source: src/ResolvExpr/ResolveAssertions.cc@ fbecee5

//
// Cforall Version 1.0.0 Copyright (C) 2015 University of Waterloo
//
// The contents of this file are covered under the licence agreement in the
// file "LICENCE" distributed with Cforall.
//
// ResolveAssertions.cc --
//
// Author           : Aaron B. Moss
// Created On       : Fri Oct 05 13:46:00 2018
// Last Modified By : Aaron B. Moss
// Last Modified On : Fri Oct 05 13:46:00 2018
// Update Count     : 1
//

#include "ResolveAssertions.h"

#include <cassert>                // for assertf
#include <list>                   // for list
#include <unordered_map>          // for unordered_map
#include <utility>                // for move
#include <vector>                 // for vector

#include "Alternative.h"          // for Alternative, AssertionItem
#include "Common/FilterCombos.h"  // for filterCombos
#include "Common/utility.h"       // for sort_mins
#include "SymTab/Indexer.h"       // for Indexer
#include "TypeEnvironment.h"      // for TypeEnvironment, etc.
#include "typeops.h"              // for adjustExprType
#include "Unify.h"                // for unify

namespace ResolvExpr {
        /// Unified assertion candidate
        struct AssnCandidate {
                SymTab::Indexer::IdData cdata;  ///< Satisfying declaration
                Type* adjType;                  ///< Satisfying type
                TypeEnvironment env;            ///< Post-unification environment
                AssertionSet have;              ///< Post-unification have-set
                AssertionSet need;              ///< Post-unification need-set
                OpenVarSet openVars;            ///< Post-unification open-var set

                AssnCandidate( const SymTab::Indexer::IdData& cdata, Type* adjType, TypeEnvironment&& env, 
                        AssertionSet&& have, AssertionSet&& need, OpenVarSet&& openVars ) 
                : cdata(cdata), adjType(adjType), env(std::move(env)), have(std::move(have)), 
                        need(std::move(need)), openVars(std::move(openVars)) {}
        };

        /// List of candidate assertion resolutions
        using CandidateList = std::vector<AssnCandidate>;

        /// Reference to single deferred item
        struct DeferRef {
                const DeclarationWithType* decl;
                const AssertionSetValue& info;
                const AssnCandidate& match;
        };

        /// Wrapper for the deferred items from a single assertion resolution.
        /// Acts like indexed list of DeferRef
        struct DeferItem {
                DeclarationWithType* decl;
                AssertionSetValue info;
                CandidateList matches;

                DeferItem( DeclarationWithType* decl, const AssertionSetValue& info, 
                        CandidateList&& matches )
                : decl(decl), info(info), matches(std::move(matches)) {}

                bool empty() const { return matches.empty(); }

                CandidateList::size_type size() const { return matches.size(); }

                DeferRef operator[] ( unsigned i ) const { return { decl, info, matches[i] }; }
        };

        /// List of deferred resolution items
        using DeferList = std::vector<DeferItem>;

        /// Combo iterator that combines candidates into an output list, merging their environments. 
        /// Rejects an appended candidate if the environments cannot be merged.
        class CandidateEnvMerger {
                /// Current list of merged candidates
                std::vector<DeferRef> crnt;
                /// Stack of environments to support backtracking
                std::vector<TypeEnvironment> envs;
                /// Stack of open variables to support backtracking
                std::vector<OpenVarSet> varSets;
                /// Indexer to use for merges
                const SymTab::Indexer& indexer;
        
        public:
                /// The merged environment/open variables and the list of candidates
                struct OutType {
                        TypeEnvironment env;
                        OpenVarSet openVars;
                        std::vector<DeferRef> assns;

                        OutType( const TypeEnvironment& env, const OpenVarSet& openVars, 
                                const std::vector<DeferRef>& assns )
                        : env(env), openVars(openVars), assns(assns) {}
                };

                CandidateEnvMerger( const TypeEnvironment& env, const OpenVarSet& openVars, 
                        const SymTab::Indexer& indexer )
                : crnt(), envs{ env }, varSets{ openVars }, indexer(indexer) {}

                bool append( DeferRef i ) {
                        TypeEnvironment env = envs.back();
                        OpenVarSet openVars = varSets.back();
                        mergeOpenVars( openVars, i.match.openVars );

                        if ( ! env.combine( i.match.env, openVars, indexer ) ) return false;

                        crnt.emplace_back( i );
                        envs.emplace_back( env );
                        varSets.emplace_back( openVars );
                        return true;
                }

                void backtrack() {
                        crnt.pop_back();
                        envs.pop_back();
                        varSets.pop_back();
                }

                OutType finalize() { return { envs.back(), varSets.back(), crnt }; }
        };

        /// Comparator for CandidateEnvMerger outputs that sums their costs and caches the stored 
        /// sums
        struct CandidateCost {
                using Element = CandidateEnvMerger::OutType;
        private:
                using Memo = std::unordered_map<const Element*, Cost>;
                mutable Memo cache;              ///< cache of element costs
                const SymTab::Indexer& indexer;  ///< Indexer for costing

        public:
                CandidateCost( const SymTab::Indexer& indexer ) : indexer(indexer) {}

                /// reports the cost of an element
                Cost get( const Element& x ) const {
                        Memo::const_iterator it = cache.find( &x );
                        if ( it == cache.end() ) {
                                Cost k = Cost::zero;
                                for ( const auto& assn : x.assns ) {
                                        k += computeConversionCost( 
                                                assn.match.adjType, assn.decl->get_type(), indexer, x.env );
                                }
                                it = cache.emplace_hint( it, &x, k );
                        }
                        return it->second;
                }
                
                /// compares elements by cost
                bool operator() ( const Element& a, const Element& b ) const {
                        return get( a ) < get( b );
                }
        };

        /// Flag for state iteration
        enum IterateFlag { IterateState };

        /// State needed to resolve a set of assertions
        struct ResnState {
                Alternative alt;                  ///< Alternative assertion is rooted on
                std::vector<AssertionItem> need;  ///< Assertions to find
                AssertionSet newNeed;             ///< New assertions for current resolutions
                DeferList deferred;               ///< Deferred matches
                SymTab::Indexer& indexer;         ///< Name lookup (depends on previous assertions)

                /// Initial resolution state for an alternative
                ResnState( Alternative& a, SymTab::Indexer& indexer )
                : alt(a), need(), newNeed(), deferred(), indexer(indexer) {
                        need.swap( a.need );
                }

                /// Updated resolution state with new need-list
                ResnState( ResnState&& o, IterateFlag )
                : alt(std::move(o.alt)), need(o.newNeed.begin(), o.newNeed.end()), newNeed(), deferred(), 
                  indexer(o.indexer) {}
        };

        /// Binds a single assertion, updating resolution state
        void bindAssertion( const DeclarationWithType* decl, AssertionSetValue info, Alternative& alt, 
                        AssnCandidate& match ) {
                
                DeclarationWithType* candidate = match.cdata.id;
                assertf( candidate->get_uniqueId(), "Assertion candidate does not have a unique ID: %s", toString( candidate ).c_str() );

                // everything with an empty idChain was pulled in by the current assertion.
                // add current assertion's idChain + current assertion's ID so that the correct 
                // inferParameters can be found.
                for ( auto& a : match.need ) {
                        if ( a.second.idChain.empty() ) {
                                a.second.idChain = info.idChain;
                                a.second.idChain.push_back( decl->get_uniqueId() );
                        }
                }

                Expression* varExpr = match.cdata.combine( alt.cvtCost );
                delete varExpr->result;
                varExpr->result = match.adjType->clone();

                // follow the current assertion's ID chain to find the correct set of inferred parameters 
                // to add the candidate o (i.e. the set of inferred parameters belonging to the entity 
                // which requested the assertion parameter)
                InferredParams* inferParams = &alt.expr->inferParams;
                for ( UniqueId id : info.idChain ) {
                        inferParams = (*inferParams)[ id ].inferParams.get();
                }

                (*inferParams)[ decl->get_uniqueId() ] = ParamEntry{
                                candidate->get_uniqueId(), match.adjType, decl->get_type()->clone(), varExpr };
        }

        /// Adds a captured assertion to the symbol table
        void addToIndexer( AssertionSet &assertSet, SymTab::Indexer &indexer ) {
                for ( AssertionSet::iterator i = assertSet.begin(); i != assertSet.end(); ++i ) {
                        if ( i->second.isUsed ) {
                                indexer.addId( i->first );
                        }
                }
        }

        /// Resolve a single assertion, in context
        bool resolveAssertion( AssertionItem& assn, ResnState& resn ) {
                // skip unused assertions
                if ( ! assn.info.isUsed ) return true;

                // lookup candidates for this assertion
                std::list< SymTab::Indexer::IdData > candidates;
                resn.indexer.lookupId( assn.decl->name, candidates );

                // find the candidates that unify with the desired type
                CandidateList matches;
                for ( const auto& cdata : candidates ) {
                        DeclarationWithType* candidate = cdata.id;

                        // build independent unification context for candidate
                        AssertionSet have, newNeed;
                        TypeEnvironment newEnv{ resn.alt.env };
                        OpenVarSet newOpenVars{ resn.alt.openVars };
                        Type* adjType = candidate->get_type()->clone();
                        adjustExprType( adjType, newEnv, resn.indexer );

                        // keep unifying candidates
                        if ( unify( assn.decl->get_type(), adjType, newEnv, newNeed, have, newOpenVars, 
                                        resn.indexer ) ) {
                                matches.emplace_back( cdata, adjType, std::move(newEnv), std::move(have), 
                                        std::move(newNeed), std::move(newOpenVars) );
                        } else {
                                delete adjType;
                        }
                }

                // break if no suitable assertion
                if ( matches.empty() ) return false;

                // defer if too many suitable assertions
                if ( matches.size() > 1 ) {
                        resn.deferred.emplace_back( assn.decl, assn.info, std::move(matches) );
                        return true;
                }

                // otherwise bind current match in ongoing scope
                AssnCandidate& match = matches.front();
                addToIndexer( match.have, resn.indexer );
                resn.newNeed.insert( match.need.begin(), match.need.end() );
                resn.alt.env = std::move(match.env);
                resn.alt.openVars = std::move(match.openVars);

                bindAssertion( assn.decl, assn.info, resn.alt, match );
                return true;
        }

        ///< Limit to depth of recursion of assertion satisfaction
        static const int recursionLimit = /* 10 */ 4;

        void resolveAssertions( Alternative& alt, const SymTab::Indexer& indexer, AltList& out ) {
                // finish early if no assertions to resolve
                if ( alt.need.empty() ) {
                        out.emplace_back( alt );
                        return;
                }

                // build list of possible resolutions
                using ResnList = std::vector<ResnState>;
                SymTab::Indexer root_indexer{ indexer };
                ResnList resns{ ResnState{ alt, root_indexer } };
                ResnList new_resns{};

                // resolve assertions in breadth-first-order up to a limited number of levels deep
                for ( unsigned level = 0; level < recursionLimit; ++level ) {
                        // scan over all mutually-compatible resolutions
                        for ( auto& resn : resns ) {
                                // make initial pass at matching assertions
                                for ( auto& assn : resn.need ) {
                                        // fail early if any assertion is not resolvable
                                        if ( ! resolveAssertion( assn, resn ) ) goto nextResn;
                                }

                                if ( resn.deferred.empty() ) {
                                        // either add successful match or push back next state
                                        if ( resn.newNeed.empty() ) {
                                                out.emplace_back( resn.alt );
                                        } else {
                                                new_resns.emplace_back( std::move(resn), IterateState );
                                        }
                                } else {
                                        // resolve deferred assertions by mutual compatibility and sort by cost
                                        std::vector<CandidateEnvMerger::OutType> compatible = filterCombos(
                                                resn.deferred, 
                                                CandidateEnvMerger{ resn.alt.env, resn.alt.openVars, resn.indexer } );
                                        auto lmin = sort_mins( compatible.begin(), compatible.end(), 
                                                CandidateCost{resn.indexer} );
                                        
                                        // for ( auto& compat : compatible ) {
                                        for ( auto it = compatible.begin(); it != lmin ; ++it ) {
                                                auto& compat = *it;
                                                ResnState new_resn = resn;

                                                // add compatible assertions to new resolution state
                                                for ( DeferRef r : compat.assns ) {
                                                        AssnCandidate match = r.match;
                                                        addToIndexer( match.have, new_resn.indexer );
                                                        new_resn.newNeed.insert( match.need.begin(), match.need.end() );

                                                        bindAssertion( r.decl, r.info, new_resn.alt, match );
                                                }

                                                // set mutual environment into resolution state
                                                new_resn.alt.env = std::move(compat.env);
                                                new_resn.alt.openVars = std::move(compat.openVars);

                                                // either add sucessful match or push back next state
                                                if ( new_resn.newNeed.empty() ) {
                                                        out.emplace_back( new_resn.alt );
                                                } else {
                                                        new_resns.emplace_back( std::move(new_resn), IterateState );
                                                }
                                        }
                                }
                        nextResn:; }

                        // finish or reset for next round
                        if ( new_resns.empty() ) return;
                        resns.swap( new_resns );
                        new_resns.clear();
                }
                
                // exceeded recursion limit if reaches here
                if ( out.empty() ) {
                        SemanticError( alt.expr->location, "Too many recursive assertions" );
                }
        }
} // namespace ResolvExpr

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// compile-command: "make install" //
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