eta_exp.h

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#pragma once
 
#include "mim/pass.h"
 
namespace mim {
 
class EtaRed;
 
/// Performs η-expansion:
/// `f -> λx.f x`, if `f` is a Lam with more than one user and does not appear in callee position.
/// This rule is a generalization of critical edge elimination.
/// It gives other Pass%es such as SSAConstr the opportunity to change `f`'s signature (e.g. adding or removingp Var%s).
class EtaExp : public FPPass<EtaExp, Lam> {
public:
    EtaExp(World& world, flags_t annex)
        : FPPass(world, annex) {}
 
    void init(PassMan*) final;
 
    /// @name interface for other passes
    ///@{
    const Proxy* proxy(Lam* lam) { return FPPass<EtaExp, Lam>::proxy(lam->type(), {lam}, 0); }
    void new2old(Lam* new_lam, Lam* old_lam) { new2old_[new_lam] = old_lam; }
    Lam* new2old(Lam* new_lam);
    ///@}
 
    /// @name Lattice
    /// EtaExp uses the following lattice:
    /// ```
    ///       expand_            <-- η-expand non-callee as the Lam occurs more than once.
    ///        /   \                 Don't η-reduce the expansion again.
    ///  Callee     Non_Callee_1 <-- Pos: Either multiple callees, **or** exactly one non-callee are okay.
    ///        \   /
    ///         Bot              <-- Never seen.
    /// ```
    /// * EtaExp::expand_ trackes if a Lam is the top element.
    /// * If it's not within EtaExp::expand_, it's either `Pos` or `Bot`.
    ///   But there is no need to differentiate this any further.
    /// * EtaExp::Pos tracks the particular element at that level and is memorized statefully.
    ///@{
    enum Pos : bool { Callee, Non_Callee_1 };
    static std::string_view pos2str(Pos pos) { return pos == Callee ? "Callee" : "Non_Callee_1"; }
 
    using Data = std::tuple<Def2Def, LamMap<Pos>>;
    auto& old2new() { return data<0>(); }
    auto& pos() { return data<1>(); }
    ///@}
 
private:
    /// @name PassMan hooks
    ///@{
    const Def* rewrite(const Def*) override;
    undo_t analyze(const Proxy*) override;
    undo_t analyze(const Def*) override;
    ///@}
    Lam* eta_exp(Lam*); ///< Helper that peforms the actual η-expansion.
 
    EtaRed* eta_red_;
    LamSet expand_;
    Lam2Lam exp2orig_;
    Lam2Lam new2old_;
    DefMap<DefVec> def2new_ops_;
};
 
} // namespace mim