source: src/ResolvExpr/ResolveAssertions.cc @ bcb311b

//
// 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 <algorithm>                // for sort
#include <cassert>                  // for assertf
#include <list>                     // for list
#include <memory>                   // for unique_ptr
#include <sstream>                  // for ostringstream
#include <string>                   // for string
#include <unordered_map>            // for unordered_map, unordered_multimap
#include <utility>                  // for move
#include <vector>                   // for vector

#include "Alternative.h"            // for Alternative, AssertionItem, AssertionList
#include "Common/FilterCombos.h"    // for filterCombos
#include "Common/Indenter.h"        // for Indenter
#include "Common/utility.h"         // for sort_mins
#include "GenPoly/GenPoly.h"        // for getFunctionType
#include "ResolvExpr/RenameVars.h"  // for renameTyVars
#include "SymTab/Indexer.h"         // for Indexer
#include "SymTab/Mangler.h"         // for Mangler
#include "SynTree/Expression.h"     // for InferredParams
#include "TypeEnvironment.h"        // for TypeEnvironment, etc.
#include "typeops.h"                // for adjustExprType, specCost
#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
                UniqueId resnSlot;              ///< Slot for any recursive assertion IDs

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

        /// 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 {
                const DeclarationWithType* decl;
                const 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;
                        Cost cost;

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

                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:
                const SymTab::Indexer& indexer;  ///< Indexer for costing

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

                /// reports the cost of an element
                Cost get( Element& x ) const {
                        // check cached cost
                        if ( x.cost != Cost::infinity ) return x.cost;

                        // generate cost
                        Cost k = Cost::zero;
                        for ( const auto& assn : x.assns ) {
                                // compute conversion cost from satisfying decl to assertion
                                k += computeConversionCost(
                                        assn.match.adjType, assn.decl->get_type(), indexer, x.env );

                                // mark vars+specialization cost on function-type assertions
                                FunctionType* func = GenPoly::getFunctionType( assn.match.cdata.id->get_type() );
                                if ( ! func ) continue;

                                for ( DeclarationWithType* formal : func->parameters ) {
                                        k.decSpec( specCost( formal->get_type() ) );
                                }
                                k.incVar( func->forall.size() );
                                for ( TypeDecl* td : func->forall ) {
                                        k.decSpec( td->assertions.size() );
                                }
                        }

                        // cache and return
                        x.cost = k;
                        return k;
                }

                /// compares elements by cost
                bool operator() ( Element& a, Element& b ) const {
                        return get( a ) < get( b );
                }
        };

        /// Set of assertion resolutions, grouped by resolution ID
        using InferCache = std::unordered_map< UniqueId, InferredParams >;

        /// Lexicographically-ordered vector of costs
        using CostVec = std::vector< Cost >;

        int compare( const CostVec & a, const CostVec & b ) {
                unsigned i = 0;
                do {
                        // lex-compare where shorter one is less
                        if ( i == a.size() ) {
                                return i == b.size() ? 0 : -1;
                        }
                        if ( i == b.size() /* && i < a.size() */ ) return 1;
                        
                        int c = a[i].compare( b[i] );
                        if ( c != 0 ) return c;
                } while ( ++i );
                assert(!"unreachable");
        }

        bool operator< ( const CostVec & a, const CostVec & b ) { return compare( a, b ) < 0; }

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

        /// State needed to resolve a set of assertions
        struct ResnState {
                Alternative alt;           ///< Alternative assertion is rooted on
                AssertionList need;        ///< Assertions to find
                AssertionSet newNeed;      ///< New assertions for current resolutions
                DeferList deferred;        ///< Deferred matches
                InferCache inferred;       ///< Cache of already-inferred parameters
                CostVec costs;             ///< Costs of recursive assertion satisfaction for disambiguation
                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(), inferred(), costs{ Cost::zero }, 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(),
                  inferred(std::move(o.inferred)), costs(o.costs), indexer(o.indexer) {
                        costs.emplace_back( Cost::zero );
                }
        };

        /// Binds a single assertion, updating resolution state
        void bindAssertion( const DeclarationWithType* decl, AssertionSetValue info, Alternative& alt,
                        AssnCandidate& match, InferCache& inferred ) {

                DeclarationWithType* candidate = match.cdata.id;
                assertf( candidate->get_uniqueId(), "Assertion candidate does not have a unique ID: %s", toString( candidate ).c_str() );

                Expression* varExpr = match.cdata.combine( alt.cvtCost );
                delete varExpr->result;
                varExpr->result = match.adjType->clone();
                if ( match.resnSlot ) { varExpr->resnSlots.push_back( match.resnSlot ); }

                // place newly-inferred assertion in proper place in cache
                inferred[ info.resnSlot ][ decl->get_uniqueId() ] = ParamEntry{
                                candidate->get_uniqueId(), candidate->clone(), match.adjType->clone(), decl->get_type()->clone(),
                                varExpr };
        }

        /// Adds a captured assertion to the symbol table
        void addToIndexer( AssertionSet &assertSet, SymTab::Indexer &indexer ) {
                for ( auto&  i : assertSet ) {
                        if ( i.second.isUsed ) {
                                indexer.addId( i.first );
                        }
                }
        }

        // in AlternativeFinder.cc; unique ID for assertion resolutions
        extern UniqueId globalResnSlot;

        /// 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 );
                        renameTyVars( adjType );

                        // keep unifying candidates
                        if ( unify( assn.decl->get_type(), adjType, newEnv, newNeed, have, newOpenVars,
                                        resn.indexer ) ) {
                                // set up binding slot for recursive assertions
                                UniqueId crntResnSlot = 0;
                                if ( ! newNeed.empty() ) {
                                        crntResnSlot = ++globalResnSlot;
                                        for ( auto& a : newNeed ) {
                                                a.second.resnSlot = crntResnSlot;
                                        }
                                }

                                matches.emplace_back( cdata, adjType, std::move(newEnv), std::move(have),
                                        std::move(newNeed), std::move(newOpenVars), crntResnSlot );
                        } 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, resn.inferred );
                return true;
        }

        /// Associates inferred parameters with an expression
        struct InferMatcher {
                InferCache& inferred;

                InferMatcher( InferCache& inferred ) : inferred( inferred ) {}

                Expression* postmutate( Expression* expr ) {
                        // defer missing inferred parameters until they are hopefully found later
                        std::vector<UniqueId> missingSlots;
                        // place inferred parameters into resolution slots
                        for ( UniqueId slot : expr->resnSlots ) {
                                // fail if no matching parameters found
                                auto it = inferred.find( slot );
                                if ( it == inferred.end() ) {
                                        missingSlots.push_back( slot );
                                        continue;
                                }
                                InferredParams& inferParams = it->second;

                                // place inferred parameters into proper place in expression
                                for ( auto& entry : inferParams ) {
                                        // recurse on inferParams of resolved expressions
                                        entry.second.expr = postmutate( entry.second.expr );
                                        // xxx - look at entry.second.inferParams?
                                        auto res = expr->inferParams.emplace( entry.first, entry.second );
                                        assert(res.second);
                                }
                        }

                        // clear resolution slots and return
                        expr->resnSlots.swap( missingSlots );
                        return expr;
                }
        };

        /// Map of alternative return types to recursive assertion satisfaction costs
        using PruneMap = std::unordered_map<std::string, CostVec>;

        /// Gets the pruning key for an alternative
        std::string pruneKey( const Alternative & alt ) {
                Type* resType = alt.expr->result->clone();
                alt.env.apply( resType );
                std::string resKey = SymTab::Mangler::mangleType( resType );
                delete resType;
                return std::move( resKey );
        }
        
        /// Replace resnSlots with inferParams and add alternative to output list, if meets pruning 
        /// threshold.
        void finalizeAssertions( ResnState& resn, PruneMap & pruneThresholds, AltList& out ) {
                // prune if cheaper alternative for same key has already been generated
                std::string resKey = pruneKey( resn.alt );
                auto it = pruneThresholds.find( resKey );
                if ( it != pruneThresholds.end() ) {
                        if ( it->second < resn.costs ) return;
                } else {
                        pruneThresholds.emplace_hint( it, resKey, resn.costs );
                }

                // replace resolution slots with inferred params, add to output
                PassVisitor<InferMatcher> matcher{ resn.inferred };
                resn.alt.expr = resn.alt.expr->acceptMutator( matcher );
                out.emplace_back( resn.alt );
        }

        /// Limit to depth of recursion of assertion satisfaction
        static const int recursionLimit = 4;
        /// Maximum number of simultaneously-deferred assertions to attempt concurrent satisfaction of
        static const int deferLimit = 10;

        void resolveAssertions( Alternative& alt, const SymTab::Indexer& indexer, AltList& out, std::list<std::string>& errors ) {
                // 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{};
                
                // Pruning thresholds by result type of the output alternatives.
                // Alternatives *should* be generated in sorted order, so no need to retroactively prune
                PruneMap thresholds;

                // 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 ) {
                                // stop this branch if already found a better option
                                auto it = thresholds.find( pruneKey( resn.alt ) );
                                if ( it != thresholds.end() && it->second < resn.costs ) goto nextResn;

                                // make initial pass at matching assertions
                                for ( auto& assn : resn.need ) {
                                        // fail early if any assertion is not resolvable
                                        if ( ! resolveAssertion( assn, resn ) ) {
                                                Indenter tabs{ 3 };
                                                std::ostringstream ss;
                                                ss << tabs << "Unsatisfiable alternative:\n";
                                                resn.alt.print( ss, ++tabs );
                                                ss << (tabs-1) << "Could not satisfy assertion:\n";
                                                assn.decl->print( ss, tabs );

                                                errors.emplace_back( ss.str() );
                                                goto nextResn;
                                        }
                                }

                                if ( resn.deferred.empty() ) {
                                        // either add successful match or push back next state
                                        if ( resn.newNeed.empty() ) {
                                                finalizeAssertions( resn, thresholds, out );
                                        } else {
                                                new_resns.emplace_back( std::move(resn), IterateState );
                                        }
                                } else if ( resn.deferred.size() > deferLimit ) {
                                        // too many deferred assertions to attempt mutual compatibility
                                        Indenter tabs{ 3 };
                                        std::ostringstream ss;
                                        ss << tabs << "Unsatisfiable alternative:\n";
                                        resn.alt.print( ss, ++tabs );
                                        ss << (tabs-1) << "Too many non-unique satisfying assignments for "
                                                "assertions:\n";
                                        for ( const auto& d : resn.deferred ) {
                                                d.decl->print( ss, tabs );
                                        }

                                        errors.emplace_back( ss.str() );
                                        goto nextResn;
                                } else {
                                        // resolve deferred assertions by mutual compatibility
                                        std::vector<CandidateEnvMerger::OutType> compatible = filterCombos(
                                                resn.deferred,
                                                CandidateEnvMerger{ resn.alt.env, resn.alt.openVars, resn.indexer } );
                                        // fail early if no mutually-compatible assertion satisfaction
                                        if ( compatible.empty() ) {
                                                Indenter tabs{ 3 };
                                                std::ostringstream ss;
                                                ss << tabs << "Unsatisfiable alternative:\n";
                                                resn.alt.print( ss, ++tabs );
                                                ss << (tabs-1) << "No mutually-compatible satisfaction for assertions:\n";
                                                for ( const auto& d : resn.deferred ) {
                                                        d.decl->print( ss, tabs );
                                                }

                                                errors.emplace_back( ss.str() );
                                                goto nextResn;
                                        }
                                        // sort by cost for overall pruning
                                        CandidateCost coster{ resn.indexer };
                                        std::sort( compatible.begin(), compatible.end(), coster );

                                        for ( auto& compat : compatible ) {
                                                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, new_resn.inferred );
                                                }

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

                                                // mark cost of this path
                                                new_resn.costs.back() += compat.cost;

                                                // either add sucessful match or push back next state
                                                if ( new_resn.newNeed.empty() ) {
                                                        finalizeAssertions( new_resn, thresholds, out );
                                                } 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

// Local Variables: //
// tab-width: 4 //
// mode: c++ //
// compile-command: "make install" //
// End: //