numbering) (rd: reg) : PMap.t (list reg) :=
match PTree.get rd n.(num_reg) with
| None => n.(num_val)
| Some v => PMap.set v (List.remove peq rd (PMap.get v n.(num_val))) n.(num_val)
end.

Definition update_reg (n: numbering) (rd: reg) (vn: valnum) : PMap.t (list reg) :=
let nv := forget_reg n rd in PMap.set vn (rd :: PMap.get vn nv) nv.

add_rhs n rd rhs updates the value numbering n to reflect the computation of the operation or load represented by rhs and the storing of the result in register rd. If an equation vn = rhs is known, register rd is set to vn. Otherwise, a fresh value number vn is generated and associated with rd, and the equation vn = rhs is added.

Definition add_rhs (n: numbering) (rd: reg) (rh: rhs) : numbering :=
match find_valnum_rhs rh n.(num_eqs) with
| Some vres =>
{| num_next := n.(num_next);
num_eqs := n.(num_eqs);
num_reg := PTree.set rd vres n.(num_reg);
num_val := update_reg n rd vres |}
| None =>
{| num_next := Pos.succ n.(num_next);
num_eqs := Eq n.(num_next) true rh :: n.(num_eqs);
num_reg := PTree.set rd n.(num_next) n.(num_reg);
num_val := update_reg n rd n.(num_next) |}
end.

add_op n rd op rs specializes add_rhs for the case of an arithmetic operation. The right-hand side corresponding to op and the value numbers for the argument registers rs is built and added to n as described in add_rhs. If op is a move instruction, we simply assign the value number of the source register to the destination register, since we know that the source and destination registers have exactly the same value. This enables more common subexpressions to be recognized. For instance:
`     z = add(x, y);  u = x; v = add(u, y);`
Since u and x have the same value number, the second add is recognized as computing the same result as the first add, and therefore u and z have the same value number.

Definition add_op (n: numbering) (rd: reg) (op: operation) (rs: list reg) :=
match is_move_operation op rs with
| Some r =>
let (n1, v) := valnum_reg n r in
{| num_next := n1.(num_next);
num_eqs := n1.(num_eqs);
num_reg := PTree.set rd v n1.(num_reg);
num_val := update_reg n1 rd v |}
| None =>
let (n1, vs) := valnum_regs n rs in
add_rhs n1 rd (Op op vs)
end.

(rs: list reg) :=
let (n1, vs) := valnum_regs n rs in

set_unknown n rd returns a numbering where rd is mapped to no value number, and no equations are added. This is useful to model instructions with unpredictable results such as Ibuiltin.

Definition set_unknown (n: numbering) (rd: reg) :=
{| num_next := n.(num_next);
num_eqs := n.(num_eqs);
num_reg := PTree.remove rd n.(num_reg);
num_val := forget_reg n rd |}.

Definition set_res_unknown (n: numbering) (res: builtin_res reg) :=
match res with
| BR r => set_unknown n r
| _ => n
end.

kill_equations pred n remove all equations satisfying predicate pred.

Fixpoint kill_eqs (pred: rhs -> bool) (eqs: list equation) : list equation :=
match eqs with
| nil => nil
| (Eq l strict r) as eq :: rem =>
if pred r then kill_eqs pred rem else eq :: kill_eqs pred rem
end.

Definition kill_equations (pred: rhs -> bool) (n: numbering) : numbering :=
{| num_next := n.(num_next);
num_eqs := kill_eqs pred n.(num_eqs);
num_reg := n.(num_reg);
num_val := n.(num_val) |}.

kill_all_loads n removes all equations involving memory loads, as well as those involving memory-dependent operators. It is used to reflect the effect of a builtin operation, which can change memory in unpredictable ways and potentially invalidate all such equations.

Definition filter_loads (r: rhs) : bool :=
match r with
| Op op _ => op_depends_on_memory op
| Load _ _ _ => true
end.

Definition kill_all_loads (n: numbering) : numbering :=

kill_loads_after_store app n chunk addr args removes all equations involving loads that could be invalidated by a store of quantity chunk at address determined by addr and args. Loads that are disjoint from this store are preserved. Equations involving memory-dependent operators are also removed.

Definition filter_after_store (app: VA.t) (n: numbering) (p: aptr) (sz: Z) (r: rhs) :=
match r with
| Op op vl =>
op_depends_on_memory op
match regs_valnums n vl with
| None => true
| Some rl =>
end
end.

(app: VA.t) (n: numbering)
kill_equations (filter_after_store app n p (size_chunk chunk)) n.

add_store_result n chunk addr rargs rsrc updates the numbering n to reflect the knowledge gained after executing an instruction Istore chunk addr rargs rsrc. An equation vsrc >= Load chunk addr vargs is added, but only if the value of rsrc is known to be normalized with respect to chunk.

Definition store_normalized_range (chunk: memory_chunk) : aval :=
match chunk with
| Mint8signed => Sgn Ptop 8
| Mint8unsigned => Uns Ptop 8
| Mint16signed => Sgn Ptop 16
| Mint16unsigned => Uns Ptop 16
| _ => Vtop
end.

(rargs: list reg) (rsrc: reg) :=
if vincl (avalue app rsrc) (store_normalized_range chunk) then
let (n1, vsrc) := valnum_reg n rsrc in
let (n2, vargs) := valnum_regs n1 rargs in
{| num_next := n2.(num_next);
num_reg := n2.(num_reg);
num_val := n2.(num_val) |}
else n.

kill_loads_after_storebyte n dst sz removes all equations involving loads that could be invalidated by a store of sz bytes starting at address dst. Loads that are disjoint from this store-bytes are preserved. Equations involving memory-dependent operators are also removed.

(app: VA.t) (n: numbering) (dst: aptr) (sz: Z) :=
kill_equations (filter_after_store app n dst sz) n.

add_memcpy app n1 n2 rsrc rdst sz adds equations to n2 that represent the effect of a memcpy block copy operation of sz bytes from the address denoted by rsrc to the address denoted by rdst. n2 is the numbering returned by kill_loads_after_storebytes and n1 is the original numbering before the memcpy operation. Valid equations (found in n1) involving loads within the source area of the memcpy are translated as equations involving loads within the destination area, and added to numbering n2. Currently, we only track memcpy operations between stack locations, as often occur when compiling assignments between local C variables of struct type.

Definition shift_memcpy_eq (src sz delta: Z) (e: equation) :=
match e with
| Eq l strict (Load chunk (Ainstack i) _) =>
let i := Ptrofs.unsigned i in
let j := i + delta in
if zle src i
&& zle (i + size_chunk chunk) (src + sz)
&& zeq (Z.modulo delta (align_chunk chunk)) 0
&& zle 0 j
&& zle j Ptrofs.max_unsigned
then Some(Eq l strict (Load chunk (Ainstack (Ptrofs.repr j)) nil))
else None
| _ => None
end.

Fixpoint add_memcpy_eqs (src sz delta: Z) (eqs1 eqs2: list equation) :=
match eqs1 with
| nil => eqs2
| e :: eqs =>
match shift_memcpy_eq src sz delta e with
| None => add_memcpy_eqs src sz delta eqs eqs2
| Some e' => e' :: add_memcpy_eqs src sz delta eqs eqs2
end
end.

| Stk src, Stk dst =>
{| num_next := n2.(num_next);
num_eqs := add_memcpy_eqs (Ptrofs.unsigned src) sz
(Ptrofs.unsigned dst - Ptrofs.unsigned src)
n1.(num_eqs) n2.(num_eqs);
num_reg := n2.(num_reg);
num_val := n2.(num_val) |}
| _, _ => n2
end.

Take advantage of known equations to select more efficient forms of operations, addressing modes, and conditions.

Section REDUCE.

Variable A: Type.
Variable f: (valnum -> option rhs) -> A -> list valnum -> option (A * list valnum).
Variable n: numbering.

Fixpoint reduce_rec (niter: nat) (op: A) (args: list valnum) : option(A * list reg) :=
match niter with
| O => None
| Datatypes.S niter' =>
match f (fun v => find_valnum_num v n.(num_eqs)) op args with
| None => None
| Some(op', args') =>
match reduce_rec niter' op' args' with
| None =>
match regs_valnums n args' with Some rl => Some(op', rl) | None => None end
| Some _ as res =>
res
end
end
end.

Definition reduce (op: A) (rl: list reg) (vl: list valnum) : A * list reg :=
match reduce_rec 4%nat op vl with
| None => (op, rl)
| Some res => res
end.

End REDUCE.

The static analysis

We now equip the type numbering with a partial order and a greatest element. The partial order is based on entailment: n1 is greater than n2 if n1 is satisfiable whenever n2 is. The greatest element is, of course, the empty numbering (no equations).

Module Numbering.
Definition t := numbering.
Definition ge (n1 n2: numbering) : Prop :=
forall valu ge sp rs m,
numbering_holds valu ge sp rs m n2 ->
numbering_holds valu ge sp rs m n1.
Definition top := empty_numbering.
Lemma top_ge: forall x, ge top x.
Proof.
intros; red; intros. unfold top. apply empty_numbering_holds.
Qed.
Lemma refl_ge: forall x, ge x x.
Proof.
intros; red; auto.
Qed.
End Numbering.

We reuse the solver for forward dataflow inequations based on propagation over extended basic blocks defined in library Kildall.

Module Solver := BBlock_solver(Numbering).

The transfer function for the dataflow analysis returns the numbering ``after'' execution of the instruction at pc, as a function of the numbering ``before''. For Iop and Iload instructions, we add equations or reuse existing value numbers as described for add_op and add_load. For Istore instructions, we forget equations involving memory loads at possibly overlapping locations, then add an equation for loads from the same location stored to. For Icall instructions, we could simply associate a fresh, unconstrained by equations value number to the result register. However, it is often undesirable to eliminate common subexpressions across a function call (there is a risk of increasing too much the register pressure across the call), so we just forget all equations and start afresh with an empty numbering. Finally, for instructions that modify neither registers nor the memory, we keep the numbering unchanged. For builtin invocations Ibuiltin, we have three strategies:
• Forget all equations. This is appropriate for builtins that can be turned into function calls (EF_external, EF_runtime, EF_malloc, EF_free).
• Forget equations involving loads but keep equations over registers. This is appropriate for builtins that can modify memory, e.g. volatile stores, or EF_builtin for unknown builtin functions.
• Keep all equations, taking advantage of the fact that neither memory nor registers are modified. This is appropriate for annotations, volatile loads, and known builtin functions.

Definition transfer (f: function) (approx: PMap.t VA.t) (pc: node) (before: numbering) :=
match f.(fn_code)!pc with
| None => before
| Some i =>
match i with
| Inop s =>
before
| Iop op args res s =>
| Istore chunk addr args src s =>
let app := approx!!pc in
| Icall sig ros args res s =>
empty_numbering
| Itailcall sig ros args =>
empty_numbering
| Ibuiltin ef args res s =>
match ef with
| EF_external _ _ | EF_runtime _ _ | EF_malloc | EF_free | EF_inline_asm _ _ _ =>
empty_numbering
| EF_vstore _ =>
| EF_builtin name sg =>
match lookup_builtin_function name sg with
| Some bf => set_res_unknown before res
| None => set_res_unknown (kill_all_loads before) res
end
| EF_memcpy sz al =>
match args with
| dst :: src :: nil =>
let app := approx!!pc in
let asrc := aaddr_arg app src in
| _ =>
empty_numbering
end
| EF_vload _ | EF_annot _ _ _ | EF_annot_val _ _ _ | EF_debug _ _ _ =>
set_res_unknown before res
end
| Icond cond args ifso ifnot =>
before
| Ijumptable arg tbl =>
before
| Ireturn optarg =>
before
end
end.

The static analysis solves the dataflow inequations implied by the transfer function using the ``extended basic block'' solver, which produces sub-optimal solutions quickly. The result is a mapping from program points to numberings.

Definition analyze (f: RTL.function) (approx: PMap.t VA.t): option (PMap.t numbering) :=
Solver.fixpoint (fn_code f) successors_instr (transfer f approx) f.(fn_entrypoint).

Code transformation

The code transformation is performed instruction by instruction. Iload instructions and non-trivial Iop instructions are turned into move instructions if their result is already available in a register, as indicated by the numbering inferred at that program point. Some operations are so cheap to compute that it is generally not worth reusing their results. These operations are detected by the function is_trivial_op in module Op.

Definition transf_instr (n: numbering) (instr: instruction) :=
match instr with
| Iop op args res s =>
if is_trivial_op op then instr else
let (n1, vl) := valnum_regs n args in
match find_rhs n1 (Op op vl) with
| Some r =>
Iop Omove (r :: nil) res s
| None =>
let (op', args') := reduce _ combine_op n1 op args vl in
Iop op' args' res s
end
let (n1, vl) := valnum_regs n args in
| Some r =>
Iop Omove (r :: nil) dst s
| None =>
end
| Istore chunk addr args src s =>
let (n1, vl) := valnum_regs n args in
Istore chunk addr' args' src s
| Icond cond args s1 s2 =>
let (n1, vl) := valnum_regs n args in
let (cond', args') := reduce _ combine_cond n1 cond args vl in
Icond cond' args' s1 s2
| _ =>
instr
end.

Definition transf_code (approxs: PMap.t numbering) (instrs: code) : code :=
PTree.map (fun pc instr => transf_instr approxs!!pc instr) instrs.

Definition vanalyze := ValueAnalysis.analyze.

Definition transf_function (rm: romem) (f: function) : res function :=
let approx := vanalyze rm f in
match analyze f approx with
| None => Error (msg "CSE failure")
| Some approxs =>
OK(mkfunction
f.(fn_sig)
f.(fn_params)
f.(fn_stacksize)
(transf_code approxs f.(fn_code))
f.(fn_entrypoint))
end.

Definition transf_fundef (rm: romem) (f: fundef) : res fundef :=
AST.transf_partial_fundef (transf_function rm) f.

Definition transf_program (p: program) : res program :=
transform_partial_program (transf_fundef (romem_for p)) p.