go/backend.h

// backend.h -- Go frontend interface to backend  -*- C++ -*-

// Copyright 2011 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

#ifndef GO_BACKEND_H
#define GO_BACKEND_H

#include <gmp.h>
#include <mpfr.h>
#include <mpc.h>

#include "operator.h"

// Pointers to these types are created by the backend, passed to the
// frontend, and passed back to the backend.  The types must be
// defined by the backend using these names.

// The backend representation of a type.
class Btype;

// The backend represention of an expression.
class Bexpression;

// The backend representation of a statement.
class Bstatement;

// The backend representation of a function definition or declaration.
class Bfunction;

// The backend representation of a block.
class Bblock;

// The backend representation of a variable.
class Bvariable;

// The backend representation of a label.
class Blabel;

// The backend interface.  This is a pure abstract class that a
// specific backend will implement.

class Backend
{
 public:
  virtual ~Backend() { }

  // Name/type/location.  Used for function parameters, struct fields,
  // interface methods.
  struct Btyped_identifier
  {
    std::string name;
    Btype* btype;
    Location location;

    Btyped_identifier()
        : name(), btype(NULL), location(Linemap::unknown_location())
    { }

    Btyped_identifier(const std::string& a_name, Btype* a_btype,
		     Location a_location)
      : name(a_name), btype(a_btype), location(a_location)
    { }
  };

  // Types.

  // Produce an error type.  Actually the backend could probably just
  // crash if this is called.
  virtual Btype*
  error_type() = 0;

  // Get a void type.  This is used in (at least) two ways: 1) as the
  // return type of a function with no result parameters; 2)
  // unsafe.Pointer is represented as *void.
  virtual Btype*
  void_type() = 0;

  // Get the unnamed boolean type.
  virtual Btype*
  bool_type() = 0;

  // Get an unnamed integer type with the given signedness and number
  // of bits.
  virtual Btype*
  integer_type(bool is_unsigned, int bits) = 0;

  // Get an unnamed floating point type with the given number of bits
  // (32 or 64).
  virtual Btype*
  float_type(int bits) = 0;

  // Get an unnamed complex type with the given number of bits (64 or 128).
  virtual Btype*
  complex_type(int bits) = 0;

  // Get a pointer type.
  virtual Btype*
  pointer_type(Btype* to_type) = 0;

  // Get a function type.  The receiver, parameter, and results are
  // generated from the types in the Function_type.  The Function_type
  // is provided so that the names are available.  This should return
  // not the type of a Go function (which is a pointer to a struct)
  // but the type of a C function pointer (which will be used as the
  // type of the first field of the struct).  If there is more than
  // one result, RESULT_STRUCT is a struct type to hold the results,
  // and RESULTS may be ignored; if there are zero or one results,
  // RESULT_STRUCT is NULL.
  virtual Btype*
  function_type(const Btyped_identifier& receiver,
		const std::vector<Btyped_identifier>& parameters,
		const std::vector<Btyped_identifier>& results,
		Btype* result_struct,
		Location location) = 0;

  // Get a struct type.
  virtual Btype*
  struct_type(const std::vector<Btyped_identifier>& fields) = 0;

  // Get an array type.
  virtual Btype*
  array_type(Btype* element_type, Bexpression* length) = 0;

  // Create a placeholder pointer type.  This is used for a named
  // pointer type, since in Go a pointer type may refer to itself.
  // NAME is the name of the type, and the location is where the named
  // type is defined.  This function is also used for unnamed function
  // types with multiple results, in which case the type has no name
  // and NAME will be empty.  FOR_FUNCTION is true if this is for a C
  // pointer to function type.  A Go func type is represented as a
  // pointer to a struct, and the first field of the struct is a C
  // pointer to function.  The return value will later be passed as
  // the first parameter to set_placeholder_pointer_type or
  // set_placeholder_function_type.
  virtual Btype*
  placeholder_pointer_type(const std::string& name, Location,
			   bool for_function) = 0;

  // Fill in a placeholder pointer type as a pointer.  This takes a
  // type returned by placeholder_pointer_type and arranges for it to
  // point to the type that TO_TYPE points to (that is, PLACEHOLDER
  // becomes the same type as TO_TYPE).  Returns true on success,
  // false on failure.
  virtual bool
  set_placeholder_pointer_type(Btype* placeholder, Btype* to_type) = 0;

  // Fill in a placeholder pointer type as a function.  This takes a
  // type returned by placeholder_pointer_type and arranges for it to
  // become a real Go function type (which corresponds to a C/C++
  // pointer to function type).  FT will be something returned by the
  // function_type method.  Returns true on success, false on failure.
  virtual bool
  set_placeholder_function_type(Btype* placeholder, Btype* ft) = 0;

  // Create a placeholder struct type.  This is used for a named
  // struct type, as with placeholder_pointer_type.  It is also used
  // for interface types, in which case NAME will be the empty string.
  virtual Btype*
  placeholder_struct_type(const std::string& name, Location) = 0;

  // Fill in a placeholder struct type.  This takes a type returned by
  // placeholder_struct_type and arranges for it to become a real
  // struct type.  The parameter is as for struct_type.  Returns true
  // on success, false on failure.
  virtual bool
  set_placeholder_struct_type(Btype* placeholder,
			      const std::vector<Btyped_identifier>& fields)
  			= 0;

  // Create a placeholder array type.  This is used for a named array
  // type, as with placeholder_pointer_type, to handle cases like
  // type A []*A.
  virtual Btype*
  placeholder_array_type(const std::string& name, Location) = 0;

  // Fill in a placeholder array type.  This takes a type returned by
  // placeholder_array_type and arranges for it to become a real array
  // type.  The parameters are as for array_type.  Returns true on
  // success, false on failure.
  virtual bool
  set_placeholder_array_type(Btype* placeholder, Btype* element_type,
			     Bexpression* length) = 0;

  // Return a named version of a type.  The location is the location
  // of the type definition.  This will not be called for a type
  // created via placeholder_pointer_type, placeholder_struct_type, or
  // placeholder_array_type..  (It may be called for a pointer,
  // struct, or array type in a case like "type P *byte; type Q P".)
  virtual Btype*
  named_type(const std::string& name, Btype*, Location) = 0;

  // Create a marker for a circular pointer type.  Go pointer and
  // function types can refer to themselves in ways that are not
  // permitted in C/C++.  When a circular type is found, this function
  // is called for the circular reference.  This permits the backend
  // to decide how to handle such a type.  PLACEHOLDER is the
  // placeholder type which has already been created; if the backend
  // is prepared to handle a circular pointer type, it may simply
  // return PLACEHOLDER.  FOR_FUNCTION is true if this is for a
  // function type.
  //
  // For "type P *P" the sequence of calls will be
  //   bt1 = placeholder_pointer_type();
  //   bt2 = circular_pointer_type(bt1, false);
  //   set_placeholder_pointer_type(bt1, bt2);
  virtual Btype*
  circular_pointer_type(Btype* placeholder, bool for_function) = 0;

  // Return whether the argument could be a special type created by
  // circular_pointer_type.  This is used to introduce explicit type
  // conversions where needed.  If circular_pointer_type returns its
  // PLACEHOLDER parameter, this may safely always return false.
  virtual bool
  is_circular_pointer_type(Btype*) = 0;

  // Return the size of a type.
  virtual int64_t
  type_size(Btype*) = 0;

  // Return the alignment of a type.
  virtual int64_t
  type_alignment(Btype*) = 0;

  // Return the alignment of a struct field of this type.  This is
  // normally the same as type_alignment, but not always.
  virtual int64_t
  type_field_alignment(Btype*) = 0;

  // Return the offset of field INDEX in a struct type.  INDEX is the
  // entry in the FIELDS std::vector parameter of struct_type or
  // set_placeholder_struct_type.
  virtual int64_t
  type_field_offset(Btype*, size_t index) = 0;

  // Expressions.

  // Return an expression for a zero value of the given type.  This is
  // used for cases such as local variable initialization and
  // converting nil to other types.
  virtual Bexpression*
  zero_expression(Btype*) = 0;

  // Create an error expression. This is used for cases which should
  // not occur in a correct program, in order to keep the compilation
  // going without crashing.
  virtual Bexpression*
  error_expression() = 0;

  // Create a nil pointer expression.
  virtual Bexpression*
  nil_pointer_expression() = 0;

  // Create a reference to a variable.
  virtual Bexpression*
  var_expression(Bvariable* var, Varexpr_context in_lvalue_pos, Location) = 0;

  // Create an expression that indirects through the pointer expression EXPR
  // (i.e., return the expression for *EXPR). KNOWN_VALID is true if the pointer
  // is known to point to a valid memory location.  BTYPE is the expected type
  // of the indirected EXPR.
  virtual Bexpression*
  indirect_expression(Btype* btype, Bexpression* expr, bool known_valid,
		      Location) = 0;

  // Return an expression that declares a constant named NAME with the
  // constant value VAL in BTYPE.
  virtual Bexpression*
  named_constant_expression(Btype* btype, const std::string& name,
                             Bexpression* val, Location) = 0;

  // Return an expression for the multi-precision integer VAL in BTYPE.
  virtual Bexpression*
  integer_constant_expression(Btype* btype, mpz_t val) = 0;

  // Return an expression for the floating point value VAL in BTYPE.
  virtual Bexpression*
  float_constant_expression(Btype* btype, mpfr_t val) = 0;

  // Return an expression for the complex value VAL in BTYPE.
  virtual Bexpression*
  complex_constant_expression(Btype* btype, mpc_t val) = 0;

  // Return an expression for the string value VAL.
  virtual Bexpression*
  string_constant_expression(const std::string& val) = 0;

  // Return an expression for the boolean value VAL.
  virtual Bexpression*
  boolean_constant_expression(bool val) = 0;

  // Return an expression for the real part of BCOMPLEX.
  virtual Bexpression*
  real_part_expression(Bexpression* bcomplex, Location) = 0;

  // Return an expression for the imaginary part of BCOMPLEX.
  virtual Bexpression*
  imag_part_expression(Bexpression* bcomplex, Location) = 0;

  // Return an expression for the complex number (BREAL, BIMAG).
  virtual Bexpression*
  complex_expression(Bexpression* breal, Bexpression* bimag, Location) = 0;

  // Return an expression that converts EXPR to TYPE.
  virtual Bexpression*
  convert_expression(Btype* type, Bexpression* expr, Location) = 0;

  // Create an expression for the address of a function.  This is used to
  // get the address of the code for a function.
  virtual Bexpression*
  function_code_expression(Bfunction*, Location) = 0;

  // Create an expression that takes the address of an expression.
  virtual Bexpression*
  address_expression(Bexpression*, Location) = 0;

  // Return an expression for the field at INDEX in BSTRUCT.
  virtual Bexpression*
  struct_field_expression(Bexpression* bstruct, size_t index, Location) = 0;

  // Create an expression that executes BSTAT before BEXPR.
  virtual Bexpression*
  compound_expression(Bstatement* bstat, Bexpression* bexpr, Location) = 0;

  // Return an expression that executes THEN_EXPR if CONDITION is true, or
  // ELSE_EXPR otherwise and returns the result as type BTYPE, within the
  // specified function FUNCTION.  ELSE_EXPR may be NULL.  BTYPE may be NULL.
  virtual Bexpression*
  conditional_expression(Bfunction* function, Btype* btype,
                         Bexpression* condition, Bexpression* then_expr,
                         Bexpression* else_expr, Location) = 0;

  // Return an expression for the unary operation OP EXPR.
  // Supported values of OP are (from operators.h):
  //    MINUS, NOT, XOR.
  virtual Bexpression*
  unary_expression(Operator op, Bexpression* expr, Location) = 0;

  // Return an expression for the binary operation LEFT OP RIGHT.
  // Supported values of OP are (from operators.h):
  //    EQEQ, NOTEQ, LT, LE, GT, GE, PLUS, MINUS, OR, XOR, MULT, DIV, MOD,
  //    LSHIFT, RSHIFT, AND, NOT.
  virtual Bexpression*
  binary_expression(Operator op, Bexpression* left, Bexpression* right,
                    Location) = 0;

  // Return an expression that constructs BTYPE with VALS.  BTYPE must be the
  // backend representation a of struct.  VALS must be in the same order as the
  // corresponding fields in BTYPE.
  virtual Bexpression*
  constructor_expression(Btype* btype, const std::vector<Bexpression*>& vals,
                         Location) = 0;

  // Return an expression that constructs an array of BTYPE with INDEXES and
  // VALS.  INDEXES and VALS must have the same amount of elements. Each index
  // in INDEXES must be in the same order as the corresponding value in VALS.
  virtual Bexpression*
  array_constructor_expression(Btype* btype,
                               const std::vector<unsigned long>& indexes,
                               const std::vector<Bexpression*>& vals,
                               Location) = 0;

  // Return an expression for the address of BASE[INDEX].
  // BASE has a pointer type.  This is used for slice indexing.
  virtual Bexpression*
  pointer_offset_expression(Bexpression* base, Bexpression* index,
                            Location) = 0;

  // Return an expression for ARRAY[INDEX] as an l-value.  ARRAY is a valid
  // fixed-length array, not a slice.
  virtual Bexpression*
  array_index_expression(Bexpression* array, Bexpression* index, Location) = 0;

  // Create an expression for a call to FN with ARGS, taking place within
  // caller CALLER.
  virtual Bexpression*
  call_expression(Bfunction *caller, Bexpression* fn,
                  const std::vector<Bexpression*>& args,
		  Bexpression* static_chain, Location) = 0;

  // Return an expression that allocates SIZE bytes on the stack.
  virtual Bexpression*
  stack_allocation_expression(int64_t size, Location) = 0;

  // Statements.

  // Create an error statement.  This is used for cases which should
  // not occur in a correct program, in order to keep the compilation
  // going without crashing.
  virtual Bstatement*
  error_statement() = 0;

  // Create an expression statement within the specified function.
  virtual Bstatement*
  expression_statement(Bfunction*, Bexpression*) = 0;

  // Create a variable initialization statement in the specified
  // function.  This initializes a local variable at the point in the
  // program flow where it is declared.
  virtual Bstatement*
  init_statement(Bfunction*, Bvariable* var, Bexpression* init) = 0;

  // Create an assignment statement within the specified function.
  virtual Bstatement*
  assignment_statement(Bfunction*, Bexpression* lhs, Bexpression* rhs,
		       Location) = 0;

  // Create a return statement, passing the representation of the
  // function and the list of values to return.
  virtual Bstatement*
  return_statement(Bfunction*, const std::vector<Bexpression*>&,
		   Location) = 0;

  // Create an if statement within a function.  ELSE_BLOCK may be NULL.
  virtual Bstatement*
  if_statement(Bfunction*, Bexpression* condition,
               Bblock* then_block, Bblock* else_block,
	       Location) = 0;

  // Create a switch statement where the case values are constants.
  // CASES and STATEMENTS must have the same number of entries.  If
  // VALUE matches any of the list in CASES[i], which will all be
  // integers, then STATEMENTS[i] is executed.  STATEMENTS[i] will
  // either end with a goto statement or will fall through into
  // STATEMENTS[i + 1].  CASES[i] is empty for the default clause,
  // which need not be last.  FUNCTION is the current function.
  virtual Bstatement*
  switch_statement(Bfunction* function, Bexpression* value,
		   const std::vector<std::vector<Bexpression*> >& cases,
		   const std::vector<Bstatement*>& statements,
		   Location) = 0;

  // Create a single statement from two statements.
  virtual Bstatement*
  compound_statement(Bstatement*, Bstatement*) = 0;

  // Create a single statement from a list of statements.
  virtual Bstatement*
  statement_list(const std::vector<Bstatement*>&) = 0;

  // Create a statement that attempts to execute BSTAT and calls EXCEPT_STMT if
  // an exception occurs. EXCEPT_STMT may be NULL.  FINALLY_STMT may be NULL and
  // if not NULL, it will always be executed.  This is used for handling defers
  // in Go functions.  In C++, the resulting code is of this form:
  //   try { BSTAT; } catch { EXCEPT_STMT; } finally { FINALLY_STMT; }
  virtual Bstatement*
  exception_handler_statement(Bstatement* bstat, Bstatement* except_stmt,
                              Bstatement* finally_stmt, Location) = 0;

  // Blocks.

  // Create a block.  The frontend will call this function when it
  // starts converting a block within a function.  FUNCTION is the
  // current function.  ENCLOSING is the enclosing block; it will be
  // NULL for the top-level block in a function.  VARS is the list of
  // local variables defined within this block; each entry will be
  // created by the local_variable function.  START_LOCATION is the
  // location of the start of the block, more or less the location of
  // the initial curly brace.  END_LOCATION is the location of the end
  // of the block, more or less the location of the final curly brace.
  // The statements will be added after the block is created.
  virtual Bblock*
  block(Bfunction* function, Bblock* enclosing,
	const std::vector<Bvariable*>& vars,
	Location start_location, Location end_location) = 0;

  // Add the statements to a block.  The block is created first.  Then
  // the statements are created.  Then the statements are added to the
  // block.  This will called exactly once per block.  The vector may
  // be empty if there are no statements.
  virtual void
  block_add_statements(Bblock*, const std::vector<Bstatement*>&) = 0;

  // Return the block as a statement.  This is used to include a block
  // in a list of statements.
  virtual Bstatement*
  block_statement(Bblock*) = 0;

  // Variables.

  // Create an error variable.  This is used for cases which should
  // not occur in a correct program, in order to keep the compilation
  // going without crashing.
  virtual Bvariable*
  error_variable() = 0;

  // Create a global variable. NAME is the package-qualified name of
  // the variable.  ASM_NAME is the encoded identifier for the
  // variable, incorporating the package, and made safe for the
  // assembler.  BTYPE is the type of the variable.  IS_EXTERNAL is
  // true if the variable is defined in some other package.  IS_HIDDEN
  // is true if the variable is not exported (name begins with a lower
  // case letter).  IN_UNIQUE_SECTION is true if the variable should
  // be put into a unique section if possible; this is intended to
  // permit the linker to garbage collect the variable if it is not
  // referenced.  LOCATION is where the variable was defined.
  virtual Bvariable*
  global_variable(const std::string& name,
                  const std::string& asm_name,
		  Btype* btype,
		  bool is_external,
		  bool is_hidden,
		  bool in_unique_section,
		  Location location) = 0;

  // A global variable will 1) be initialized to zero, or 2) be
  // initialized to a constant value, or 3) be initialized in the init
  // function.  In case 2, the frontend will call
  // global_variable_set_init to set the initial value.  If this is
  // not called, the backend should initialize a global variable to 0.
  // The init function may then assign a value to it.
  virtual void
  global_variable_set_init(Bvariable*, Bexpression*) = 0;

  // Create a local variable.  The frontend will create the local
  // variables first, and then create the block which contains them.
  // FUNCTION is the function in which the variable is defined.  NAME
  // is the name of the variable.  TYPE is the type.  IS_ADDRESS_TAKEN
  // is true if the address of this variable is taken (this implies
  // that the address does not escape the function, as otherwise the
  // variable would be on the heap).  LOCATION is where the variable
  // is defined.  For each local variable the frontend will call
  // init_statement to set the initial value.
  virtual Bvariable*
  local_variable(Bfunction* function, const std::string& name, Btype* type,
		 bool is_address_taken, Location location) = 0;

  // Create a function parameter.  This is an incoming parameter, not
  // a result parameter (result parameters are treated as local
  // variables).  The arguments are as for local_variable.
  virtual Bvariable*
  parameter_variable(Bfunction* function, const std::string& name,
		     Btype* type, bool is_address_taken,
		     Location location) = 0;

  // Create a static chain parameter.  This is the closure parameter.
  virtual Bvariable*
  static_chain_variable(Bfunction* function, const std::string& name,
		        Btype* type, Location location) = 0;

  // Create a temporary variable.  A temporary variable has no name,
  // just a type.  We pass in FUNCTION and BLOCK in case they are
  // needed.  If INIT is not NULL, the variable should be initialized
  // to that value.  Otherwise the initial value is irrelevant--the
  // backend does not have to explicitly initialize it to zero.
  // ADDRESS_IS_TAKEN is true if the programs needs to take the
  // address of this temporary variable.  LOCATION is the location of
  // the statement or expression which requires creating the temporary
  // variable, and may not be very useful.  This function should
  // return a variable which can be referenced later and should set
  // *PSTATEMENT to a statement which initializes the variable.
  virtual Bvariable*
  temporary_variable(Bfunction*, Bblock*, Btype*, Bexpression* init,
		     bool address_is_taken, Location location,
		     Bstatement** pstatement) = 0;

  // Create an implicit variable that is compiler-defined.  This is
  // used when generating GC data and roots, when storing the values
  // of a slice constructor, and for the zero value of types.  This returns a
  // Bvariable because it corresponds to an initialized variable in C.
  //
  // NAME is the name to use for the initialized variable this will create.
  //
  // ASM_NAME is encoded assembler-friendly version of the name, or the
  // empty string if no encoding is needed.
  //
  // TYPE is the type of the implicit variable. 
  //
  // IS_HIDDEN will be true if the descriptor should only be visible
  // within the current object.
  //
  // IS_CONSTANT is true if the implicit variable should be treated like it is
  // immutable.  For slice initializers, if the values must be copied to the
  // heap, the variable IS_CONSTANT.
  //
  // IS_COMMON is true if the implicit variable should
  // be treated as a common variable (multiple definitions with
  // different sizes permitted in different object files, all merged
  // into the largest definition at link time); this will be true for
  // the zero value.  IS_HIDDEN and IS_COMMON will never both be true.
  //
  // If ALIGNMENT is not zero, it is the desired alignment of the variable.
  virtual Bvariable*
  implicit_variable(const std::string& name, const std::string& asm_name,
                    Btype* type, bool is_hidden, bool is_constant,
                    bool is_common, int64_t alignment) = 0;


  // Set the initial value of a variable created by implicit_variable.
  // This must be called even if there is no initializer, i.e., INIT is NULL.
  // The NAME, TYPE, IS_HIDDEN, IS_CONSTANT, and IS_COMMON parameters are
  // the same ones passed to implicit_variable.  INIT will be a composite
  // literal of type TYPE.  It will not contain any function calls or anything
  // else that can not be put into a read-only data section.
  // It may contain the address of variables created by implicit_variable.
  //
  // If IS_COMMON is true, INIT will be NULL, and the
  // variable should be initialized to all zeros.
  virtual void
  implicit_variable_set_init(Bvariable*, const std::string& name, Btype* type,
			     bool is_hidden, bool is_constant, bool is_common,
			     Bexpression* init) = 0;

  // Create a reference to a named implicit variable defined in some
  // other package.  This will be a variable created by a call to
  // implicit_variable with the same NAME, ASM_NAME and TYPE and with
  // IS_COMMON passed as false.  This corresponds to an extern global
  // variable in C.
  virtual Bvariable*
  implicit_variable_reference(const std::string& name,
                              const std::string& asm_name,
                              Btype* type) = 0;

  // Create a named immutable initialized data structure.  This is
  // used for type descriptors, map descriptors, and function
  // descriptors.  This returns a Bvariable because it corresponds to
  // an initialized const variable in C.
  //
  // NAME is the name to use for the initialized global variable which
  // this call will create.
  //
  // ASM_NAME is the encoded, assembler-friendly version of NAME, or
  // the empty string if no encoding is needed.
  //
  // IS_HIDDEN will be true if the descriptor should only be visible
  // within the current object.
  //
  // IS_COMMON is true if NAME may be defined by several packages, and
  // the linker should merge all such definitions.  If IS_COMMON is
  // false, NAME should be defined in only one file.  In general
  // IS_COMMON will be true for the type descriptor of an unnamed type
  // or a builtin type.  IS_HIDDEN and IS_COMMON will never both be
  // true.
  //
  // TYPE will be a struct type; the type of the returned expression
  // must be a pointer to this struct type.
  // 
  // We must create the named structure before we know its
  // initializer, because the initializer may refer to its own
  // address.  After calling this the frontend will call
  // immutable_struct_set_init.
  virtual Bvariable*
  immutable_struct(const std::string& name,
                   const std::string& asm_name,
                   bool is_hidden, bool is_common,
		   Btype* type, Location) = 0;

  // Set the initial value of a variable created by immutable_struct.
  // The NAME, IS_HIDDEN, IS_COMMON, TYPE, and location parameters are
  // the same ones passed to immutable_struct.  INITIALIZER will be a
  // composite literal of type TYPE.  It will not contain any function
  // calls or anything else that can not be put into a read-only data
  // section.  It may contain the address of variables created by
  // immutable_struct.
  virtual void
  immutable_struct_set_init(Bvariable*, const std::string& name,
			    bool is_hidden, bool is_common, Btype* type,
			    Location, Bexpression* initializer) = 0;

  // Create a reference to a named immutable initialized data
  // structure defined in some other package.  This will be a
  // structure created by a call to immutable_struct with the same
  // NAME, ASM_NAME and TYPE and with IS_COMMON passed as false.  This
  // corresponds to an extern const global variable in C.
  virtual Bvariable*
  immutable_struct_reference(const std::string& name,
                             const std::string& asm_name,
                             Btype* type, Location) = 0;

  // Labels.
  
  // Create a new label.  NAME will be empty if this is a label
  // created by the frontend for a loop construct.  The location is
  // where the label is defined.
  virtual Blabel*
  label(Bfunction*, const std::string& name, Location) = 0;

  // Create a statement which defines a label.  This statement will be
  // put into the codestream at the point where the label should be
  // defined.
  virtual Bstatement*
  label_definition_statement(Blabel*) = 0;

  // Create a goto statement to a label.
  virtual Bstatement*
  goto_statement(Blabel*, Location) = 0;

  // Create an expression for the address of a label.  This is used to
  // get the return address of a deferred function which may call
  // recover.
  virtual Bexpression*
  label_address(Blabel*, Location) = 0;

  // Functions.

  // Create an error function.  This is used for cases which should
  // not occur in a correct program, in order to keep the compilation
  // going without crashing.
  virtual Bfunction*
  error_function() = 0;

  // Declare or define a function of FNTYPE.
  // NAME is the Go name of the function. ASM_NAME, if not the empty string, is
  // the name that should be used in the symbol table; this will be non-empty if
  // a magic extern comment is used.
  // IS_VISIBLE is true if this function should be visible outside of the
  // current compilation unit. IS_DECLARATION is true if this is a function
  // declaration rather than a definition; the function definition will be in
  // another compilation unit.
  // IS_INLINABLE is true if the function can be inlined.
  // DISABLE_SPLIT_STACK is true if this function may not split the stack; this
  // is used for the implementation of recover.
  // IN_UNIQUE_SECTION is true if this function should be put into a unique
  // location if possible; this is used for field tracking.
  virtual Bfunction*
  function(Btype* fntype, const std::string& name, const std::string& asm_name,
           bool is_visible, bool is_declaration, bool is_inlinable,
           bool disable_split_stack, bool in_unique_section, Location) = 0;

  // Create a statement that runs all deferred calls for FUNCTION.  This should
  // be a statement that looks like this in C++:
  //   finish:
  //     try { DEFER_RETURN; } catch { CHECK_DEFER; goto finish; }
  virtual Bstatement*
  function_defer_statement(Bfunction* function, Bexpression* undefer,
                           Bexpression* check_defer, Location) = 0;

  // Record PARAM_VARS as the variables to use for the parameters of FUNCTION.
  // This will only be called for a function definition.  Returns true on
  // success, false on failure.
  virtual bool
  function_set_parameters(Bfunction* function,
                         const std::vector<Bvariable*>& param_vars) = 0;

  // Set the function body for FUNCTION using the code in CODE_STMT.  Returns
  // true on success, false on failure.
  virtual bool
  function_set_body(Bfunction* function, Bstatement* code_stmt) = 0;

  // Look up a named built-in function in the current backend implementation.
  // Returns NULL if no built-in function by that name exists.
  virtual Bfunction*
  lookup_builtin(const std::string&) = 0;

  // Utility.

  // Write the definitions for all TYPE_DECLS, CONSTANT_DECLS,
  // FUNCTION_DECLS, and VARIABLE_DECLS declared globally.
  virtual void
  write_global_definitions(const std::vector<Btype*>& type_decls,
                           const std::vector<Bexpression*>& constant_decls,
                           const std::vector<Bfunction*>& function_decls,
                           const std::vector<Bvariable*>& variable_decls) = 0;

  // Write SIZE bytes of export data from BYTES to the proper
  // section in the output object file.
  virtual void
  write_export_data(const char* bytes, unsigned int size) = 0;
};

#endif // !defined(GO_BACKEND_H)