c-tree.texi

理解和实践操作系统的一本好书

@cindex copy constructor
@cindex assignment operator
@cindex linkage
@findex DECL_NAME
@findex DECL_ASSEMBLER_NAME
@findex TREE_PUBLIC
@findex DECL_LINKONCE_P
@findex DECL_FUNCTION_MEMBER_P
@findex DECL_CONSTRUCTOR_P
@findex DECL_DESTRUCTOR_P
@findex DECL_OVERLOADED_OPERATOR_P
@findex DECL_CONV_FN_P
@findex DECL_ARTIFICIAL
@findex DECL_GLOBAL_CTOR_P
@findex DECL_GLOBAL_DTOR_P
@findex GLOBAL_INIT_PRIORITY

The following macros and functions can be used on a @code{FUNCTION_DECL}:
@ftable @code
@item DECL_MAIN_P
This predicate holds for a function that is the program entry point
@code{::code}.

@item DECL_NAME
This macro returns the unqualified name of the function, as an
@code{IDENTIFIER_NODE}.  For an instantiation of a function template,
the @code{DECL_NAME} is the unqualified name of the template, not
something like @code{f<int>}.  The value of @code{DECL_NAME} is
undefined when used on a constructor, destructor, overloaded operator,
or type-conversion operator, or any function that is implicitly
generated by the compiler.  See below for macros that can be used to
distinguish these cases.

@item DECL_ASSEMBLER_NAME
This macro returns the mangled name of the function, also an
@code{IDENTIFIER_NODE}.  This name does not contain leading underscores
on systems that prefix all identifiers with underscores.  The mangled
name is computed in the same way on all platforms; if special processing
is required to deal with the object file format used on a particular
platform, it is the responsibility of the back end to perform those
modifications.  (Of course, the back end should not modify
@code{DECL_ASSEMBLER_NAME} itself.)

Using @code{DECL_ASSEMBLER_NAME} will cause additional memory to be
allocated (for the mangled name of the entity) so it should be used
only when emitting assembly code.  It should not be used within the
optimizers to determine whether or not two declarations are the same,
even though some of the existing optimizers do use it in that way.
These uses will be removed over time.

@item DECL_EXTERNAL
This predicate holds if the function is undefined.

@item TREE_PUBLIC
This predicate holds if the function has external linkage.

@item DECL_LOCAL_FUNCTION_P
This predicate holds if the function was declared at block scope, even
though it has a global scope.

@item DECL_ANTICIPATED
This predicate holds if the function is a built-in function but its
prototype is not yet explicitly declared.

@item DECL_EXTERN_C_FUNCTION_P
This predicate holds if the function is declared as an
`@code{extern "C"}' function.

@item DECL_LINKONCE_P
This macro holds if multiple copies of this function may be emitted in
various translation units.  It is the responsibility of the linker to
merge the various copies.  Template instantiations are the most common
example of functions for which @code{DECL_LINKONCE_P} holds; G++
instantiates needed templates in all translation units which require them,
and then relies on the linker to remove duplicate instantiations.

FIXME: This macro is not yet implemented.

@item DECL_FUNCTION_MEMBER_P
This macro holds if the function is a member of a class, rather than a
member of a namespace.

@item DECL_STATIC_FUNCTION_P
This predicate holds if the function a static member function.

@item DECL_NONSTATIC_MEMBER_FUNCTION_P
This macro holds for a non-static member function.

@item DECL_CONST_MEMFUNC_P
This predicate holds for a @code{const}-member function.

@item DECL_VOLATILE_MEMFUNC_P
This predicate holds for a @code{volatile}-member function.

@item DECL_CONSTRUCTOR_P
This macro holds if the function is a constructor.

@item DECL_NONCONVERTING_P
This predicate holds if the constructor is a non-converting constructor.

@item DECL_COMPLETE_CONSTRUCTOR_P
This predicate holds for a function which is a constructor for an object
of a complete type.

@item DECL_BASE_CONSTRUCTOR_P
This predicate holds for a function which is a constructor for a base
class sub-object.

@item DECL_COPY_CONSTRUCTOR_P
This predicate holds for a function which is a copy-constructor.

@item DECL_DESTRUCTOR_P
This macro holds if the function is a destructor.

@item DECL_COMPLETE_DESTRUCTOR_P
This predicate holds if the function is the destructor for an object a
complete type.

@item DECL_OVERLOADED_OPERATOR_P
This macro holds if the function is an overloaded operator.

@item DECL_CONV_FN_P
This macro holds if the function is a type-conversion operator.

@item DECL_GLOBAL_CTOR_P
This predicate holds if the function is a file-scope initialization
function.

@item DECL_GLOBAL_DTOR_P
This predicate holds if the function is a file-scope finalization
function.

@item DECL_THUNK_P
This predicate holds if the function is a thunk.

These functions represent stub code that adjusts the @code{this} pointer
and then jumps to another function.  When the jumped-to function
returns, control is transferred directly to the caller, without
returning to the thunk.  The first parameter to the thunk is always the
@code{this} pointer; the thunk should add @code{THUNK_DELTA} to this
value.  (The @code{THUNK_DELTA} is an @code{int}, not an
@code{INTEGER_CST}.)

Then, if @code{THUNK_VCALL_OFFSET} (an @code{INTEGER_CST}) is nonzero
the adjusted @code{this} pointer must be adjusted again.  The complete
calculation is given by the following pseudo-code:

@smallexample
this += THUNK_DELTA
if (THUNK_VCALL_OFFSET)
  this += (*((ptrdiff_t **) this))[THUNK_VCALL_OFFSET]
@end smallexample

Finally, the thunk should jump to the location given
by @code{DECL_INITIAL}; this will always be an expression for the
address of a function.

@item DECL_NON_THUNK_FUNCTION_P
This predicate holds if the function is @emph{not} a thunk function.

@item GLOBAL_INIT_PRIORITY
If either @code{DECL_GLOBAL_CTOR_P} or @code{DECL_GLOBAL_DTOR_P} holds,
then this gives the initialization priority for the function.  The
linker will arrange that all functions for which
@code{DECL_GLOBAL_CTOR_P} holds are run in increasing order of priority
before @code{main} is called.  When the program exits, all functions for
which @code{DECL_GLOBAL_DTOR_P} holds are run in the reverse order.

@item DECL_ARTIFICIAL
This macro holds if the function was implicitly generated by the
compiler, rather than explicitly declared.  In addition to implicitly
generated class member functions, this macro holds for the special
functions created to implement static initialization and destruction, to
compute run-time type information, and so forth.

@item DECL_ARGUMENTS
This macro returns the @code{PARM_DECL} for the first argument to the
function.  Subsequent @code{PARM_DECL} nodes can be obtained by
following the @code{TREE_CHAIN} links.

@item DECL_RESULT
This macro returns the @code{RESULT_DECL} for the function.

@item TREE_TYPE
This macro returns the @code{FUNCTION_TYPE} or @code{METHOD_TYPE} for
the function.

@item TYPE_RAISES_EXCEPTIONS
This macro returns the list of exceptions that a (member-)function can
raise.  The returned list, if non @code{NULL}, is comprised of nodes
whose @code{TREE_VALUE} represents a type.

@item TYPE_NOTHROW_P
This predicate holds when the exception-specification of its arguments
if of the form `@code{()}'.

@item DECL_ARRAY_DELETE_OPERATOR_P
This predicate holds if the function an overloaded
@code{operator delete[]}.

@end ftable

@c ---------------------------------------------------------------------
@c Function Bodies
@c ---------------------------------------------------------------------

@node Function Bodies
@subsection Function Bodies
@cindex function body
@cindex statements
@tindex BREAK_STMT
@tindex CLEANUP_STMT
@findex CLEANUP_DECL
@findex CLEANUP_EXPR
@tindex CONTINUE_STMT
@tindex DECL_STMT
@findex DECL_STMT_DECL
@tindex DO_STMT
@findex DO_BODY
@findex DO_COND
@tindex EMPTY_CLASS_EXPR
@tindex EXPR_STMT
@findex EXPR_STMT_EXPR
@tindex FOR_STMT
@findex FOR_INIT_STMT
@findex FOR_COND
@findex FOR_EXPR
@findex FOR_BODY
@tindex HANDLER
@tindex IF_STMT
@findex IF_COND
@findex THEN_CLAUSE
@findex ELSE_CLAUSE
@tindex RETURN_STMT
@findex RETURN_EXPR
@tindex SUBOBJECT
@findex SUBOBJECT_CLEANUP
@tindex SWITCH_STMT
@findex SWITCH_COND
@findex SWITCH_BODY
@tindex TRY_BLOCK
@findex TRY_STMTS
@findex TRY_HANDLERS
@findex HANDLER_PARMS
@findex HANDLER_BODY
@findex USING_STMT
@tindex WHILE_STMT
@findex WHILE_BODY
@findex WHILE_COND

A function that has a definition in the current translation unit will
have a non-@code{NULL} @code{DECL_INITIAL}.  However, back ends should not make
use of the particular value given by @code{DECL_INITIAL}.

The @code{DECL_SAVED_TREE} macro will give the complete body of the
function.

@subsubsection Statements

There are tree nodes corresponding to all of the source-level
statement constructs, used within the C and C++ frontends.  These are
enumerated here, together with a list of the various macros that can
be used to obtain information about them.  There are a few macros that
can be used with all statements:

@ftable @code
@item STMT_IS_FULL_EXPR_P
In C++, statements normally constitute ``full expressions''; temporaries
created during a statement are destroyed when the statement is complete.
However, G++ sometimes represents expressions by statements; these
statements will not have @code{STMT_IS_FULL_EXPR_P} set.  Temporaries
created during such statements should be destroyed when the innermost
enclosing statement with @code{STMT_IS_FULL_EXPR_P} set is exited.

@end ftable

Here is the list of the various statement nodes, and the macros used to
access them.  This documentation describes the use of these nodes in
non-template functions (including instantiations of template functions).
In template functions, the same nodes are used, but sometimes in
slightly different ways.

Many of the statements have substatements.  For example, a @code{while}
loop will have a body, which is itself a statement.  If the substatement
is @code{NULL_TREE}, it is considered equivalent to a statement
consisting of a single @code{;}, i.e., an expression statement in which
the expression has been omitted.  A substatement may in fact be a list
of statements, connected via their @code{TREE_CHAIN}s.  So, you should
always process the statement tree by looping over substatements, like
this:
@smallexample
void process_stmt (stmt)
     tree stmt;
@{
  while (stmt)
    @{
      switch (TREE_CODE (stmt))
        @{
        case IF_STMT:
          process_stmt (THEN_CLAUSE (stmt));
          /* @r{More processing here.}  */
          break;

        @dots{}
        @}

      stmt = TREE_CHAIN (stmt);
    @}
@}
@end smallexample
In other words, while the @code{then} clause of an @code{if} statement
in C++ can be only one statement (although that one statement may be a
compound statement), the intermediate representation will sometimes use
several statements chained together.

@table @code
@item ASM_EXPR

Used to represent an inline assembly statement.  For an inline assembly
statement like:
@smallexample
asm ("mov x, y");
@end smallexample
The @code{ASM_STRING} macro will return a @code{STRING_CST} node for
@code{"mov x, y"}.  If the original statement made use of the
extended-assembly syntax, then @code{ASM_OUTPUTS},
@code{ASM_INPUTS}, and @code{ASM_CLOBBERS} will be the outputs, inputs,
and clobbers for the statement, represent