source: src/ResolvExpr/SatisfyAssertions.cpp @ 42a99b0

//
// 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.
//
// SatisfyAssertions.cpp --
//
// Author           : Aaron B. Moss
// Created On       : Mon Jun 10 17:45:00 2019
// Last Modified By : Aaron B. Moss
// Last Modified On : Mon Jun 10 17:45:00 2019
// Update Count     : 1
//

#include "SatisfyAssertions.hpp"

#include <algorithm>
#include <cassert>
#include <sstream>
#include <string>
#include <unordered_map>
#include <vector>

#include "Candidate.hpp"
#include "CandidateFinder.hpp"
#include "Cost.h"
#include "RenameVars.h"
#include "typeops.h"
#include "Unify.h"
#include "AST/Decl.hpp"
#include "AST/Expr.hpp"
#include "AST/Node.hpp"
#include "AST/Pass.hpp"
#include "AST/Print.hpp"
#include "AST/SymbolTable.hpp"
#include "AST/TypeEnvironment.hpp"
#include "Common/FilterCombos.h"
#include "Common/Indenter.h"
#include "GenPoly/GenPoly.h"
#include "SymTab/Mangler.h"

namespace ResolvExpr {

// in CandidateFinder.cpp; unique ID for assertion satisfaction
extern UniqueId globalResnSlot;

namespace {
        /// Post-unification assertion satisfaction candidate
        struct AssnCandidate {
                ast::SymbolTable::IdData cdata;  ///< Satisfying declaration
                ast::ptr< ast::Type > adjType;   ///< Satisfying type
                ast::TypeEnvironment env;        ///< Post-unification environment
                ast::AssertionSet have;          ///< Post-unification have-set
                ast::AssertionSet need;          ///< Post-unification need-set
                ast::OpenVarSet open;            ///< Post-unification open-var-set
                ast::UniqueId resnSlot;          ///< Slot for any recursive assertion IDs

                AssnCandidate( 
                        const ast::SymbolTable::IdData c, const ast::Type * at, ast::TypeEnvironment && e, 
                        ast::AssertionSet && h, ast::AssertionSet && n, ast::OpenVarSet && o, ast::UniqueId rs )
                : cdata( c ), adjType( at ), env( std::move( e ) ), have( std::move( h ) ), 
                  need( std::move( n ) ), open( std::move( o ) ), resnSlot( rs ) {}
        };

        /// List of assertion satisfaction candidates
        using AssnCandidateList = std::vector< AssnCandidate >;

        /// Reference to a single deferred item
        struct DeferRef {
                const ast::DeclWithType * decl;
                const ast::AssertionSetValue & info;
                const AssnCandidate & match;
        };
        
        /// Wrapper for the deferred items from a single assertion satisfaction. 
        /// Acts like an indexed list of DeferRef
        struct DeferItem {
                const ast::DeclWithType * decl;
                const ast::AssertionSetValue & info;
                AssnCandidateList matches;

                DeferItem( 
                        const ast::DeclWithType * d, const ast::AssertionSetValue & i, AssnCandidateList && ms )
                : decl( d ), info( i ), matches( std::move( ms ) ) {}

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

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

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

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

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

        /// Lexicographically-ordered vector of costs.
        /// Lexicographic order comes from default operator< on std::vector.
        using CostVec = std::vector< Cost >;

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

        /// Intermediate state for satisfying a set of assertions
        struct SatState {
                CandidateRef cand;          ///< Candidate assertion is rooted on
                ast::AssertionList need;    ///< Assertions to find
                ast::AssertionSet newNeed;  ///< Recursive assertions from current satisfied assertions
                DeferList deferred;         ///< Deferred matches
                InferCache inferred;        ///< Cache of already-inferred assertions
                CostVec costs;              ///< Disambiguating costs of recursive assertion satisfaction
                ast::SymbolTable symtab;    ///< Name lookup (depends on previous assertions)

                /// Initial satisfaction state for a candidate
                SatState( CandidateRef & c, const ast::SymbolTable & syms )
                : cand( c ), need(), newNeed(), deferred(), inferred(), costs{ Cost::zero }, 
                  symtab( syms ) { need.swap( c->need ); }
                
                /// Update satisfaction state for next step after previous state
                SatState( SatState && o, IterateFlag )
                : cand( std::move( o.cand ) ), need( o.newNeed.begin(), o.newNeed.end() ), newNeed(), 
                  deferred(), inferred( std::move( o.inferred ) ), costs( std::move( o.costs ) ), 
                  symtab( o.symtab ) { costs.emplace_back( Cost::zero ); }
                
                /// Field-wise next step constructor
                SatState(
                        CandidateRef && c, ast::AssertionSet && nn, InferCache && i, CostVec && cs, 
                        ast::SymbolTable && syms )
                : cand( std::move( c ) ), need( nn.begin(), nn.end() ), newNeed(), deferred(), 
                  inferred( std::move( i ) ), costs( std::move( cs ) ), symtab( std::move( syms ) )
                  { costs.emplace_back( Cost::zero ); }
        };

        /// Adds a captured assertion to the symbol table
        void addToSymbolTable( const ast::AssertionSet & have, ast::SymbolTable & symtab ) {
                for ( auto & i : have ) {
                        if ( i.second.isUsed ) { symtab.addId( i.first ); }
                }
        }

        /// Binds a single assertion, updating satisfaction state
        void bindAssertion( 
                const ast::DeclWithType * decl, const ast::AssertionSetValue & info, CandidateRef & cand, 
                AssnCandidate & match, InferCache & inferred
        ) {
                const ast::DeclWithType * candidate = match.cdata.id;
                assertf( candidate->uniqueId, 
                        "Assertion candidate does not have a unique ID: %s", toString( candidate ).c_str() );
                
                ast::Expr * varExpr = match.cdata.combine( cand->expr->location, cand->cvtCost );
                varExpr->result = match.adjType;
                if ( match.resnSlot ) { varExpr->inferred.resnSlots().emplace_back( match.resnSlot ); }

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

        /// Satisfy a single assertion
        bool satisfyAssertion( ast::AssertionList::value_type & assn, SatState & sat ) {
                // skip unused assertions
                if ( ! assn.second.isUsed ) return true;

                // find candidates that unify with the desired type
                AssnCandidateList matches;
                for ( const ast::SymbolTable::IdData & cdata : sat.symtab.lookupId( assn.first->name ) ) {
                        const ast::DeclWithType * candidate = cdata.id;

                        // build independent unification context for candidate
                        ast::AssertionSet have, newNeed;
                        ast::TypeEnvironment newEnv{ sat.cand->env };
                        ast::OpenVarSet newOpen{ sat.cand->open };
                        ast::ptr< ast::Type > toType = assn.first->get_type();
                        ast::ptr< ast::Type > adjType = 
                                renameTyVars( adjustExprType( candidate->get_type(), newEnv, sat.symtab ) );

                        // only keep candidates which unify
                        if ( unify( toType, adjType, newEnv, newNeed, have, newOpen, sat.symtab ) ) {
                                // set up binding slot for recursive assertions
                                ast::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( newNeed ), std::move( have ), 
                                        std::move( newOpen ), crntResnSlot );
                        }
                }

                // break if no satisfying match
                if ( matches.empty() ) return false;

                // defer if too many satisfying matches
                if ( matches.size() > 1 ) {
                        sat.deferred.emplace_back( assn.first, assn.second, std::move( matches ) );
                        return true;
                }

                // otherwise bind unique match in ongoing scope
                AssnCandidate & match = matches.front();
                addToSymbolTable( match.have, sat.symtab );
                sat.newNeed.insert( match.need.begin(), match.need.end() );
                sat.cand->env = std::move( match.env );
                sat.cand->open = std::move( match.open );

                bindAssertion( assn.first, assn.second, sat.cand, match, sat.inferred );
                return true;
        }

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

        /// Gets the pruning key for a candidate (derived from environment-adjusted return type)
        std::string pruneKey( const Candidate & cand ) {
                ast::ptr< ast::Type > resType = cand.expr->result;
                cand.env.apply( resType );
                return Mangle::mangle( resType, Mangle::typeMode() );
        }

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

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

                const ast::Expr * postmutate( const ast::Expr * expr ) {
                        // Skip if no slots to find
                        if ( expr->inferred.mode != ast::Expr::InferUnion::Slots ) return expr;

                        // find inferred parameters for resolution slots
                        ast::InferredParams newInferred;
                        for ( UniqueId slot : expr->inferred.resnSlots() ) {
                                // fail if no matching assertions found
                                auto it = inferred.find( slot );
                                if ( it == inferred.end() ) {
                                        assert(!"missing assertion");
                                }

                                // place inferred parameters into new map
                                for ( auto & entry : it->second ) {
                                        // recurse on inferParams of resolved expressions
                                        entry.second.expr = postmutate( entry.second.expr );
                                        auto res = newInferred.emplace( entry );
                                        assert( res.second && "all assertions newly placed" );
                                }
                        }

                        ast::Expr * ret = mutate( expr );
                        ret->inferred.set_inferParams( std::move( newInferred ) );
                        return ret;
                }
        };

        /// Replace ResnSlots with InferParams and add alternative to output list, if it meets pruning 
        /// threshold.
        void finalizeAssertions( 
                CandidateRef & cand, InferCache & inferred, PruneMap & thresholds, CostVec && costs, 
                CandidateList & out 
        ) {
                // prune if cheaper alternative for same key has already been generated
                std::string key = pruneKey( *cand );
                auto it = thresholds.find( key );
                if ( it != thresholds.end() ) {
                        if ( it->second < costs ) return;
                } else {
                        thresholds.emplace_hint( it, key, std::move( costs ) );
                }

                // replace resolution slots with inferred parameters, add to output
                ast::Pass< InferMatcher > matcher{ inferred };
                cand->expr = cand->expr->accept( matcher );
                out.emplace_back( cand );
        }

        /// Combo iterator that combines candidates into an output list, merging their environments. 
        /// Rejects an appended candidate if environments cannot be merged. See `Common/FilterCombos.h` 
        /// for description of "combo iterator".
        class CandidateEnvMerger {
                /// Current list of merged candidates
                std::vector< DeferRef > crnt;
                /// Stack of environments to support backtracking
                std::vector< ast::TypeEnvironment > envs;
                /// Stack of open variables to support backtracking
                std::vector< ast::OpenVarSet > opens;
                /// Symbol table to use for merges
                const ast::SymbolTable & symtab;

        public:
                /// The merged environment/open variables and the list of candidates
                struct OutType {
                        ast::TypeEnvironment env;
                        ast::OpenVarSet open;
                        std::vector< DeferRef > assns;
                        Cost cost;

                        OutType( 
                                const ast::TypeEnvironment & e, const ast::OpenVarSet & o, 
                                const std::vector< DeferRef > & as, const ast::SymbolTable & symtab )
                        : env( e ), open( o ), assns( as ), cost( Cost::zero ) {
                                // compute combined conversion cost
                                for ( const DeferRef & assn : assns ) {
                                        // compute conversion cost from satisfying decl to assertion
                                        cost += computeConversionCost( 
                                                assn.match.adjType, assn.decl->get_type(), symtab, env );
                                        
                                        // mark vars+specialization on function-type assertions
                                        const ast::FunctionType * func = 
                                                GenPoly::getFunctionType( assn.match.cdata.id->get_type() );
                                        if ( ! func ) continue;

                                        for ( const ast::DeclWithType * param : func->params ) {
                                                cost.decSpec( specCost( param->get_type() ) );
                                        }
                                        
                                        cost.incVar( func->forall.size() );
                                        
                                        for ( const ast::TypeDecl * td : func->forall ) {
                                                cost.decSpec( td->assertions.size() );
                                        }
                                }
                        }

                        bool operator< ( const OutType & o ) const { return cost < o.cost; }
                };

                CandidateEnvMerger( 
                        const ast::TypeEnvironment & env, const ast::OpenVarSet & open, 
                        const ast::SymbolTable & syms )
                : crnt(), envs{ env }, opens{ open }, symtab( syms ) {}

                bool append( DeferRef i ) {
                        ast::TypeEnvironment env = envs.back();
                        ast::OpenVarSet open = opens.back();
                        mergeOpenVars( open, i.match.open );

                        if ( ! env.combine( i.match.env, open, symtab ) ) return false;

                        crnt.emplace_back( i );
                        envs.emplace_back( std::move( env ) );
                        opens.emplace_back( std::move( open ) );
                        return true;
                }

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

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

        /// 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;
} // anonymous namespace

void satisfyAssertions( 
        CandidateRef & cand, const ast::SymbolTable & symtab, CandidateList & out, 
        std::vector<std::string> & errors
) {
        // finish early if no assertions to satisfy
        if ( cand->need.empty() ) {
                out.emplace_back( cand );
                return;
        }

        // build list of possible combinations of satisfying declarations
        std::vector< SatState > sats{ SatState{ cand, symtab } };
        std::vector< SatState > nextSats{};

        // pruning thresholds by result type of output candidates.
        // Candidates *should* be generated in sorted order, so no need to retroactively prune
        PruneMap thresholds;

        // satisfy assertions in breadth-first order over the recursion tree of assertion satisfaction.
        // Stop recursion at a limited number of levels deep to avoid infinite loops.
        for ( unsigned level = 0; level < recursionLimit; ++level ) {
                // for each current mutually-compatible set of assertions
                for ( SatState & sat : sats ) {
                        // stop this branch if a better option is already found
                        auto it = thresholds.find( pruneKey( *sat.cand ) );
                        if ( it != thresholds.end() && it->second < sat.costs ) goto nextSat;

                        // make initial pass at matching assertions
                        for ( auto & assn : sat.need ) {
                                // fail early if any assertion is not satisfiable
                                if ( ! satisfyAssertion( assn, sat ) ) {
                                        Indenter tabs{ 3 };
                                        std::ostringstream ss;
                                        ss << tabs << "Unsatisfiable alternative:\n";
                                        print( ss, *sat.cand, ++tabs );
                                        ss << (tabs-1) << "Could not satisfy assertion:\n";
                                        ast::print( ss, assn.first, tabs );

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

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

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

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

                                // sort by cost (for overall pruning order)
                                std::sort( compatible.begin(), compatible.end() );

                                // process mutually-compatible combinations
                                for ( auto & compat : compatible ) {
                                        // set up next satisfaction state
                                        CandidateRef nextCand = std::make_shared<Candidate>(
                                                sat.cand->expr, std::move( compat.env ), std::move( compat.open ), 
                                                ast::AssertionSet{} /* need moved into satisfaction state */,
                                                sat.cand->cost, sat.cand->cvtCost );

                                        ast::AssertionSet nextNewNeed{ sat.newNeed };
                                        InferCache nextInferred{ sat.inferred };
                                        
                                        CostVec nextCosts{ sat.costs };
                                        nextCosts.back() += compat.cost;
                                                                
                                        ast::SymbolTable nextSymtab{ sat.symtab };

                                        // add compatible assertions to new satisfaction state
                                        for ( DeferRef r : compat.assns ) {
                                                AssnCandidate match = r.match;
                                                addToSymbolTable( match.have, nextSymtab );
                                                nextNewNeed.insert( match.need.begin(), match.need.end() );

                                                bindAssertion( r.decl, r.info, nextCand, match, nextInferred );
                                        }

                                        // either add successful match or push back next state
                                        if ( nextNewNeed.empty() ) {
                                                finalizeAssertions( 
                                                        nextCand, nextInferred, thresholds, std::move( nextCosts ), out );
                                        } else {
                                                nextSats.emplace_back( 
                                                        std::move( nextCand ), std::move( nextNewNeed ), 
                                                        std::move( nextInferred ), std::move( nextCosts ), 
                                                        std::move( nextSymtab ) );
                                        }
                                }
                        }
                nextSat:; }

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

} // namespace ResolvExpr

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