src/sksl/SkSLIRGenerator.cpp

/*
 * Copyright 2016 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
#include "SkSLIRGenerator.h"

#include "limits.h"

#include "ast/SkSLASTBoolLiteral.h"
#include "ast/SkSLASTFieldSuffix.h"
#include "ast/SkSLASTFloatLiteral.h"
#include "ast/SkSLASTIndexSuffix.h"
#include "ast/SkSLASTIntLiteral.h"
#include "ir/SkSLBinaryExpression.h"
#include "ir/SkSLBoolLiteral.h"
#include "ir/SkSLBreakStatement.h"
#include "ir/SkSLConstructor.h"
#include "ir/SkSLContinueStatement.h"
#include "ir/SkSLDiscardStatement.h"
#include "ir/SkSLDoStatement.h"
#include "ir/SkSLExpressionStatement.h"
#include "ir/SkSLField.h"
#include "ir/SkSLFieldAccess.h"
#include "ir/SkSLFloatLiteral.h"
#include "ir/SkSLForStatement.h"
#include "ir/SkSLFunctionCall.h"
#include "ir/SkSLFunctionDeclaration.h"
#include "ir/SkSLFunctionDefinition.h"
#include "ir/SkSLFunctionReference.h"
#include "ir/SkSLIfStatement.h"
#include "ir/SkSLIndexExpression.h"
#include "ir/SkSLInterfaceBlock.h"
#include "ir/SkSLIntLiteral.h"
#include "ir/SkSLLayout.h"
#include "ir/SkSLPostfixExpression.h"
#include "ir/SkSLPrefixExpression.h"
#include "ir/SkSLReturnStatement.h"
#include "ir/SkSLSwizzle.h"
#include "ir/SkSLTernaryExpression.h"
#include "ir/SkSLUnresolvedFunction.h"
#include "ir/SkSLVariable.h"
#include "ir/SkSLVarDeclaration.h"
#include "ir/SkSLVarDeclarationStatement.h"
#include "ir/SkSLVariableReference.h"
#include "ir/SkSLWhileStatement.h"

namespace SkSL {

class AutoSymbolTable {
public:
    AutoSymbolTable(IRGenerator* ir) 
    : fIR(ir)
    , fPrevious(fIR->fSymbolTable) {
        fIR->pushSymbolTable();
    }

    ~AutoSymbolTable() {
        fIR->popSymbolTable();
        ASSERT(fPrevious == fIR->fSymbolTable);
    }

    IRGenerator* fIR;
    std::shared_ptr<SymbolTable> fPrevious;
};

IRGenerator::IRGenerator(const Context* context, std::shared_ptr<SymbolTable> symbolTable, 
                         ErrorReporter& errorReporter)
: fContext(*context)
, fCurrentFunction(nullptr)
, fSymbolTable(std::move(symbolTable))
, fErrors(errorReporter) {}

void IRGenerator::pushSymbolTable() {
    fSymbolTable.reset(new SymbolTable(std::move(fSymbolTable), fErrors));
}

void IRGenerator::popSymbolTable() {
    fSymbolTable = fSymbolTable->fParent;
}

std::unique_ptr<Extension> IRGenerator::convertExtension(const ASTExtension& extension) {
    return std::unique_ptr<Extension>(new Extension(extension.fPosition, extension.fName));
}

std::unique_ptr<Statement> IRGenerator::convertStatement(const ASTStatement& statement) {
    switch (statement.fKind) {
        case ASTStatement::kBlock_Kind:
            return this->convertBlock((ASTBlock&) statement);
        case ASTStatement::kVarDeclaration_Kind:
            return this->convertVarDeclarationStatement((ASTVarDeclarationStatement&) statement);
        case ASTStatement::kExpression_Kind:
            return this->convertExpressionStatement((ASTExpressionStatement&) statement);
        case ASTStatement::kIf_Kind:
            return this->convertIf((ASTIfStatement&) statement);
        case ASTStatement::kFor_Kind:
            return this->convertFor((ASTForStatement&) statement);
        case ASTStatement::kWhile_Kind:
            return this->convertWhile((ASTWhileStatement&) statement);
        case ASTStatement::kDo_Kind:
            return this->convertDo((ASTDoStatement&) statement);
        case ASTStatement::kReturn_Kind:
            return this->convertReturn((ASTReturnStatement&) statement);
        case ASTStatement::kBreak_Kind:
            return this->convertBreak((ASTBreakStatement&) statement);
        case ASTStatement::kContinue_Kind:
            return this->convertContinue((ASTContinueStatement&) statement);
        case ASTStatement::kDiscard_Kind:
            return this->convertDiscard((ASTDiscardStatement&) statement);
        default:
            ABORT("unsupported statement type: %d\n", statement.fKind);
    }
}

std::unique_ptr<Block> IRGenerator::convertBlock(const ASTBlock& block) {
    AutoSymbolTable table(this);
    std::vector<std::unique_ptr<Statement>> statements;
    for (size_t i = 0; i < block.fStatements.size(); i++) {
        std::unique_ptr<Statement> statement = this->convertStatement(*block.fStatements[i]);
        if (!statement) {
            return nullptr;
        }
        statements.push_back(std::move(statement));
    }
    return std::unique_ptr<Block>(new Block(block.fPosition, std::move(statements), fSymbolTable));
}

std::unique_ptr<Statement> IRGenerator::convertVarDeclarationStatement(
                                                              const ASTVarDeclarationStatement& s) {
    auto decl = this->convertVarDeclarations(*s.fDeclarations, Variable::kLocal_Storage);
    if (!decl) {
        return nullptr;
    }
    return std::unique_ptr<Statement>(new VarDeclarationsStatement(std::move(decl)));
}

Modifiers IRGenerator::convertModifiers(const ASTModifiers& modifiers) {
    return Modifiers(modifiers);
}

std::unique_ptr<VarDeclarations> IRGenerator::convertVarDeclarations(const ASTVarDeclarations& decl,
                                                                     Variable::Storage storage) {
    std::vector<VarDeclaration> variables;
    const Type* baseType = this->convertType(*decl.fType);
    if (!baseType) {
        return nullptr;
    }
    for (const auto& varDecl : decl.fVars) {
        Modifiers modifiers = this->convertModifiers(decl.fModifiers);
        const Type* type = baseType;
        ASSERT(type->kind() != Type::kArray_Kind);
        std::vector<std::unique_ptr<Expression>> sizes;
        for (const auto& rawSize : varDecl.fSizes) {
            if (rawSize) {
                auto size = this->coerce(this->convertExpression(*rawSize), *fContext.fInt_Type);
                if (!size) {
                    return nullptr;
                }
                std::string name = type->fName;
                uint64_t count;
                if (size->fKind == Expression::kIntLiteral_Kind) {
                    count = ((IntLiteral&) *size).fValue;
                    if (count <= 0) {
                        fErrors.error(size->fPosition, "array size must be positive");
                    }
                    name += "[" + to_string(count) + "]";
                } else {
                    count = -1;
                    name += "[]";
                }
                type = new Type(name, Type::kArray_Kind, *type, (int) count);
                fSymbolTable->takeOwnership((Type*) type);
                sizes.push_back(std::move(size));
            } else {
                type = new Type(type->fName + "[]", Type::kArray_Kind, *type, -1);
                fSymbolTable->takeOwnership((Type*) type);
                sizes.push_back(nullptr);
            }
        }
        auto var = std::unique_ptr<Variable>(new Variable(decl.fPosition, modifiers, varDecl.fName, 
                                                          *type, storage));
        std::unique_ptr<Expression> value;
        if (varDecl.fValue) {
            value = this->convertExpression(*varDecl.fValue);
            if (!value) {
                return nullptr;
            }
            value = this->coerce(std::move(value), *type);
        }
        if ("sk_FragColor" == varDecl.fName && (*fSymbolTable)[varDecl.fName]) {
            // already defined, ignore
        } else if ((*fSymbolTable)[varDecl.fName] && 
                   (*fSymbolTable)[varDecl.fName]->fKind == Symbol::kVariable_Kind && 
                   ((Variable*) (*fSymbolTable)[varDecl.fName])->fModifiers.fLayout.fBuiltin >= 0) {
            // already defined, just update the modifiers
            Variable* old = (Variable*) (*fSymbolTable)[varDecl.fName];
            old->fModifiers = var->fModifiers;
        } else {
            variables.emplace_back(var.get(), std::move(sizes), std::move(value));
            fSymbolTable->add(varDecl.fName, std::move(var));
        }
    }
    return std::unique_ptr<VarDeclarations>(new VarDeclarations(decl.fPosition, 
                                                                baseType,
                                                                std::move(variables)));
}

std::unique_ptr<ModifiersDeclaration> IRGenerator::convertModifiersDeclaration(
                                                                 const ASTModifiersDeclaration& m) {
    Modifiers modifiers = this->convertModifiers(m.fModifiers);
    return std::unique_ptr<ModifiersDeclaration>(new ModifiersDeclaration(modifiers));
}

std::unique_ptr<Statement> IRGenerator::convertIf(const ASTIfStatement& s) {
    std::unique_ptr<Expression> test = this->coerce(this->convertExpression(*s.fTest), 
                                                    *fContext.fBool_Type);
    if (!test) {
        return nullptr;
    }
    std::unique_ptr<Statement> ifTrue = this->convertStatement(*s.fIfTrue);
    if (!ifTrue) {
        return nullptr;
    }
    std::unique_ptr<Statement> ifFalse;
    if (s.fIfFalse) {
        ifFalse = this->convertStatement(*s.fIfFalse);
        if (!ifFalse) {
            return nullptr;
        }
    }
    return std::unique_ptr<Statement>(new IfStatement(s.fPosition, std::move(test), 
                                                      std::move(ifTrue), std::move(ifFalse)));
}

std::unique_ptr<Statement> IRGenerator::convertFor(const ASTForStatement& f) {
    AutoSymbolTable table(this);
    std::unique_ptr<Statement> initializer = this->convertStatement(*f.fInitializer);
    if (!initializer) {
        return nullptr;
    }
    std::unique_ptr<Expression> test = this->coerce(this->convertExpression(*f.fTest), 
                                                    *fContext.fBool_Type);
    if (!test) {
        return nullptr;
    }
    std::unique_ptr<Expression> next = this->convertExpression(*f.fNext);
    if (!next) {
        return nullptr;
    }
    this->checkValid(*next);
    std::unique_ptr<Statement> statement = this->convertStatement(*f.fStatement);
    if (!statement) {
        return nullptr;
    }
    return std::unique_ptr<Statement>(new ForStatement(f.fPosition, std::move(initializer), 
                                                       std::move(test), std::move(next),
                                                       std::move(statement), fSymbolTable));
}

std::unique_ptr<Statement> IRGenerator::convertWhile(const ASTWhileStatement& w) {
    std::unique_ptr<Expression> test = this->coerce(this->convertExpression(*w.fTest), 
                                                    *fContext.fBool_Type);
    if (!test) {
        return nullptr;
    }
    std::unique_ptr<Statement> statement = this->convertStatement(*w.fStatement);
    if (!statement) {
        return nullptr;
    }
    return std::unique_ptr<Statement>(new WhileStatement(w.fPosition, std::move(test),
                                                         std::move(statement)));
}

std::unique_ptr<Statement> IRGenerator::convertDo(const ASTDoStatement& d) {
    std::unique_ptr<Expression> test = this->coerce(this->convertExpression(*d.fTest),
                                                    *fContext.fBool_Type);
    if (!test) {
        return nullptr;
    }
    std::unique_ptr<Statement> statement = this->convertStatement(*d.fStatement);
    if (!statement) {
        return nullptr;
    }
    return std::unique_ptr<Statement>(new DoStatement(d.fPosition, std::move(statement), 
                                                      std::move(test)));
}

std::unique_ptr<Statement> IRGenerator::convertExpressionStatement(
                                                                  const ASTExpressionStatement& s) {
    std::unique_ptr<Expression> e = this->convertExpression(*s.fExpression);
    if (!e) {
        return nullptr;
    }
    this->checkValid(*e);
    return std::unique_ptr<Statement>(new ExpressionStatement(std::move(e)));
}

std::unique_ptr<Statement> IRGenerator::convertReturn(const ASTReturnStatement& r) {
    ASSERT(fCurrentFunction);
    if (r.fExpression) {
        std::unique_ptr<Expression> result = this->convertExpression(*r.fExpression);
        if (!result) {
            return nullptr;
        }
        if (fCurrentFunction->fReturnType == *fContext.fVoid_Type) {
            fErrors.error(result->fPosition, "may not return a value from a void function");
        } else {
            result = this->coerce(std::move(result), fCurrentFunction->fReturnType);
            if (!result) {
                return nullptr;
            }
        }
        return std::unique_ptr<Statement>(new ReturnStatement(std::move(result)));
    } else {
        if (fCurrentFunction->fReturnType != *fContext.fVoid_Type) {
            fErrors.error(r.fPosition, "expected function to return '" +
                                       fCurrentFunction->fReturnType.description() + "'");
        }
        return std::unique_ptr<Statement>(new ReturnStatement(r.fPosition));
    }
}

std::unique_ptr<Statement> IRGenerator::convertBreak(const ASTBreakStatement& b) {
    return std::unique_ptr<Statement>(new BreakStatement(b.fPosition));
}

std::unique_ptr<Statement> IRGenerator::convertContinue(const ASTContinueStatement& c) {
    return std::unique_ptr<Statement>(new ContinueStatement(c.fPosition));
}

std::unique_ptr<Statement> IRGenerator::convertDiscard(const ASTDiscardStatement& d) {
    return std::unique_ptr<Statement>(new DiscardStatement(d.fPosition));
}

static const Type& expand_generics(const Type& type, int i) {
    if (type.kind() == Type::kGeneric_Kind) {
        return *type.coercibleTypes()[i];
    }
    return type;
}

static void expand_generics(const FunctionDeclaration& decl, 
                            std::shared_ptr<SymbolTable> symbolTable) {
    for (int i = 0; i < 4; i++) {
        const Type& returnType = expand_generics(decl.fReturnType, i);
        std::vector<const Variable*> parameters;
        for (const auto& p : decl.fParameters) {
            Variable* var = new Variable(p->fPosition, Modifiers(p->fModifiers), p->fName,
                                         expand_generics(p->fType, i),
                                         Variable::kParameter_Storage);
            symbolTable->takeOwnership(var);
            parameters.push_back(var);
        }
        symbolTable->add(decl.fName, std::unique_ptr<FunctionDeclaration>(new FunctionDeclaration(
                                                                           decl.fPosition,
                                                                           decl.fName,
                                                                           std::move(parameters),
                                                                           std::move(returnType))));
    }
}

std::unique_ptr<FunctionDefinition> IRGenerator::convertFunction(const ASTFunction& f) {
    bool isGeneric;
    const Type* returnType = this->convertType(*f.fReturnType);
    if (!returnType) {
        return nullptr;
    }
    isGeneric = returnType->kind() == Type::kGeneric_Kind;
    std::vector<const Variable*> parameters;
    for (const auto& param : f.fParameters) {
        const Type* type = this->convertType(*param->fType);
        if (!type) {
            return nullptr;
        }
        for (int j = (int) param->fSizes.size() - 1; j >= 0; j--) {
            int size = param->fSizes[j];
            std::string name = type->name() + "[" + to_string(size) + "]";
            Type* newType = new Type(std::move(name), Type::kArray_Kind, *type, size);
            fSymbolTable->takeOwnership(newType);
            type = newType;
        }
        std::string name = param->fName;
        Modifiers modifiers = this->convertModifiers(param->fModifiers);
        Position pos = param->fPosition;
        Variable* var = new Variable(pos, modifiers, std::move(name), *type,
                                     Variable::kParameter_Storage);
        fSymbolTable->takeOwnership(var);
        parameters.push_back(var);
        isGeneric |= type->kind() == Type::kGeneric_Kind;
    }

    // find existing declaration
    const FunctionDeclaration* decl = nullptr;
    auto entry = (*fSymbolTable)[f.fName];
    if (entry) {
        std::vector<const FunctionDeclaration*> functions;
        switch (entry->fKind) {
            case Symbol::kUnresolvedFunction_Kind:
                functions = ((UnresolvedFunction*) entry)->fFunctions;
                break;
            case Symbol::kFunctionDeclaration_Kind:
                functions.push_back((FunctionDeclaration*) entry);
                break;
            default:
                fErrors.error(f.fPosition, "symbol '" + f.fName + "' was already defined");
                return nullptr;
        }
        for (const auto& other : functions) {
            ASSERT(other->fName == f.fName);
            if (parameters.size() == other->fParameters.size()) {
                bool match = true;
                for (size_t i = 0; i < parameters.size(); i++) {
                    if (parameters[i]->fType != other->fParameters[i]->fType) {
                        match = false;
                        break;
                    }
                }
                if (match) {
                    if (*returnType != other->fReturnType) {
                        FunctionDeclaration newDecl(f.fPosition, f.fName, parameters, *returnType);
                        fErrors.error(f.fPosition, "functions '" + newDecl.description() +
                                                   "' and '" + other->description() + 
                                                   "' differ only in return type");
                        return nullptr;
                    }
                    decl = other;
                    for (size_t i = 0; i < parameters.size(); i++) {
                        if (parameters[i]->fModifiers != other->fParameters[i]->fModifiers) {
                            fErrors.error(f.fPosition, "modifiers on parameter " + 
                                                       to_string((uint64_t) i + 1) + 
                                                       " differ between declaration and "
                                                       "definition");
                            return nullptr;
                        }
                    }
                    if (other->fDefined) {
                        fErrors.error(f.fPosition, "duplicate definition of " + 
                                                   other->description());
                    }
                    break;
                }
            }
        }
    }
    if (!decl) {
        // couldn't find an existing declaration
        if (isGeneric) {
            ASSERT(!f.fBody);
            expand_generics(FunctionDeclaration(f.fPosition, f.fName, parameters, *returnType), 
                            fSymbolTable);
        } else {
            auto newDecl = std::unique_ptr<FunctionDeclaration>(new FunctionDeclaration(
                                                                                     f.fPosition, 
                                                                                     f.fName, 
                                                                                     parameters, 
                                                                                     *returnType));
            decl = newDecl.get();
            fSymbolTable->add(decl->fName, std::move(newDecl));
        }
    }
    if (f.fBody) {
        ASSERT(!fCurrentFunction);
        fCurrentFunction = decl;
        decl->fDefined = true;
        std::shared_ptr<SymbolTable> old = fSymbolTable;
        AutoSymbolTable table(this);
        for (size_t i = 0; i < parameters.size(); i++) {
            fSymbolTable->addWithoutOwnership(parameters[i]->fName, decl->fParameters[i]);
        }
        std::unique_ptr<Block> body = this->convertBlock(*f.fBody);
        fCurrentFunction = nullptr;
        if (!body) {
            return nullptr;
        }
        return std::unique_ptr<FunctionDefinition>(new FunctionDefinition(f.fPosition, *decl, 
                                                                          std::move(body)));
    }
    return nullptr;
}

std::unique_ptr<InterfaceBlock> IRGenerator::convertInterfaceBlock(const ASTInterfaceBlock& intf) {
    std::shared_ptr<SymbolTable> old = fSymbolTable;
    AutoSymbolTable table(this);
    Modifiers mods = this->convertModifiers(intf.fModifiers);
    std::vector<Type::Field> fields;
    for (size_t i = 0; i < intf.fDeclarations.size(); i++) {
        std::unique_ptr<VarDeclarations> decl = this->convertVarDeclarations(
                                                                         *intf.fDeclarations[i], 
                                                                         Variable::kGlobal_Storage);
        for (const auto& var : decl->fVars) {
            fields.push_back(Type::Field(var.fVar->fModifiers, var.fVar->fName, 
                                         &var.fVar->fType));
            if (var.fValue) {
                fErrors.error(decl->fPosition, 
                              "initializers are not permitted on interface block fields");
            }
            if (var.fVar->fModifiers.fFlags & (Modifiers::kIn_Flag |
                                               Modifiers::kOut_Flag |
                                               Modifiers::kUniform_Flag |
                                               Modifiers::kConst_Flag)) {
                fErrors.error(decl->fPosition,
                              "interface block fields may not have storage qualifiers");
            }
        }        
    }
    Type* type = new Type(intf.fInterfaceName, fields);
    fSymbolTable->takeOwnership(type);
    std::string name = intf.fValueName.length() > 0 ? intf.fValueName : intf.fInterfaceName;
    Variable* var = new Variable(intf.fPosition, mods, name, *type, Variable::kGlobal_Storage);
    fSymbolTable->takeOwnership(var);
    if (intf.fValueName.length()) {
        old->addWithoutOwnership(intf.fValueName, var);
    } else {
        for (size_t i = 0; i < fields.size(); i++) {
            old->add(fields[i].fName, std::unique_ptr<Field>(new Field(intf.fPosition, *var, 
                                                                       (int) i)));
        }
    }
    return std::unique_ptr<InterfaceBlock>(new InterfaceBlock(intf.fPosition, *var, fSymbolTable));
}

const Type* IRGenerator::convertType(const ASTType& type) {
    const Symbol* result = (*fSymbolTable)[type.fName];
    if (result && result->fKind == Symbol::kType_Kind) {
        return (const Type*) result;
    }
    fErrors.error(type.fPosition, "unknown type '" + type.fName + "'");
    return nullptr;
}

std::unique_ptr<Expression> IRGenerator::convertExpression(const ASTExpression& expr) {
    switch (expr.fKind) {
        case ASTExpression::kIdentifier_Kind:
            return this->convertIdentifier((ASTIdentifier&) expr);
        case ASTExpression::kBool_Kind:
            return std::unique_ptr<Expression>(new BoolLiteral(fContext, expr.fPosition,
                                                               ((ASTBoolLiteral&) expr).fValue));
        case ASTExpression::kInt_Kind:
            return std::unique_ptr<Expression>(new IntLiteral(fContext, expr.fPosition,
                                                              ((ASTIntLiteral&) expr).fValue));
        case ASTExpression::kFloat_Kind:
            return std::unique_ptr<Expression>(new FloatLiteral(fContext, expr.fPosition,
                                                                ((ASTFloatLiteral&) expr).fValue));
        case ASTExpression::kBinary_Kind:
            return this->convertBinaryExpression((ASTBinaryExpression&) expr);
        case ASTExpression::kPrefix_Kind:
            return this->convertPrefixExpression((ASTPrefixExpression&) expr);
        case ASTExpression::kSuffix_Kind:
            return this->convertSuffixExpression((ASTSuffixExpression&) expr);
        case ASTExpression::kTernary_Kind:
            return this->convertTernaryExpression((ASTTernaryExpression&) expr);
        default:
            ABORT("unsupported expression type: %d\n", expr.fKind);
    }
}

std::unique_ptr<Expression> IRGenerator::convertIdentifier(const ASTIdentifier& identifier) {
    const Symbol* result = (*fSymbolTable)[identifier.fText];
    if (!result) {
        fErrors.error(identifier.fPosition, "unknown identifier '" + identifier.fText + "'");
        return nullptr;
    }
    switch (result->fKind) {
        case Symbol::kFunctionDeclaration_Kind: {
            std::vector<const FunctionDeclaration*> f = {
                (const FunctionDeclaration*) result
            };
            return std::unique_ptr<FunctionReference>(new FunctionReference(fContext,
                                                                            identifier.fPosition,
                                                                            f));
        }
        case Symbol::kUnresolvedFunction_Kind: {
            const UnresolvedFunction* f = (const UnresolvedFunction*) result;
            return std::unique_ptr<FunctionReference>(new FunctionReference(fContext,
                                                                            identifier.fPosition,
                                                                            f->fFunctions));
        }
        case Symbol::kVariable_Kind: {
            const Variable* var = (const Variable*) result;
            this->markReadFrom(*var);
            return std::unique_ptr<VariableReference>(new VariableReference(identifier.fPosition,
                                                                            *var));
        }
        case Symbol::kField_Kind: {
            const Field* field = (const Field*) result;
            VariableReference* base = new VariableReference(identifier.fPosition, field->fOwner);
            return std::unique_ptr<Expression>(new FieldAccess(
                                                  std::unique_ptr<Expression>(base),
                                                  field->fFieldIndex,
                                                  FieldAccess::kAnonymousInterfaceBlock_OwnerKind));
        }
        case Symbol::kType_Kind: {
            const Type* t = (const Type*) result;
            return std::unique_ptr<TypeReference>(new TypeReference(fContext, identifier.fPosition, 
                                                                    *t));
        }
        default:
            ABORT("unsupported symbol type %d\n", result->fKind);
    }

}

std::unique_ptr<Expression> IRGenerator::coerce(std::unique_ptr<Expression> expr, 
                                                const Type& type) {
    if (!expr) {
        return nullptr;
    }
    if (expr->fType == type) {
        return expr;
    }
    this->checkValid(*expr);
    if (expr->fType == *fContext.fInvalid_Type) {
        return nullptr;
    }
    if (!expr->fType.canCoerceTo(type)) {
        fErrors.error(expr->fPosition, "expected '" + type.description() + "', but found '" + 
                                        expr->fType.description() + "'");
        return nullptr;
    }
    if (type.kind() == Type::kScalar_Kind) {
        std::vector<std::unique_ptr<Expression>> args;
        args.push_back(std::move(expr));
        ASTIdentifier id(Position(), type.description());
        std::unique_ptr<Expression> ctor = this->convertIdentifier(id);
        ASSERT(ctor);
        return this->call(Position(), std::move(ctor), std::move(args));
    }
    std::vector<std::unique_ptr<Expression>> args;
    args.push_back(std::move(expr));
    return std::unique_ptr<Expression>(new Constructor(Position(), type, std::move(args)));
}

static bool is_matrix_multiply(const Type& left, const Type& right) {
    if (left.kind() == Type::kMatrix_Kind) {
        return right.kind() == Type::kMatrix_Kind || right.kind() == Type::kVector_Kind;
    }
    return left.kind() == Type::kVector_Kind && right.kind() == Type::kMatrix_Kind;
}
/**
 * Determines the operand and result types of a binary expression. Returns true if the expression is
 * legal, false otherwise. If false, the values of the out parameters are undefined.
 */
static bool determine_binary_type(const Context& context, 
                                  Token::Kind op, 
                                  const Type& left, 
                                  const Type& right, 
                                  const Type** outLeftType,
                                  const Type** outRightType,
                                  const Type** outResultType,
                                  bool tryFlipped) {
    bool isLogical;
    switch (op) {
        case Token::EQEQ: // fall through
        case Token::NEQ:  // fall through
        case Token::LT:   // fall through
        case Token::GT:   // fall through
        case Token::LTEQ: // fall through
        case Token::GTEQ:
            isLogical = true;
            break;
        case Token::LOGICALOR: // fall through
        case Token::LOGICALAND: // fall through
        case Token::LOGICALXOR: // fall through
        case Token::LOGICALOREQ: // fall through
        case Token::LOGICALANDEQ: // fall through
        case Token::LOGICALXOREQ:
            *outLeftType = context.fBool_Type.get();
            *outRightType = context.fBool_Type.get();
            *outResultType = context.fBool_Type.get();
            return left.canCoerceTo(*context.fBool_Type) && 
                   right.canCoerceTo(*context.fBool_Type);
        case Token::STAR: // fall through
        case Token::STAREQ: 
            if (is_matrix_multiply(left, right)) {
                // determine final component type
                if (determine_binary_type(context, Token::STAR, left.componentType(),
                                          right.componentType(), outLeftType, outRightType,
                                          outResultType, false)) {
                    *outLeftType = &(*outResultType)->toCompound(context, left.columns(), 
                                                                 left.rows());;
                    *outRightType = &(*outResultType)->toCompound(context, right.columns(), 
                                                                  right.rows());;
                    int leftColumns = left.columns();
                    int leftRows = left.rows();
                    int rightColumns;
                    int rightRows;
                    if (right.kind() == Type::kVector_Kind) {
                        // matrix * vector treats the vector as a column vector, so we need to
                        // transpose it
                        rightColumns = right.rows();
                        rightRows = right.columns();
                        ASSERT(rightColumns == 1);
                    } else {
                        rightColumns = right.columns();
                        rightRows = right.rows();
                    }
                    if (rightColumns > 1) {
                        *outResultType = &(*outResultType)->toCompound(context, rightColumns,
                                                                       leftRows);
                    } else {
                        // result was a column vector, transpose it back to a row
                        *outResultType = &(*outResultType)->toCompound(context, leftRows,
                                                                       rightColumns);
                    }
                    return leftColumns == rightRows;
                } else {
                    return false;
                }
            }
            // fall through
        default:
            isLogical = false;
    }
    // FIXME: need to disallow illegal operations like vec3 > vec3. Also do not currently have
    // full support for numbers other than float.
    if (left == right) {
        *outLeftType = &left;
        *outRightType = &left;
        if (isLogical) {
            *outResultType = context.fBool_Type.get();
        } else {
            *outResultType = &left;
        }
        return true;
    }
    // FIXME: incorrect for shift operations
    if (left.canCoerceTo(right)) {
        *outLeftType = &right;
        *outRightType = &right;
        if (isLogical) {
            *outResultType = context.fBool_Type.get();
        } else {
            *outResultType = &right;
        }
        return true;
    }
    if ((left.kind() == Type::kVector_Kind || left.kind() == Type::kMatrix_Kind) && 
        (right.kind() == Type::kScalar_Kind)) {
        if (determine_binary_type(context, op, left.componentType(), right, outLeftType, 
                                  outRightType, outResultType, false)) {
            *outLeftType = &(*outLeftType)->toCompound(context, left.columns(), left.rows());
            if (!isLogical) {
                *outResultType = &(*outResultType)->toCompound(context, left.columns(), 
                                                               left.rows());
            }
            return true;
        }
        return false;
    }
    if (tryFlipped) {
        return determine_binary_type(context, op, right, left, outRightType, outLeftType, 
                                     outResultType, false);
    }
    return false;
}

std::unique_ptr<Expression> IRGenerator::convertBinaryExpression(
                                                            const ASTBinaryExpression& expression) {
    std::unique_ptr<Expression> left = this->convertExpression(*expression.fLeft);
    if (!left) {
        return nullptr;
    }
    std::unique_ptr<Expression> right = this->convertExpression(*expression.fRight);
    if (!right) {
        return nullptr;
    }
    const Type* leftType;
    const Type* rightType;
    const Type* resultType;
    if (!determine_binary_type(fContext, expression.fOperator, left->fType, right->fType, &leftType,
                               &rightType, &resultType, true)) {
        fErrors.error(expression.fPosition, "type mismatch: '" + 
                                            Token::OperatorName(expression.fOperator) + 
                                            "' cannot operate on '" + left->fType.fName + 
                                            "', '" + right->fType.fName + "'");
        return nullptr;
    }
    switch (expression.fOperator) {
        case Token::EQ:           // fall through
        case Token::PLUSEQ:       // fall through
        case Token::MINUSEQ:      // fall through
        case Token::STAREQ:       // fall through
        case Token::SLASHEQ:      // fall through
        case Token::PERCENTEQ:    // fall through
        case Token::SHLEQ:        // fall through
        case Token::SHREQ:        // fall through
        case Token::BITWISEOREQ:  // fall through
        case Token::BITWISEXOREQ: // fall through
        case Token::BITWISEANDEQ: // fall through
        case Token::LOGICALOREQ:  // fall through
        case Token::LOGICALXOREQ: // fall through
        case Token::LOGICALANDEQ: 
            this->markWrittenTo(*left);
        default:
            break;
    }
    return std::unique_ptr<Expression>(new BinaryExpression(expression.fPosition, 
                                                            this->coerce(std::move(left), 
                                                                         *leftType), 
                                                            expression.fOperator, 
                                                            this->coerce(std::move(right), 
                                                                         *rightType), 
                                                            *resultType));
}

std::unique_ptr<Expression> IRGenerator::convertTernaryExpression(  
                                                           const ASTTernaryExpression& expression) {
    std::unique_ptr<Expression> test = this->coerce(this->convertExpression(*expression.fTest), 
                                                    *fContext.fBool_Type);
    if (!test) {
        return nullptr;
    }
    std::unique_ptr<Expression> ifTrue = this->convertExpression(*expression.fIfTrue);
    if (!ifTrue) {
        return nullptr;
    }
    std::unique_ptr<Expression> ifFalse = this->convertExpression(*expression.fIfFalse);
    if (!ifFalse) {
        return nullptr;
    }
    const Type* trueType;
    const Type* falseType;
    const Type* resultType;
    if (!determine_binary_type(fContext, Token::EQEQ, ifTrue->fType, ifFalse->fType, &trueType,
                               &falseType, &resultType, true)) {
        fErrors.error(expression.fPosition, "ternary operator result mismatch: '" + 
                                            ifTrue->fType.fName + "', '" + 
                                            ifFalse->fType.fName + "'");
        return nullptr;
    }
    ASSERT(trueType == falseType);
    ifTrue = this->coerce(std::move(ifTrue), *trueType);
    ifFalse = this->coerce(std::move(ifFalse), *falseType);
    return std::unique_ptr<Expression>(new TernaryExpression(expression.fPosition, 
                                                             std::move(test),
                                                             std::move(ifTrue), 
                                                             std::move(ifFalse)));
}

std::unique_ptr<Expression> IRGenerator::call(Position position, 
                                              const FunctionDeclaration& function, 
                                              std::vector<std::unique_ptr<Expression>> arguments) {
    if (function.fParameters.size() != arguments.size()) {
        std::string msg = "call to '" + function.fName + "' expected " + 
                                 to_string((uint64_t) function.fParameters.size()) + 
                                 " argument";
        if (function.fParameters.size() != 1) {
            msg += "s";
        }
        msg += ", but found " + to_string((uint64_t) arguments.size());
        fErrors.error(position, msg);
        return nullptr;
    }
    for (size_t i = 0; i < arguments.size(); i++) {
        arguments[i] = this->coerce(std::move(arguments[i]), function.fParameters[i]->fType);
        if (arguments[i] && (function.fParameters[i]->fModifiers.fFlags & Modifiers::kOut_Flag)) {
            this->markWrittenTo(*arguments[i]);
        }
    }
    return std::unique_ptr<FunctionCall>(new FunctionCall(position, function,
                                                          std::move(arguments)));
}

/**
 * Determines the cost of coercing the arguments of a function to the required types. Returns true 
 * if the cost could be computed, false if the call is not valid. Cost has no particular meaning 
 * other than "lower costs are preferred".
 */
bool IRGenerator::determineCallCost(const FunctionDeclaration& function, 
                                    const std::vector<std::unique_ptr<Expression>>& arguments,
                                    int* outCost) {
    if (function.fParameters.size() != arguments.size()) {
        return false;
    }
    int total = 0;
    for (size_t i = 0; i < arguments.size(); i++) {
        int cost;
        if (arguments[i]->fType.determineCoercionCost(function.fParameters[i]->fType, &cost)) {
            total += cost;
        } else {
            return false;
        }
    }
    *outCost = total;
    return true;
}

std::unique_ptr<Expression> IRGenerator::call(Position position, 
                                              std::unique_ptr<Expression> functionValue, 
                                              std::vector<std::unique_ptr<Expression>> arguments) {
    if (functionValue->fKind == Expression::kTypeReference_Kind) {
        return this->convertConstructor(position, 
                                        ((TypeReference&) *functionValue).fValue, 
                                        std::move(arguments));
    }
    if (functionValue->fKind != Expression::kFunctionReference_Kind) {
        fErrors.error(position, "'" + functionValue->description() + "' is not a function");
        return nullptr;
    }
    FunctionReference* ref = (FunctionReference*) functionValue.get();
    int bestCost = INT_MAX;
    const FunctionDeclaration* best = nullptr;
    if (ref->fFunctions.size() > 1) {
        for (const auto& f : ref->fFunctions) {
            int cost;
            if (this->determineCallCost(*f, arguments, &cost) && cost < bestCost) {
                bestCost = cost;
                best = f;
            }
        }
        if (best) {
            return this->call(position, *best, std::move(arguments));
        }
        std::string msg = "no match for " + ref->fFunctions[0]->fName + "(";
        std::string separator = "";
        for (size_t i = 0; i < arguments.size(); i++) {
            msg += separator;
            separator = ", ";
            msg += arguments[i]->fType.description();
        }
        msg += ")";
        fErrors.error(position, msg);
        return nullptr;
    }
    return this->call(position, *ref->fFunctions[0], std::move(arguments));
}

std::unique_ptr<Expression> IRGenerator::convertConstructor(
                                                    Position position, 
                                                    const Type& type, 
                                                    std::vector<std::unique_ptr<Expression>> args) {
    // FIXME: add support for structs and arrays
    Type::Kind kind = type.kind();
    if (!type.isNumber() && kind != Type::kVector_Kind && kind != Type::kMatrix_Kind && 
        kind != Type::kArray_Kind) {
        fErrors.error(position, "cannot construct '" + type.description() + "'");
        return nullptr;
    }
    if (type == *fContext.fFloat_Type && args.size() == 1 && 
        args[0]->fKind == Expression::kIntLiteral_Kind) {
        int64_t value = ((IntLiteral&) *args[0]).fValue;
        return std::unique_ptr<Expression>(new FloatLiteral(fContext, position, (double) value));
    }
    if (args.size() == 1 && args[0]->fType == type) {
        // argument is already the right type, just return it
        return std::move(args[0]);
    }
    if (type.isNumber()) {
        if (args.size() != 1) {
            fErrors.error(position, "invalid arguments to '" + type.description() + 
                                    "' constructor, (expected exactly 1 argument, but found " +
                                    to_string((uint64_t) args.size()) + ")");
        }
        if (args[0]->fType == *fContext.fBool_Type) {
            std::unique_ptr<IntLiteral> zero(new IntLiteral(fContext, position, 0));
            std::unique_ptr<IntLiteral> one(new IntLiteral(fContext, position, 1));
            return std::unique_ptr<Expression>(
                                         new TernaryExpression(position, std::move(args[0]),
                                                               this->coerce(std::move(one), type),
                                                               this->coerce(std::move(zero), 
                                                                            type)));
        } else if (!args[0]->fType.isNumber()) {
            fErrors.error(position, "invalid argument to '" + type.description() + 
                                    "' constructor (expected a number or bool, but found '" +
                                    args[0]->fType.description() + "')");
        }
        if (args[0]->fKind == Expression::kIntLiteral_Kind && (type == *fContext.fInt_Type || 
            type == *fContext.fUInt_Type)) {
            return std::unique_ptr<Expression>(new IntLiteral(fContext, 
                                                              position, 
                                                              ((IntLiteral&) *args[0]).fValue,
                                                              &type));
        }
    } else if (kind == Type::kArray_Kind) {
        const Type& base = type.componentType();
        for (size_t i = 0; i < args.size(); i++) {
            args[i] = this->coerce(std::move(args[i]), base);
        }
    } else {
        ASSERT(kind == Type::kVector_Kind || kind == Type::kMatrix_Kind);
        int actual = 0;
        for (size_t i = 0; i < args.size(); i++) {
            if (args[i]->fType.kind() == Type::kVector_Kind || 
                args[i]->fType.kind() == Type::kMatrix_Kind) {
                int columns = args[i]->fType.columns();
                int rows = args[i]->fType.rows();
                args[i] = this->coerce(std::move(args[i]), 
                                       type.componentType().toCompound(fContext, columns, rows));
                actual += args[i]->fType.rows() * args[i]->fType.columns();
            } else if (args[i]->fType.kind() == Type::kScalar_Kind) {
                actual += 1;
                if (type.kind() != Type::kScalar_Kind) {
                    args[i] = this->coerce(std::move(args[i]), type.componentType());
                }
            } else {
                fErrors.error(position, "'" + args[i]->fType.description() + "' is not a valid "
                                        "parameter to '" + type.description() + "' constructor");
                return nullptr;
            }
        }
        int min = type.rows() * type.columns();
        int max = type.columns() > 1 ? INT_MAX : min;
        if ((actual < min || actual > max) &&
            !((kind == Type::kVector_Kind || kind == Type::kMatrix_Kind) && (actual == 1))) {
            fErrors.error(position, "invalid arguments to '" + type.description() + 
                                    "' constructor (expected " + to_string(min) + " scalar" + 
                                    (min == 1 ? "" : "s") + ", but found " + to_string(actual) + 
                                    ")");
            return nullptr;
        }
    }
    return std::unique_ptr<Expression>(new Constructor(position, std::move(type), std::move(args)));
}

std::unique_ptr<Expression> IRGenerator::convertPrefixExpression(
                                                            const ASTPrefixExpression& expression) {
    std::unique_ptr<Expression> base = this->convertExpression(*expression.fOperand);
    if (!base) {
        return nullptr;
    }
    switch (expression.fOperator) {
        case Token::PLUS:
            if (!base->fType.isNumber() && base->fType.kind() != Type::kVector_Kind) {
                fErrors.error(expression.fPosition, 
                              "'+' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            return base;
        case Token::MINUS:
            if (!base->fType.isNumber() && base->fType.kind() != Type::kVector_Kind) {
                fErrors.error(expression.fPosition, 
                              "'-' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            if (base->fKind == Expression::kIntLiteral_Kind) {
                return std::unique_ptr<Expression>(new IntLiteral(fContext, base->fPosition,
                                                                  -((IntLiteral&) *base).fValue));
            }
            if (base->fKind == Expression::kFloatLiteral_Kind) {
                double value = -((FloatLiteral&) *base).fValue;
                return std::unique_ptr<Expression>(new FloatLiteral(fContext, base->fPosition, 
                                                                    value));
            }
            return std::unique_ptr<Expression>(new PrefixExpression(Token::MINUS, std::move(base)));
        case Token::PLUSPLUS:
            if (!base->fType.isNumber()) {
                fErrors.error(expression.fPosition, 
                              "'" + Token::OperatorName(expression.fOperator) + 
                              "' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            this->markWrittenTo(*base);
            break;
        case Token::MINUSMINUS: 
            if (!base->fType.isNumber()) {
                fErrors.error(expression.fPosition, 
                              "'" + Token::OperatorName(expression.fOperator) + 
                              "' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            this->markWrittenTo(*base);
            break;
        case Token::LOGICALNOT:
            if (base->fType != *fContext.fBool_Type) {
                fErrors.error(expression.fPosition, 
                              "'" + Token::OperatorName(expression.fOperator) + 
                              "' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            break;
        case Token::BITWISENOT:
            if (base->fType != *fContext.fInt_Type) {
                fErrors.error(expression.fPosition, 
                              "'" + Token::OperatorName(expression.fOperator) + 
                              "' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            break;
        default: 
            ABORT("unsupported prefix operator\n");
    }
    return std::unique_ptr<Expression>(new PrefixExpression(expression.fOperator, 
                                                            std::move(base)));
}

std::unique_ptr<Expression> IRGenerator::convertIndex(std::unique_ptr<Expression> base,
                                                      const ASTExpression& index) {
    if (base->fType.kind() != Type::kArray_Kind && base->fType.kind() != Type::kMatrix_Kind &&
            base->fType.kind() != Type::kVector_Kind) {
        fErrors.error(base->fPosition, "expected array, but found '" + base->fType.description() + 
                                       "'");
        return nullptr;
    }
    std::unique_ptr<Expression> converted = this->convertExpression(index);
    if (!converted) {
        return nullptr;
    }
    if (converted->fType != *fContext.fUInt_Type) {
        converted = this->coerce(std::move(converted), *fContext.fInt_Type);
        if (!converted) {
            return nullptr;
        }
    }
    return std::unique_ptr<Expression>(new IndexExpression(fContext, std::move(base), 
                                                           std::move(converted)));
}

std::unique_ptr<Expression> IRGenerator::convertField(std::unique_ptr<Expression> base,
                                                      const std::string& field) {
    auto fields = base->fType.fields();
    for (size_t i = 0; i < fields.size(); i++) {
        if (fields[i].fName == field) {
            return std::unique_ptr<Expression>(new FieldAccess(std::move(base), (int) i));
        }
    }
    fErrors.error(base->fPosition, "type '" + base->fType.description() + "' does not have a "
                                   "field named '" + field + "");
    return nullptr;
}

std::unique_ptr<Expression> IRGenerator::convertSwizzle(std::unique_ptr<Expression> base,
                                                        const std::string& fields) {
    if (base->fType.kind() != Type::kVector_Kind) {
        fErrors.error(base->fPosition, "cannot swizzle type '" + base->fType.description() + "'");
        return nullptr;
    }
    std::vector<int> swizzleComponents;
    for (char c : fields) {
        switch (c) {
            case 'x': // fall through
            case 'r': // fall through
            case 's': 
                swizzleComponents.push_back(0);
                break;
            case 'y': // fall through
            case 'g': // fall through
            case 't':
                if (base->fType.columns() >= 2) {
                    swizzleComponents.push_back(1);
                    break;
                }
                // fall through
            case 'z': // fall through
            case 'b': // fall through
            case 'p': 
                if (base->fType.columns() >= 3) {
                    swizzleComponents.push_back(2);
                    break;
                }
                // fall through
            case 'w': // fall through
            case 'a': // fall through
            case 'q':
                if (base->fType.columns() >= 4) {
                    swizzleComponents.push_back(3);
                    break;
                }
                // fall through
            default:
                fErrors.error(base->fPosition, "invalid swizzle component '" + std::string(1, c) +
                                               "'");
                return nullptr;
        }
    }
    ASSERT(swizzleComponents.size() > 0);
    if (swizzleComponents.size() > 4) {
        fErrors.error(base->fPosition, "too many components in swizzle mask '" + fields + "'");
        return nullptr;
    }
    return std::unique_ptr<Expression>(new Swizzle(fContext, std::move(base), swizzleComponents));
}

std::unique_ptr<Expression> IRGenerator::convertSuffixExpression(
                                                            const ASTSuffixExpression& expression) {
    std::unique_ptr<Expression> base = this->convertExpression(*expression.fBase);
    if (!base) {
        return nullptr;
    }
    switch (expression.fSuffix->fKind) {
        case ASTSuffix::kIndex_Kind: {
            const ASTExpression* expr = ((ASTIndexSuffix&) *expression.fSuffix).fExpression.get();
            if (expr) {
                return this->convertIndex(std::move(base), *expr);
            } else if (base->fKind == Expression::kTypeReference_Kind) {
                const Type& oldType = ((TypeReference&) *base).fValue;
                Type* newType = new Type(oldType.name() + "[]", Type::kArray_Kind, oldType, 
                                         -1);
                fSymbolTable->takeOwnership(newType);
                return std::unique_ptr<Expression>(new TypeReference(fContext, base->fPosition, 
                                                                     *newType));
            } else {
                fErrors.error(expression.fPosition, "'[]' must follow a type name");
            }
        }
        case ASTSuffix::kCall_Kind: {
            auto rawArguments = &((ASTCallSuffix&) *expression.fSuffix).fArguments;
            std::vector<std::unique_ptr<Expression>> arguments;
            for (size_t i = 0; i < rawArguments->size(); i++) {
                std::unique_ptr<Expression> converted = 
                        this->convertExpression(*(*rawArguments)[i]);
                if (!converted) {
                    return nullptr;
                }
                arguments.push_back(std::move(converted));
            }
            return this->call(expression.fPosition, std::move(base), std::move(arguments));
        }
        case ASTSuffix::kField_Kind: {
            switch (base->fType.kind()) {
                case Type::kVector_Kind:
                    return this->convertSwizzle(std::move(base), 
                                                ((ASTFieldSuffix&) *expression.fSuffix).fField);
                case Type::kStruct_Kind:
                    return this->convertField(std::move(base),
                                              ((ASTFieldSuffix&) *expression.fSuffix).fField);
                default:
                    fErrors.error(base->fPosition, "cannot swizzle value of type '" + 
                                                   base->fType.description() + "'");
                    return nullptr;
            }
        }
        case ASTSuffix::kPostIncrement_Kind:
            if (!base->fType.isNumber()) {
                fErrors.error(expression.fPosition, 
                              "'++' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            this->markWrittenTo(*base);
            return std::unique_ptr<Expression>(new PostfixExpression(std::move(base), 
                                                                     Token::PLUSPLUS));
        case ASTSuffix::kPostDecrement_Kind:
            if (!base->fType.isNumber()) {
                fErrors.error(expression.fPosition, 
                              "'--' cannot operate on '" + base->fType.description() + "'");
                return nullptr;
            }
            this->markWrittenTo(*base);
            return std::unique_ptr<Expression>(new PostfixExpression(std::move(base), 
                                                                     Token::MINUSMINUS));
        default:
            ABORT("unsupported suffix operator");
    }
}

void IRGenerator::checkValid(const Expression& expr) {
    switch (expr.fKind) {
        case Expression::kFunctionReference_Kind:
            fErrors.error(expr.fPosition, "expected '(' to begin function call");
            break;
        case Expression::kTypeReference_Kind:
            fErrors.error(expr.fPosition, "expected '(' to begin constructor invocation");
            break;
        default:
            ASSERT(expr.fType != *fContext.fInvalid_Type);
            break;
    }
}

void IRGenerator::markReadFrom(const Variable& var) {
    var.fIsReadFrom = true;
}

static bool has_duplicates(const Swizzle& swizzle) {
    int bits = 0;
    for (int idx : swizzle.fComponents) {
        ASSERT(idx >= 0 && idx <= 3);
        int bit = 1 << idx;
        if (bits & bit) {
            return true;
        }
        bits |= bit;
    }
    return false;
}

void IRGenerator::markWrittenTo(const Expression& expr) {
    switch (expr.fKind) {
        case Expression::kVariableReference_Kind: {
            const Variable& var = ((VariableReference&) expr).fVariable;
            if (var.fModifiers.fFlags & (Modifiers::kConst_Flag | Modifiers::kUniform_Flag)) {
                fErrors.error(expr.fPosition, 
                              "cannot modify immutable variable '" + var.fName + "'");
            }
            var.fIsWrittenTo = true;
            break;
        }
        case Expression::kFieldAccess_Kind:
            this->markWrittenTo(*((FieldAccess&) expr).fBase);
            break;
        case Expression::kSwizzle_Kind:
            if (has_duplicates((Swizzle&) expr)) {
                fErrors.error(expr.fPosition, 
                              "cannot write to the same swizzle field more than once");
            }
            this->markWrittenTo(*((Swizzle&) expr).fBase);
            break;
        case Expression::kIndex_Kind:
            this->markWrittenTo(*((IndexExpression&) expr).fBase);
            break;
        default:
            fErrors.error(expr.fPosition, "cannot assign to '" + expr.description() + "'");
            break;
    }
}

}