Library compcert.backend.Tunneling

Branch tunneling (optimization of branches to branches).

Require Import Coqlib.
Require Import Maps.
Require Import UnionFind.
Require Import AST.
Require Import LTL.

Branch tunneling shortens sequences of branches (with no intervening computations) by rewriting the branch and conditional branch instructions so that they jump directly to the end of the branch sequence. For example:

     L1: nop L2;                          L1: nop L3;
     L2; nop L3;               becomes    L2: nop L3;
     L3: instr;                           L3: instr;
     L4: if (cond) goto L1;               L4: if (cond) goto L3;

This optimization can be applied to several of our intermediate languages. We choose to perform it on the LTL language, after register allocation but before code linearization. Register allocation can delete instructions (such as dead computations or useless moves), therefore there are more opportunities for tunneling after allocation than before. Symmetrically, prior tunneling helps linearization to produce better code, e.g. by revealing that some nop instructions are dead code (as the "nop L3" in the example above).

The naive implementation of branch tunneling would replace any branch to a node pc by a branch to the node branch_target f pc, defined as follows:

  branch_target f pc = branch_target f pc'  if f(pc) = nop pc'
                     = pc                   otherwise

However, this definition can fail to terminate if the program can contain loops consisting only of branches, as in

     L1: nop L1;

L1: nop L2;
     L2: nop L1;

Coq warns us of this fact by not accepting the definition of branch_target above.

To handle this problem, we proceed in two passes. The first pass populates a union-find data structure, adding equalities pc = pc´ for every instruction pc: nop pc´ in the function.

Module U := UnionFind.UF(PTree).

Definition record_goto (uf: U.t) (pc: node) (b: bblock) : U.t :=
  match b with
  | Lbranch s :: _ => U.union uf pc s
  | _ => uf
  end.

Definition record_gotos (f: LTL.function) : U.t :=
  PTree.fold record_goto f.(fn_code) U.empty.

The second pass rewrites all LTL instructions, replacing every successor s of every instruction by the canonical representative of its equivalence class in the union-find data structure.

Definition tunnel_instr (uf: U.t) (i: instruction) : instruction :=
  match i with
  | Lbranch s => Lbranch (U.repr uf s)
  | Lcond cond args s1 s2 => Lcond cond args (U.repr uf s1) (U.repr uf s2)
  | Ljumptable arg tbl => Ljumptable arg (List.map (U.repr uf) tbl)
  | _ => i
  end.

Definition tunnel_block (uf: U.t) (b: bblock) : bblock :=
  List.map (tunnel_instr uf) b.

Definition tunnel_function (f: LTL.function) : LTL.function :=
  let uf := record_gotos f in
  mkfunction
    (fn_sig f)
    (fn_stacksize f)
    (PTree.map1 (tunnel_block uf) (fn_code f))
    (U.repr uf (fn_entrypoint f)).

Definition tunnel_fundef (f: LTL.fundef) : LTL.fundef :=
  transf_fundef tunnel_function f.

Definition tunnel_program (p: LTL.program) : LTL.program :=
  transform_program tunnel_fundef p.