Changeset 21300d7 for src/ResolvExpr/CandidateFinder.cpp

Timestamp:

Jun 12, 2019, 4:06:37 PM (5 years ago)

Author:

Thierry Delisle <tdelisle@…>

Branches:

ADT, arm-eh, ast-experimental, enum, forall-pointer-decay, jacob/cs343-translation, jenkins-sandbox, master, new-ast, new-ast-unique-expr, pthread-emulation, qualifiedEnum

Children:

462a7c7, d60780c

Parents:

aaeacf4 (diff), 6625727 (diff)
Note: this is a merge changeset, the changes displayed below correspond to the merge itself.
Use the (diff) links above to see all the changes relative to each parent.

Message:

Merge branch 'master' of plg.uwaterloo.ca:software/cfa/cfa-cc

File:

• src/ResolvExpr/CandidateFinder.cpp (modified) (1 diff)

src/ResolvExpr/CandidateFinder.cpp

@@ -16 +16 @@
 #include "CandidateFinder.hpp"
 
+#include <deque>
+#include <iterator>               // for back_inserter
+#include <sstream>
+#include <string>
+#include <unordered_map>
+#include <vector>
+
+#include "Candidate.hpp"
+#include "CompilationState.h"
+#include "Cost.h"
+#include "ExplodedArg.hpp"
+#include "Resolver.h"
+#include "SatisfyAssertions.hpp"
+#include "typeops.h"              // for adjustExprType
+#include "Unify.h"
 #include "AST/Expr.hpp"
+#include "AST/Node.hpp"
+#include "AST/Pass.hpp"
+#include "AST/Print.hpp"
+#include "AST/SymbolTable.hpp"
+#include "AST/Type.hpp"
+#include "SymTab/Mangler.h"
+#include "Tuples/Tuples.h"        // for handleTupleAssignment
+
+#define PRINT( text ) if ( resolvep ) { text }
 
 namespace ResolvExpr {
 
+namespace {
+
+        /// First index is which argument, second is which alternative, third is which exploded element
+        using ExplodedArgs_new = std::deque< std::vector< ExplodedArg > >;
+
+        /// Returns a list of alternatives with the minimum cost in the given list
+        CandidateList findMinCost( const CandidateList & candidates ) {
+                CandidateList out;
+                Cost minCost = Cost::infinity;
+                for ( const CandidateRef & r : candidates ) {
+                        if ( r->cost < minCost ) {
+                                minCost = r->cost;
+                                out.clear();
+                                out.emplace_back( r );
+                        } else if ( r->cost == minCost ) {
+                                out.emplace_back( r );
+                        }
+                }
+                return out;
+        }
+
+        /// Computes conversion cost for a given candidate
+        Cost computeApplicationConversionCost( 
+                const CandidateRef & cand, const ast::SymbolTable & symtab 
+        ) {
+                #warning unimplemented
+                (void)cand; (void)symtab;
+                assert(false);
+                return Cost::infinity;
+        }
+
+        /// Actually visits expressions to find their candidate interpretations
+        struct Finder final : public ast::WithShortCircuiting {
+                CandidateFinder & selfFinder;
+                const ast::SymbolTable & symtab;
+                CandidateList & candidates;
+                const ast::TypeEnvironment & tenv;
+                ast::ptr< ast::Type > & targetType;
+
+                Finder( CandidateFinder & f )
+                : selfFinder( f ), symtab( f.symtab ), candidates( f.candidates ), tenv( f.env ), 
+                  targetType( f.targetType ) {}
+                
+                void previsit( const ast::Node * ) { visit_children = false; }
+
+                /// Convenience to add candidate to list
+                template<typename... Args>
+                void addCandidate( Args &&... args ) {
+                        candidates.emplace_back( new Candidate{ std::forward<Args>( args )... } );
+                }
+
+                void postvisit( const ast::ApplicationExpr * applicationExpr ) {
+                        addCandidate( applicationExpr, tenv );
+                }
+
+                /// Builds a list of candidates for a function, storing them in out
+                void makeFunctionCandidates(
+                        const CandidateRef & func, const ast::FunctionType * funcType, 
+                        const ExplodedArgs_new & args, CandidateList & out
+                ) {
+                        #warning unimplemented
+                        (void)func; (void)funcType; (void)args; (void)out;
+                        assert(false);
+                }
+
+                /// Adds implicit struct-conversions to the alternative list
+                void addAnonConversions( const CandidateRef & cand ) {
+                        #warning unimplemented
+                        (void)cand;
+                        assert(false);
+                }
+
+                void postvisit( const ast::UntypedExpr * untypedExpr ) {
+                        CandidateFinder funcFinder{ symtab, tenv };
+                        funcFinder.find( untypedExpr->func, ResolvMode::withAdjustment() );
+                        // short-circuit if no candidates
+                        if ( funcFinder.candidates.empty() ) return;
+
+                        std::vector< CandidateFinder > argCandidates = 
+                                selfFinder.findSubExprs( untypedExpr->args );
+                        
+                        // take care of possible tuple assignments
+                        // if not tuple assignment, handled as normal function call
+                        Tuples::handleTupleAssignment( selfFinder, untypedExpr, argCandidates );
+
+                        // find function operators
+                        ast::ptr< ast::Expr > opExpr = new ast::NameExpr{ untypedExpr->location, "?()" };
+                        CandidateFinder opFinder{ symtab, tenv };
+                        // okay if there aren't any function operations
+                        opFinder.find( opExpr, ResolvMode::withoutFailFast() );
+                        PRINT(
+                                std::cerr << "known function ops:" << std::endl;
+                                print( std::cerr, opFinder.candidates, 1 );
+                        )
+
+                        // pre-explode arguments
+                        ExplodedArgs_new argExpansions;
+                        for ( const CandidateFinder & args : argCandidates ) {
+                                argExpansions.emplace_back();
+                                auto & argE = argExpansions.back();
+                                for ( const CandidateRef & arg : args ) { argE.emplace_back( *arg, symtab ); }
+                        }
+
+                        // Find function matches
+                        CandidateList found;
+                        SemanticErrorException errors;
+                        for ( CandidateRef & func : funcFinder ) {
+                                try {
+                                        PRINT(
+                                                std::cerr << "working on alternative:" << std::endl;
+                                                print( std::cerr, *func, 2 );
+                                        )
+
+                                        // check if the type is a pointer to function
+                                        const ast::Type * funcResult = func->expr->result->stripReferences();
+                                        if ( auto pointer = dynamic_cast< const ast::PointerType * >( funcResult ) ) {
+                                                if ( auto function = pointer->base.as< ast::FunctionType >() ) {
+                                                        CandidateRef newFunc{ new Candidate{ *func } };
+                                                        newFunc->expr = 
+                                                                referenceToRvalueConversion( newFunc->expr, newFunc->cost );
+                                                        makeFunctionCandidates( newFunc, function, argExpansions, found );
+                                                }
+                                        } else if ( 
+                                                auto inst = dynamic_cast< const ast::TypeInstType * >( funcResult ) 
+                                        ) {
+                                                if ( const ast::EqvClass * clz = func->env.lookup( inst->name ) ) {
+                                                        if ( auto function = clz->bound.as< ast::FunctionType >() ) {
+                                                                CandidateRef newFunc{ new Candidate{ *func } };
+                                                                newFunc->expr = 
+                                                                        referenceToRvalueConversion( newFunc->expr, newFunc->cost );
+                                                                makeFunctionCandidates( newFunc, function, argExpansions, found );
+                                                        }
+                                                }
+                                        }
+                                } catch ( SemanticErrorException & e ) { errors.append( e ); }
+                        }
+
+                        // Find matches on function operators `?()`
+                        if ( ! opFinder.candidates.empty() ) {
+                                // add exploded function alternatives to front of argument list
+                                std::vector< ExplodedArg > funcE;
+                                funcE.reserve( funcFinder.candidates.size() );
+                                for ( const CandidateRef & func : funcFinder ) { 
+                                        funcE.emplace_back( *func, symtab );
+                                }
+                                argExpansions.emplace_front( std::move( funcE ) );
+
+                                for ( const CandidateRef & op : opFinder ) {
+                                        try {
+                                                // check if type is pointer-to-function
+                                                const ast::Type * opResult = op->expr->result->stripReferences();
+                                                if ( auto pointer = dynamic_cast< const ast::PointerType * >( opResult ) ) {
+                                                        if ( auto function = pointer->base.as< ast::FunctionType >() ) {
+                                                                CandidateRef newOp{ new Candidate{ *op} };
+                                                                newOp->expr = 
+                                                                        referenceToRvalueConversion( newOp->expr, newOp->cost );
+                                                                makeFunctionCandidates( newOp, function, argExpansions, found );
+                                                        }
+                                                }
+                                        } catch ( SemanticErrorException & e ) { errors.append( e ); }
+                                }
+                        }
+
+                        // Implement SFINAE; resolution errors are only errors if there aren't any non-error 
+                        // candidates
+                        if ( found.empty() && ! errors.isEmpty() ) { throw errors; }
+
+                        // Compute conversion costs
+                        for ( CandidateRef & withFunc : found ) {
+                                Cost cvtCost = computeApplicationConversionCost( withFunc, symtab );
+
+                                PRINT(
+                                        auto appExpr = withFunc->expr.strict_as< ast::ApplicationExpr >();
+                                        auto pointer = appExpr->func->result.strict_as< ast::PointerType >();
+                                        auto function = pointer->base.strict_as< ast::FunctionType >();
+                                        
+                                        std::cerr << "Case +++++++++++++ " << appExpr->func << std::endl;
+                                        std::cerr << "parameters are:" << std::endl;
+                                        ast::printAll( std::cerr, function->params, 2 );
+                                        std::cerr << "arguments are:" << std::endl;
+                                        ast::printAll( std::cerr, appExpr->args, 2 );
+                                        std::cerr << "bindings are:" << std::endl;
+                                        ast::print( std::cerr, withFunc->env, 2 );
+                                        std::cerr << "cost is: " << withFunc->cost << std::endl;
+                                        std::cerr << "cost of conversion is:" << cvtCost << std::endl;
+                                )
+
+                                if ( cvtCost != Cost::infinity ) {
+                                        withFunc->cvtCost = cvtCost;
+                                        candidates.emplace_back( std::move( withFunc ) );
+                                }
+                        }
+                        found = std::move( candidates );
+
+                        // use a new list so that candidates are not examined by addAnonConversions twice
+                        CandidateList winners = findMinCost( found );
+                        promoteCvtCost( winners );
+
+                        // function may return a struct/union value, in which case we need to add candidates 
+                        // for implicit conversions to each of the anonymous members, which must happen after 
+                        // `findMinCost`, since anon conversions are never the cheapest
+                        for ( const CandidateRef & c : winners ) {
+                                addAnonConversions( c );
+                        }
+                        spliceBegin( candidates, winners );
+
+                        if ( candidates.empty() && targetType && ! targetType->isVoid() ) {
+                                // If resolution is unsuccessful with a target type, try again without, since it 
+                                // will sometimes succeed when it wouldn't with a target type binding.
+                                // For example:
+                                //   forall( otype T ) T & ?[]( T *, ptrdiff_t );
+                                //   const char * x = "hello world";
+                                //   unsigned char ch = x[0];
+                                // Fails with simple return type binding (xxx -- check this!) as follows:
+                                // * T is bound to unsigned char
+                                // * (x: const char *) is unified with unsigned char *, which fails
+                                // xxx -- fix this better
+                                targetType = nullptr;
+                                postvisit( untypedExpr );
+                        }
+                }
+
+                /// true if expression is an lvalue
+                static bool isLvalue( const ast::Expr * x ) {
+                        return x->result && ( x->result->is_lvalue() || x->result.as< ast::ReferenceType >() );
+                }
+
+                void postvisit( const ast::AddressExpr * addressExpr ) {
+                        CandidateFinder finder{ symtab, tenv };
+                        finder.find( addressExpr->arg );
+                        for ( CandidateRef & r : finder.candidates ) {
+                                if ( ! isLvalue( r->expr ) ) continue;
+                                addCandidate( *r, new ast::AddressExpr{ addressExpr->location, r->expr } );
+                        }
+                }
+
+                void postvisit( const ast::LabelAddressExpr * labelExpr ) {
+                        addCandidate( labelExpr, tenv );
+                }
+
+                void postvisit( const ast::CastExpr * castExpr ) {
+                        #warning unimplemented
+                        (void)castExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::VirtualCastExpr * castExpr ) {
+                        assertf( castExpr->result, "Implicit virtual cast targets not yet supported." );
+                        CandidateFinder finder{ symtab, tenv };
+                        // don't prune here, all alternatives guaranteed to have same type
+                        finder.find( castExpr->arg, ResolvMode::withoutPrune() );
+                        for ( CandidateRef & r : finder.candidates ) {
+                                addCandidate( 
+                                        *r, new ast::VirtualCastExpr{ castExpr->location, r->expr, castExpr->result } );
+                        }
+                }
+
+                void postvisit( const ast::UntypedMemberExpr * memberExpr ) {
+                        #warning unimplemented
+                        (void)memberExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::MemberExpr * memberExpr ) {
+                        addCandidate( memberExpr, tenv );
+                }
+
+                void postvisit( const ast::NameExpr * variableExpr ) {
+                        #warning unimplemented
+                        (void)variableExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::VariableExpr * variableExpr ) {
+                        // not sufficient to just pass `variableExpr` here, type might have changed since
+                        // creation
+                        addCandidate( 
+                                new ast::VariableExpr{ variableExpr->location, variableExpr->var }, tenv );
+                }
+
+                void postvisit( const ast::ConstantExpr * constantExpr ) {
+                        addCandidate( constantExpr, tenv );
+                }
+
+                void postvisit( const ast::SizeofExpr * sizeofExpr ) {
+                        #warning unimplemented
+                        (void)sizeofExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::AlignofExpr * alignofExpr ) {
+                        #warning unimplemented
+                        (void)alignofExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::UntypedOffsetofExpr * offsetofExpr ) {
+                        #warning unimplemented
+                        (void)offsetofExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::OffsetofExpr * offsetofExpr ) {
+                        addCandidate( offsetofExpr, tenv );
+                }
+
+                void postvisit( const ast::OffsetPackExpr * offsetPackExpr ) {
+                        addCandidate( offsetPackExpr, tenv );
+                }
+
+                void postvisit( const ast::LogicalExpr * logicalExpr ) {
+                        CandidateFinder finder1{ symtab, tenv };
+                        finder1.find( logicalExpr->arg1, ResolvMode::withAdjustment() );
+                        if ( finder1.candidates.empty() ) return;
+
+                        CandidateFinder finder2{ symtab, tenv };
+                        finder2.find( logicalExpr->arg2, ResolvMode::withAdjustment() );
+                        if ( finder2.candidates.empty() ) return;
+
+                        for ( const CandidateRef & r1 : finder1.candidates ) {
+                                for ( const CandidateRef & r2 : finder2.candidates ) {
+                                        ast::TypeEnvironment env{ r1->env };
+                                        env.simpleCombine( r2->env );
+                                        ast::OpenVarSet open{ r1->open };
+                                        mergeOpenVars( open, r2->open );
+                                        ast::AssertionSet need;
+                                        mergeAssertionSet( need, r1->need );
+                                        mergeAssertionSet( need, r2->need );
+
+                                        addCandidate(
+                                                new ast::LogicalExpr{ 
+                                                        logicalExpr->location, r1->expr, r2->expr, logicalExpr->isAnd },
+                                                std::move( env ), std::move( open ), std::move( need ), 
+                                                r1->cost + r2->cost );
+                                }
+                        }
+                }
+
+                void postvisit( const ast::ConditionalExpr * conditionalExpr ) {
+                        // candidates for condition
+                        CandidateFinder finder1{ symtab, tenv };
+                        finder1.find( conditionalExpr->arg1, ResolvMode::withAdjustment() );
+                        if ( finder1.candidates.empty() ) return;
+
+                        // candidates for true result
+                        CandidateFinder finder2{ symtab, tenv };
+                        finder2.find( conditionalExpr->arg2, ResolvMode::withAdjustment() );
+                        if ( finder2.candidates.empty() ) return;
+
+                        // candidates for false result
+                        CandidateFinder finder3{ symtab, tenv };
+                        finder3.find( conditionalExpr->arg3, ResolvMode::withAdjustment() );
+                        if ( finder3.candidates.empty() ) return;
+
+                        for ( const CandidateRef & r1 : finder1.candidates ) {
+                                for ( const CandidateRef & r2 : finder2.candidates ) {
+                                        for ( const CandidateRef & r3 : finder3.candidates ) {
+                                                ast::TypeEnvironment env{ r1->env };
+                                                env.simpleCombine( r2->env );
+                                                env.simpleCombine( r3->env );
+                                                ast::OpenVarSet open{ r1->open };
+                                                mergeOpenVars( open, r2->open );
+                                                mergeOpenVars( open, r3->open );
+                                                ast::AssertionSet need;
+                                                mergeAssertionSet( need, r1->need );
+                                                mergeAssertionSet( need, r2->need );
+                                                mergeAssertionSet( need, r3->need );
+                                                ast::AssertionSet have;
+
+                                                // unify true and false results, then infer parameters to produce new 
+                                                // candidates
+                                                ast::ptr< ast::Type > common;
+                                                if ( 
+                                                        unify( 
+                                                                r2->expr->result, r3->expr->result, env, need, have, open, symtab, 
+                                                                common ) 
+                                                ) {
+                                                        #warning unimplemented
+                                                        assert(false);
+                                                }
+                                        }
+                                }
+                        }
+                }
+
+                void postvisit( const ast::CommaExpr * commaExpr ) {
+                        ast::TypeEnvironment env{ tenv };
+                        ast::ptr< ast::Expr > arg1 = resolveInVoidContext( commaExpr->arg1, symtab, env );
+                        
+                        CandidateFinder finder2{ symtab, env };
+                        finder2.find( commaExpr->arg2, ResolvMode::withAdjustment() );
+
+                        for ( const CandidateRef & r2 : finder2.candidates ) {
+                                addCandidate( *r2, new ast::CommaExpr{ commaExpr->location, arg1, r2->expr } );
+                        }
+                }
+
+                void postvisit( const ast::ImplicitCopyCtorExpr * ctorExpr ) {
+                        addCandidate( ctorExpr, tenv );
+                }
+
+                void postvisit( const ast::ConstructorExpr * ctorExpr ) {
+                        CandidateFinder finder{ symtab, tenv };
+                        finder.find( ctorExpr->callExpr, ResolvMode::withoutPrune() );
+                        for ( CandidateRef & r : finder.candidates ) {
+                                addCandidate( *r, new ast::ConstructorExpr{ ctorExpr->location, r->expr } );
+                        }
+                }
+
+                void postvisit( const ast::RangeExpr * rangeExpr ) {
+                        // resolve low and high, accept candidates where low and high types unify
+                        CandidateFinder finder1{ symtab, tenv };
+                        finder1.find( rangeExpr->low, ResolvMode::withAdjustment() );
+                        if ( finder1.candidates.empty() ) return;
+
+                        CandidateFinder finder2{ symtab, tenv };
+                        finder2.find( rangeExpr->high, ResolvMode::withAdjustment() );
+                        if ( finder2.candidates.empty() ) return;
+
+                        for ( const CandidateRef & r1 : finder1.candidates ) {
+                                for ( const CandidateRef & r2 : finder2.candidates ) {
+                                        ast::TypeEnvironment env{ r1->env };
+                                        env.simpleCombine( r2->env );
+                                        ast::OpenVarSet open{ r1->open };
+                                        mergeOpenVars( open, r2->open );
+                                        ast::AssertionSet need;
+                                        mergeAssertionSet( need, r1->need );
+                                        mergeAssertionSet( need, r2->need );
+                                        ast::AssertionSet have;
+
+                                        ast::ptr< ast::Type > common;
+                                        if ( 
+                                                unify( 
+                                                        r1->expr->result, r2->expr->result, env, need, have, open, symtab, 
+                                                        common ) 
+                                        ) {
+                                                ast::RangeExpr * newExpr = 
+                                                        new ast::RangeExpr{ rangeExpr->location, r1->expr, r2->expr };
+                                                newExpr->result = common ? common : r1->expr->result;
+                                                
+                                                #warning unimplemented
+                                                assert(false);
+                                        }
+                                }
+                        }
+                }
+
+                void postvisit( const ast::UntypedTupleExpr * tupleExpr ) {
+                        std::vector< CandidateFinder > subCandidates = 
+                                selfFinder.findSubExprs( tupleExpr->exprs );
+                        std::vector< CandidateList > possibilities;
+                        combos( subCandidates.begin(), subCandidates.end(), back_inserter( possibilities ) );
+
+                        for ( const CandidateList & subs : possibilities ) {
+                                std::vector< ast::ptr< ast::Expr > > exprs;
+                                exprs.reserve( subs.size() );
+                                for ( const CandidateRef & sub : subs ) { exprs.emplace_back( sub->expr ); }
+
+                                ast::TypeEnvironment env;
+                                ast::OpenVarSet open;
+                                ast::AssertionSet need;
+                                for ( const CandidateRef & sub : subs ) {
+                                        env.simpleCombine( sub->env );
+                                        mergeOpenVars( open, sub->open );
+                                        mergeAssertionSet( need, sub->need );
+                                }
+
+                                addCandidate(
+                                        new ast::TupleExpr{ tupleExpr->location, std::move( exprs ) }, 
+                                        std::move( env ), std::move( open ), std::move( need ), sumCost( subs ) );
+                        }
+                }
+
+                void postvisit( const ast::TupleExpr * tupleExpr ) {
+                        addCandidate( tupleExpr, tenv );
+                }
+
+                void postvisit( const ast::TupleIndexExpr * tupleExpr ) {
+                        addCandidate( tupleExpr, tenv );
+                }
+
+                void postvisit( const ast::TupleAssignExpr * tupleExpr ) {
+                        addCandidate( tupleExpr, tenv );
+                }
+
+                void postvisit( const ast::UniqueExpr * unqExpr ) {
+                        CandidateFinder finder{ symtab, tenv };
+                        finder.find( unqExpr->expr, ResolvMode::withAdjustment() );
+                        for ( CandidateRef & r : finder.candidates ) {
+                                // ensure that the the id is passed on so that the expressions are "linked"
+                                addCandidate( *r, new ast::UniqueExpr{ unqExpr->location, r->expr, unqExpr->id } );
+                        }
+                }
+
+                void postvisit( const ast::StmtExpr * stmtExpr ) {
+                        #warning unimplemented
+                        (void)stmtExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::UntypedInitExpr * initExpr ) {
+                        #warning unimplemented
+                        (void)initExpr;
+                        assert(false);
+                }
+
+                void postvisit( const ast::InitExpr * ) {
+                        assertf( false, "CandidateFinder should never see a resolved InitExpr." );
+                }
+
+                void postvisit( const ast::DeletedExpr * ) {
+                        assertf( false, "CandidateFinder should never see a DeletedExpr." );
+                }
+
+                void postvisit( const ast::GenericExpr * ) {
+                        assertf( false, "_Generic is not yet supported." );
+                }
+        };
+
+        /// Prunes a list of candidates down to those that have the minimum conversion cost for a given 
+        /// return type. Skips ambiguous candidates.
+        CandidateList pruneCandidates( CandidateList & candidates ) {
+                struct PruneStruct {
+                        CandidateRef candidate;
+                        bool ambiguous;
+
+                        PruneStruct() = default;
+                        PruneStruct( const CandidateRef & c ) : candidate( c ), ambiguous( false ) {}
+                };
+
+                // find lowest-cost candidate for each type
+                std::unordered_map< std::string, PruneStruct > selected;
+                for ( CandidateRef & candidate : candidates ) {
+                        std::string mangleName;
+                        {
+                                ast::ptr< ast::Type > newType = candidate->expr->result;
+                                candidate->env.apply( newType );
+                                mangleName = Mangle::mangle( newType );
+                        }
+
+                        auto found = selected.find( mangleName );
+                        if ( found != selected.end() ) {
+                                if ( candidate->cost < found->second.candidate->cost ) {
+                                        PRINT(
+                                                std::cerr << "cost " << candidate->cost << " beats " 
+                                                        << found->second.candidate->cost << std::endl;
+                                        )
+
+                                        found->second = PruneStruct{ candidate };
+                                } else if ( candidate->cost == found->second.candidate->cost ) {
+                                        // if one of the candidates contains a deleted identifier, can pick the other, 
+                                        // since deleted expressions should not be ambiguous if there is another option 
+                                        // that is at least as good
+                                        if ( findDeletedExpr( candidate->expr ) ) {
+                                                // do nothing
+                                                PRINT( std::cerr << "candidate is deleted" << std::endl; )
+                                        } else if ( findDeletedExpr( found->second.candidate->expr ) ) {
+                                                PRINT( std::cerr << "current is deleted" << std::endl; )
+                                                found->second = PruneStruct{ candidate };
+                                        } else {
+                                                PRINT( std::cerr << "marking ambiguous" << std::endl; )
+                                                found->second.ambiguous = true;
+                                        }
+                                } else {
+                                        PRINT(
+                                                std::cerr << "cost " << candidate->cost << " loses to " 
+                                                        << found->second.candidate->cost << std::endl;
+                                        )
+                                }
+                        } else {
+                                selected.emplace_hint( found, mangleName, candidate );
+                        }
+                }
+
+                // report unambiguous min-cost candidates
+                CandidateList out;
+                for ( auto & target : selected ) {
+                        if ( target.second.ambiguous ) continue;
+
+                        CandidateRef cand = target.second.candidate;
+                        
+                        ast::ptr< ast::Type > newResult = cand->expr->result;
+                        cand->env.applyFree( newResult );
+                        cand->expr = ast::mutate_field(
+                                cand->expr.get(), &ast::Expr::result, std::move( newResult ) );
+                        
+                        out.emplace_back( cand );
+                }
+                return out;
+        }
+
+} // anonymous namespace
+
 void CandidateFinder::find( const ast::Expr * expr, ResolvMode mode ) {
-        #warning unimplemented
-        (void)expr; (void)mode;
-        assert(false);
+        // Find alternatives for expression
+        ast::Pass<Finder> finder{ *this };
+        expr->accept( finder );
+
+        if ( mode.failFast && candidates.empty() ) {
+                SemanticError( expr, "No reasonable alternatives for expression " );
+        }
+
+        if ( mode.satisfyAssns || mode.prune ) {
+                // trim candidates to just those where the assertions are satisfiable
+                // - necessary pre-requisite to pruning
+                CandidateList satisfied;
+                std::vector< std::string > errors;
+                for ( auto & candidate : candidates ) {
+                        satisfyAssertions( *candidate, symtab, satisfied, errors );
+                }
+
+                // fail early if none such
+                if ( mode.failFast && satisfied.empty() ) {
+                        std::ostringstream stream;
+                        stream << "No alternatives with satisfiable assertions for " << expr << "\n";
+                        for ( const auto& err : errors ) {
+                                stream << err;
+                        }
+                        SemanticError( expr->location, stream.str() );
+                }
+
+                // reset candidates
+                candidates = std::move( satisfied );
+        }
+
+        if ( mode.prune ) {
+                // trim candidates to single best one
+                PRINT(
+                        std::cerr << "alternatives before prune:" << std::endl;
+                        print( std::cerr, candidates );
+                )
+
+                CandidateList pruned = pruneCandidates( candidates );
+                
+                if ( mode.failFast && pruned.empty() ) {
+                        std::ostringstream stream;
+                        CandidateList winners = findMinCost( candidates );
+                        stream << "Cannot choose between " << winners.size() << " alternatives for "
+                                "expression\n";
+                        ast::print( stream, expr );
+                        stream << " Alternatives are:\n";
+                        print( stream, winners, 1 );
+                        SemanticError( expr->location, stream.str() );
+                }
+
+                auto oldsize = candidates.size();
+                candidates = std::move( pruned );
+
+                PRINT(
+                        std::cerr << "there are " << oldsize << " alternatives before elimination" << std::endl;
+                )
+                PRINT(
+                        std::cerr << "there are " << candidates.size() << " alternatives after elimination" 
+                                << std::endl;
+                )
+        }
+
+        // adjust types after pruning so that types substituted by pruneAlternatives are correctly 
+        // adjusted
+        if ( mode.adjust ) {
+                for ( CandidateRef & r : candidates ) {
+                        r->expr = ast::mutate_field( 
+                                r->expr.get(), &ast::Expr::result, 
+                                adjustExprType( r->expr->result, r->env, symtab ) );
+                }
+        }
+
+        // Central location to handle gcc extension keyword, etc. for all expressions
+        for ( CandidateRef & r : candidates ) {
+                if ( r->expr->extension != expr->extension ) {
+                        r->expr.get_and_mutate()->extension = expr->extension;
+                }
+        }
+}
+
+std::vector< CandidateFinder > CandidateFinder::findSubExprs( 
+        const std::vector< ast::ptr< ast::Expr > > & xs 
+) {
+        std::vector< CandidateFinder > out;
+
+        for ( const auto & x : xs ) {
+                out.emplace_back( symtab, env );
+                out.back().find( x, ResolvMode::withAdjustment() );
+                
+                PRINT(
+                        std::cerr << "findSubExprs" << std::endl;
+                        print( std::cerr, out.back().candidates );
+                )
+        }
+
+        return out;
 }