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* gccint: (gccint).            Internals of the GNU Compiler Collection.
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   This file documents the internals of the GNU compilers.

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File: gccint.info,  Node: Function Bodies,  Prev: Function Basics,  Up: Functions

Function Bodies
---------------

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

   The `DECL_SAVED_TREE' macro will give the complete body of the
function.  This node will usually be a `COMPOUND_STMT' representing the
outermost block of the function, but it may also be a `TRY_BLOCK', a
`RETURN_INIT', or any other valid statement.

Statements
..........

   There are tree nodes corresponding to all of the source-level
statement constructs.  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:

`STMT_LINENO'
     This macro returns the line number for the statement.  If the
     statement spans multiple lines, this value will be the number of
     the first line on which the statement occurs.  Although we mention
     `CASE_LABEL' below as if it were a statement, they do not allow
     the use of `STMT_LINENO'.  There is no way to obtain the line
     number for a `CASE_LABEL'.

     Statements do not contain information about the file from which
     they came; that information is implicit in the `FUNCTION_DECL'
     from which the statements originate.

`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
     `STMT_IS_FULL_EXPR_P' set.  Temporaries created during such
     statements should be destroyed when the innermost enclosing
     statement with `STMT_IS_FULL_EXPR_P' set is exited.

   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 `while'
loop will have a body, which is itself a statement.  If the substatement
is `NULL_TREE', it is considered equivalent to a statement consisting
of a single `;', 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 `TREE_CHAIN's.  So, you should always process the
statement tree by looping over substatements, like this:
     void process_stmt (stmt)
          tree stmt;
     {
       while (stmt)
         {
           switch (TREE_CODE (stmt))
             {
             case IF_STMT:
               process_stmt (THEN_CLAUSE (stmt));
               /* More processing here.  */
               break;
     
             ...
             }
     
           stmt = TREE_CHAIN (stmt);
         }
     }
   In other words, while the `then' clause of an `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.

`ASM_STMT'
     Used to represent an inline assembly statement.  For an inline
     assembly statement like:
          asm ("mov x, y");
     The `ASM_STRING' macro will return a `STRING_CST' node for `"mov
     x, y"'.  If the original statement made use of the
     extended-assembly syntax, then `ASM_OUTPUTS', `ASM_INPUTS', and
     `ASM_CLOBBERS' will be the outputs, inputs, and clobbers for the
     statement, represented as `STRING_CST' nodes.  The
     extended-assembly syntax looks like:
          asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
     The first string is the `ASM_STRING', containing the instruction
     template.  The next two strings are the output and inputs,
     respectively; this statement has no clobbers.  As this example
     indicates, "plain" assembly statements are merely a special case
     of extended assembly statements; they have no cv-qualifiers,
     outputs, inputs, or clobbers.  All of the strings will be
     `NUL'-terminated, and will contain no embedded `NUL'-characters.

     If the assembly statement is declared `volatile', or if the
     statement was not an extended assembly statement, and is therefore
     implicitly volatile, then the predicate `ASM_VOLATILE_P' will hold
     of the `ASM_STMT'.

`BREAK_STMT'
     Used to represent a `break' statement.  There are no additional
     fields.

`CASE_LABEL'
     Use to represent a `case' label, range of `case' labels, or a
     `default' label.  If `CASE_LOW' is `NULL_TREE', then this is a
     `default' label.  Otherwise, if `CASE_HIGH' is `NULL_TREE', then
     this is an ordinary `case' label.  In this case, `CASE_LOW' is an
     expression giving the value of the label.  Both `CASE_LOW' and
     `CASE_HIGH' are `INTEGER_CST' nodes.  These values will have the
     same type as the condition expression in the switch statement.

     Otherwise, if both `CASE_LOW' and `CASE_HIGH' are defined, the
     statement is a range of case labels.  Such statements originate
     with the extension that allows users to write things of the form:
          case 2 ... 5:
     The first value will be `CASE_LOW', while the second will be
     `CASE_HIGH'.

`CLEANUP_STMT'
     Used to represent an action that should take place upon exit from
     the enclosing scope.  Typically, these actions are calls to
     destructors for local objects, but back ends cannot rely on this
     fact.  If these nodes are in fact representing such destructors,
     `CLEANUP_DECL' will be the `VAR_DECL' destroyed.  Otherwise,
     `CLEANUP_DECL' will be `NULL_TREE'.  In any case, the
     `CLEANUP_EXPR' is the expression to execute.  The cleanups
     executed on exit from a scope should be run in the reverse order
     of the order in which the associated `CLEANUP_STMT's were
     encountered.

`COMPOUND_STMT'
     Used to represent a brace-enclosed block.  The first substatement
     is given by `COMPOUND_BODY'.  Subsequent substatements are found by
     following the `TREE_CHAIN' link from one substatement to the next.
     The `COMPOUND_BODY' will be `NULL_TREE' if there are no
     substatements.

`CONTINUE_STMT'
     Used to represent a `continue' statement.  There are no additional
     fields.

`CTOR_STMT'
     Used to mark the beginning (if `CTOR_BEGIN_P' holds) or end (if
     `CTOR_END_P' holds of the main body of a constructor.  See also
     `SUBOBJECT' for more information on how to use these nodes.

`DECL_STMT'
     Used to represent a local declaration.  The `DECL_STMT_DECL' macro
     can be used to obtain the entity declared.  This declaration may
     be a `LABEL_DECL', indicating that the label declared is a local
     label.  (As an extension, GCC allows the declaration of labels
     with scope.)  In C, this declaration may be a `FUNCTION_DECL',
     indicating the use of the GCC nested function extension.  For more
     information, *note Functions::.

`DO_STMT'
     Used to represent a `do' loop.  The body of the loop is given by
     `DO_BODY' while the termination condition for the loop is given by
     `DO_COND'.  The condition for a `do'-statement is always an
     expression.

`EMPTY_CLASS_EXPR'
     Used to represent a temporary object of a class with no data whose
     address is never taken.  (All such objects are interchangeable.)
     The `TREE_TYPE' represents the type of the object.

`EXPR_STMT'
     Used to represent an expression statement.  Use `EXPR_STMT_EXPR' to
     obtain the expression.

`FILE_STMT'
     Used to record a change in filename within the body of a function.
     Use `FILE_STMT_FILENAME' to obtain the new filename.

`FOR_STMT'
     Used to represent a `for' statement.  The `FOR_INIT_STMT' is the
     initialization statement for the loop.  The `FOR_COND' is the
     termination condition.  The `FOR_EXPR' is the expression executed
     right before the `FOR_COND' on each loop iteration; often, this
     expression increments a counter.  The body of the loop is given by
     `FOR_BODY'.  Note that `FOR_INIT_STMT' and `FOR_BODY' return
     statements, while `FOR_COND' and `FOR_EXPR' return expressions.

`GOTO_STMT'
     Used to represent a `goto' statement.  The `GOTO_DESTINATION' will
     usually be a `LABEL_DECL'.  However, if the "computed goto"
     extension has been used, the `GOTO_DESTINATION' will be an
     arbitrary expression indicating the destination.  This expression
     will always have pointer type.  Additionally the `GOTO_FAKE_P'
     flag is set whenever the goto statement does not come from source
     code, but it is generated implicitly by the compiler.  This is
     used for branch prediction.

`HANDLER'
     Used to represent a C++ `catch' block.  The `HANDLER_TYPE' is the
     type of exception that will be caught by this handler; it is equal
     (by pointer equality) to `CATCH_ALL_TYPE' if this handler is for
     all types.  `HANDLER_PARMS' is the `DECL_STMT' for the catch
     parameter, and `HANDLER_BODY' is the `COMPOUND_STMT' for the block
     itself.

`IF_STMT'
     Used to represent an `if' statement.  The `IF_COND' is the
     expression.

     If the condition is a `TREE_LIST', then the `TREE_PURPOSE' is a
     statement (usually a `DECL_STMT').  Each time the condition is
     evaluated, the statement should be executed.  Then, the
     `TREE_VALUE' should be used as the conditional expression itself.
     This representation is used to handle C++ code like this:

          if (int i = 7) ...

     where there is a new local variable (or variables) declared within
     the condition.

     The `THEN_CLAUSE' represents the statement given by the `then'
     condition, while the `ELSE_CLAUSE' represents the statement given
     by the `else' condition.

`LABEL_STMT'
     Used to represent a label.  The `LABEL_DECL' declared by this
     statement can be obtained with the `LABEL_STMT_LABEL' macro.  The
     `IDENTIFIER_NODE' giving the name of the label can be obtained from
     the `LABEL_DECL' with `DECL_NAME'.

`RETURN_INIT'
     If the function uses the G++ "named return value" extension,
     meaning that the function has been defined like:
          S f(int) return s {...}
     then there will be a `RETURN_INIT'.  There is never a named
     returned value for a constructor.  The first argument to the
     `RETURN_INIT' is the name of the object returned; the second
     argument is the initializer for the object.  The object is
     initialized when the `RETURN_INIT' is encountered.  The object
     referred to is the actual object returned; this extension is a
     manual way of doing the "return-value optimization."  Therefore,
     the object must actually be constructed in the place where the
     object will be returned.

`RETURN_STMT'
     Used to represent a `return' statement.  The `RETURN_EXPR' is the
     expression returned; it will be `NULL_TREE' if the statement was
     just
          return;

`SCOPE_STMT'
     A scope-statement represents the beginning or end of a scope.  If
     `SCOPE_BEGIN_P' holds, this statement represents the beginning of a
     scope; if `SCOPE_END_P' holds this statement represents the end of
     a scope.  On exit from a scope, all cleanups from `CLEANUP_STMT's
     occurring in the scope must be run, in reverse order to the order
     in which they were encountered.  If `SCOPE_NULLIFIED_P' or
     `SCOPE_NO_CLEANUPS_P' holds of the scope, back ends should behave
     as if the `SCOPE_STMT' were not present at all.

`SUBOBJECT'
     In a constructor, these nodes are used to mark the point at which a
     subobject of `this' is fully constructed.  If, after this point, an
     exception is thrown before a `CTOR_STMT' with `CTOR_END_P' set is
     encountered, the `SUBOBJECT_CLEANUP' must be executed.  The
     cleanups must be executed in the reverse order in which they
     appear.

`SWITCH_STMT'
     Used to represent a `switch' statement.  The `SWITCH_COND' is the
     expression on which the switch is occurring.  See the documentation
     for an `IF_STMT' for more information on the representation used
     for the condition.  The `SWITCH_BODY' is the body of the switch
     statement.   The `SWITCH_TYPE' is the original type of switch
     expression as given in the source, before any compiler conversions.

`TRY_BLOCK'
     Used to represent a `try' block.  The body of the try block is
     given by `TRY_STMTS'.  Each of the catch blocks is a `HANDLER'
     node.  The first handler is given by `TRY_HANDLERS'.  Subsequent
     handlers are obtained by following the `TREE_CHAIN' link from one
     handler to the next.  The body of the handler is given by
     `HANDLER_BODY'.

     If `CLEANUP_P' holds of the `TRY_BLOCK', then the `TRY_HANDLERS'
     will not be a `HANDLER' node.  Instead, it will be an expression
     that should be executed if an exception is thrown in the try
     block.  It must rethrow the exception after executing that code.
     And, if an exception is thrown while the expression is executing,
     `terminate' must be called.

`USING_STMT'
     Used to represent a `using' directive.  The namespace is given by
     `USING_STMT_NAMESPACE', which will be a NAMESPACE_DECL.  This node
     is needed inside template functions, to implement using directives
     during instantiation.

`WHILE_STMT'
     Used to represent a `while' loop.  The `WHILE_COND' is the
     termination condition for the loop.  See the documentation for an
     `IF_STMT' for more information on the representation used for the
     condition.

     The `WHILE_BODY' is the body of the loop.


File: gccint.info,  Node: Attributes,  Next: Expression trees,  Prev: Declarations,  Up: Trees

Attributes in trees
===================

   Attributes, as specified using the `__attribute__' keyword, are
represented internally as a `TREE_LIST'.  The `TREE_PURPOSE' is the
name of the attribute, as an `IDENTIFIER_NODE'.  The `TREE_VALUE' is a
`TREE_LIST' of the arguments of the attribute, if any, or `NULL_TREE'
if there are no arguments; the arguments are stored as the `TREE_VALUE'
of successive entries in the list, and may be identifiers or
expressions.  The `TREE_CHAIN' of the attribute is the next attribute
in a list of attributes applying to the same declaration or type, or
`NULL_TREE' if there are no further attributes in the list.

   Attributes may be attached to declarations and to types; these
attributes may be accessed with the following macros.  All attributes
are stored in this way, and many also cause other changes to the
declaration or type or to other internal compiler data structures.

 - Tree Macro: tree DECL_ATTRIBUTES (tree DECL)
     This macro returns the attributes on the declaration DECL.

 - Tree Macro: tree TYPE_ATTRIBUTES (tree TYPE)
     This macro returns the attributes on the type TYPE.


File: gccint.info,  Node: Expression trees,  Prev: Attributes,  Up: Trees

Expressions
===========

   The internal representation for expressions is for the most part
quite straightforward.  However, there are a few facts that one must
bear in mind.  In particular, the expression "tree" is actually a
directed acyclic graph.  (For example there may be many references to
the integer constant zero throughout the source program; many of these
will be represented by the same expression node.)  You should not rely
on certain kinds of node being shared, nor should rely on certain kinds
of nodes being unshared.

   The following macros can be used with all expression nodes:

`TREE_TYPE'
     Returns the type of the expression.  This value may not be
     precisely the same type that would be given the expression in the
     original program.

   In what follows, some nodes that one might expect to always have type
`bool' are documented to have either integral or boolean type.  At some
point in the future, the C front end may also make use of this same
intermediate representation, and at this point these nodes will
certainly have integral type.  The previous sentence is not meant to
imply that the C++ front end does not or will not give these nodes
integral type.

   Below, we list the various kinds of expression nodes.  Except where
noted otherwise, the operands to an expression are accessed using the
`TREE_OPERAND' macro.  For example, to access the first operand to a
binary plus expression `expr', use:

     TREE_OPERAND (expr, 0)

As this example indicates, the operands are zero-indexed.

   The table below begins with constants, moves on to unary expressions,
then proceeds to binary expressions, and concludes with various other
kinds of expressions:

`INTEGER_CST'
     These nodes represent integer constants.  Note that the type of
     these constants is obtained with `TREE_TYPE'; they are not always
     of type `int'.  In particular, `char' constants are represented
     with `INTEGER_CST' nodes.  The value of the integer constant `e' is
     given by
          ((TREE_INT_CST_HIGH (e) << HOST_BITS_PER_WIDE_INT)
          + TREE_INST_CST_LOW (e))

     HOST_BITS_PER_WIDE_INT is at least thirty-two on all platforms.
     Both `TREE_INT_CST_HIGH' and `TREE_INT_CST_LOW' return a
     `HOST_WIDE_INT'.  The value of an `INTEGER_CST' is interpreted as
     a signed or unsigned quantity depending on the type of the
     constant.  In general, the expression given above will overflow,
     so it should not be used to calculate the value of the constant.

     The variable `integer_zero_node' is an integer constant with value
     zero.  Similarly, `integer_one_node' is an integer constant with
     value one.  The `size_zero_node' and `size_one_node' variables are
     analogous, but have type `size_t' rather than `int'.

     The function `tree_int_cst_lt' is a predicate which holds if its
     first argument is less than its second.  Both constants are
     assumed to have the same signedness (i.e., either both should be
     signed or both should be unsigned.)  The full width of the
     constant is used when doing the comparison; the usual rules about
     promotions and conversions are ignored.  Similarly,
     `tree_int_cst_equal' holds if the two constants are equal.  The
     `tree_int_cst_sgn' function returns the sign of a constant.  The
     value is `1', `0', or `-1' according on whether the constant is
     greater than, equal to, or less than zero.  Again, the signedness
     of the constant's type is taken into account; an unsigned constant
     is never less than zero, no matter what its bit-pattern.

`REAL_CST'
     FIXME: Talk about how to obtain representations of this constant,
     do comparisons, and so forth.

`COMPLEX_CST'
     These nodes are used to represent complex number constants, that
     is a `__complex__' whose parts are constant nodes.  The
     `TREE_REALPART' and `TREE_IMAGPART' return the real and the
     imaginary parts respectively.

`VECTOR_CST'
     These nodes are used to represent vector constants, whose parts are
     constant nodes.  Each individual constant node is either an
     integer or a double constant node.  The first operand is a
     `TREE_LIST' of the constant nodes and is accessed through
     `TREE_VECTOR_CST_ELTS'.

`STRING_CST'
     These nodes represent string-constants.  The `TREE_STRING_LENGTH'
     returns the length of the string, as an `int'.  The
     `TREE_STRING_POINTER' is a `char*' containing the string itself.
     The string may not be `NUL'-terminated, and it may contain
     embedded `NUL' characters.  Therefore, the `TREE_STRING_LENGTH'
     includes the trailing `NUL' if it is present.

     For wide string constants, the `TREE_STRING_LENGTH' is the number
     of bytes in the string, and the `TREE_STRING_POINTER' points to an
     array of the bytes of the string, as represented on the target
     system (that is, as integers in the target endianness).  Wide and
     non-wide string constants are distinguished only by the `TREE_TYPE'
     of the `STRING_CST'.

     FIXME: The formats of string constants are not well-defined when
     the target system bytes are not the same width as host system
     bytes.

`PTRMEM_CST'
     These nodes are used to represent pointer-to-member constants.  The
     `PTRMEM_CST_CLASS' is the class type (either a `RECORD_TYPE' or
     `UNION_TYPE' within which the pointer points), and the
     `PTRMEM_CST_MEMBER' is the declaration for the pointed to object.
     Note that the `DECL_CONTEXT' for the `PTRMEM_CST_MEMBER' is in
     general different from the `PTRMEM_CST_CLASS'.  For example, given:
          struct B { int i; };
          struct D : public B {};
          int D::*dp = &D::i;

     The `PTRMEM_CST_CLASS' for `&D::i' is `D', even though the
     `DECL_CONTEXT' for the `PTRMEM_CST_MEMBER' is `B', since `B::i' is
     a member of `B', not `D'.

`VAR_DECL'
     These nodes represent variables, including static data members.
     For more information, *note Declarations::.

`NEGATE_EXPR'
     These nodes represent unary negation of the single operand, for
     both integer and floating-point types.  The type of negation can be
     determined by looking at the type of the expression.

`BIT_NOT_EXPR'
     These nodes represent bitwise complement, and will always have
     integral type.  The only operand is the value to be complemented.

`TRUTH_NOT_EXPR'
     These nodes represent logical negation, and will always have
     integral (or boolean) type.  The operand is the value being
     negated.

`PREDECREMENT_EXPR'
`PREINCREMENT_EXPR'
`POSTDECREMENT_EXPR'
`POSTINCREMENT_EXPR'
     These nodes represent increment and decrement expressions.  The
     value of the single operand is computed, and the operand
     incremented or decremented.  In the case of `PREDECREMENT_EXPR' and
     `PREINCREMENT_EXPR', the value of the expression is the value
     resulting after the increment or decrement; in the case of
     `POSTDECREMENT_EXPR' and `POSTINCREMENT_EXPR' is the value before
     the increment or decrement occurs.  The type of the operand, like
     that of the result, will be either integral, boolean, or
     floating-point.

`ADDR_EXPR'
     These nodes are used to represent the address of an object.  (These
     expressions will always have pointer or reference type.)  The
     operand may be another expression, or it may be a declaration.

     As an extension, GCC allows users to take the address of a label.
     In this case, the operand of the `ADDR_EXPR' will be a
     `LABEL_DECL'.  The type of such an expression is `void*'.

     If the object addressed is not an lvalue, a temporary is created,
     and the address of the temporary is used.

`INDIRECT_REF'
     These nodes are used to represent the object pointed to by a
     pointer.  The operand is the pointer being dereferenced; it will
     always have pointer or reference type.

`FIX_TRUNC_EXPR'
     These nodes represent conversion of a floating-point value to an
     integer.  The single operand will have a floating-point type,
     while the the complete expression will have an integral (or
     boolean) type.  The operand is rounded towards zero.

`FLOAT_EXPR'
     These nodes represent conversion of an integral (or boolean) value
     to a floating-point value.  The single operand will have integral
     type, while the complete expression will have a floating-point
     type.

     FIXME: How is the operand supposed to be rounded?  Is this
     dependent on `-mieee'?

`COMPLEX_EXPR'
     These nodes are used to represent complex numbers constructed from
     two expressions of the same (integer or real) type.  The first
     operand is the real part and the second operand is the imaginary
     part.

`CONJ_EXPR'
     These nodes represent the conjugate of their operand.

`REALPART_EXPR'

`IMAGPART_EXPR'
     These nodes represent respectively the real and the imaginary parts
     of complex numbers (their sole argument).

`NON_LVALUE_EXPR'
     These nodes indicate that their one and only operand is not an
     lvalue.  A back end can treat these identically to the single
     operand.

`NOP_EXPR'
     These nodes are used to represent conversions that do not require
     any code-generation.  For example, conversion of a `char*' to an
     `int*' does not require any code be generated; such a conversion is
     represented by a `NOP_EXPR'.  The single operand is the expression
     to be converted.  The conversion from a pointer to a reference is
     also represented with a `NOP_EXPR'.

`CONVERT_EXPR'
     These nodes are similar to `NOP_EXPR's, but are used in those
     situations where code may need to be generated.  For example, if an
     `int*' is converted to an `int' code may need to be generated on
     some platforms.  These nodes are never used for C++-specific
     conversions, like conversions between pointers to different
     classes in an inheritance hierarchy.  Any adjustments that need to
     be made in such cases are always indicated explicitly.  Similarly,
     a user-defined conversion is never represented by a
     `CONVERT_EXPR'; instead, the function calls are made explicit.

`THROW_EXPR'
     These nodes represent `throw' expressions.  The single operand is
     an expression for the code that should be executed to throw the
     exception.  However, there is one implicit action not represented
     in that expression; namely the call to `__throw'.  This function
     takes no arguments.  If `setjmp'/`longjmp' exceptions are used, the
     function `__sjthrow' is called instead.  The normal GCC back end
     uses the function `emit_throw' to generate this code; you can
     examine this function to see what needs to be done.

`LSHIFT_EXPR'
`RSHIFT_EXPR'
     These nodes represent left and right shifts, respectively.  The
     first operand is the value to shift; it will always be of integral
     type.  The second operand is an expression for the number of bits
     by which to shift.  Right shift should be treated as arithmetic,
     i.e., the high-order bits should be zero-filled when the
     expression has unsigned type and filled with the sign bit when the
     expression has signed type.  Note that the result is undefined if
     the second operand is larger than the first operand's type size.

`BIT_IOR_EXPR'
`BIT_XOR_EXPR'
`BIT_AND_EXPR'
     These nodes represent bitwise inclusive or, bitwise exclusive or,
     and bitwise and, respectively.  Both operands will always have
     integral type.

`TRUTH_ANDIF_EXPR'
`TRUTH_ORIF_EXPR'
     These nodes represent logical and and logical or, respectively.
     These operators are not strict; i.e., the second operand is
     evaluated only if the value of the expression is not determined by
     evaluation of the first operand.  The type of the operands, and
     the result type, is always of boolean or integral type.

`TRUTH_AND_EXPR'
`TRUTH_OR_EXPR'
`TRUTH_XOR_EXPR'
     These nodes represent logical and, logical or, and logical
     exclusive or.  They are strict; both arguments are always
     evaluated.  There are no corresponding operators in C or C++, but
     the front end will sometimes generate these expressions anyhow, if
     it can tell that strictness does not matter.

`PLUS_EXPR'
`MINUS_EXPR'
`MULT_EXPR'
`TRUNC_DIV_EXPR'
`TRUNC_MOD_EXPR'
`RDIV_EXPR'
     These nodes represent various binary arithmetic operations.
     Respectively, these operations are addition, subtraction (of the
     second operand from the first), multiplication, integer division,
     integer remainder, and floating-point division.  The operands to
     the first three of these may have either integral or floating
     type, but there will never be case in which one operand is of
     floating type and the other is of integral type.

     The result of a `TRUNC_DIV_EXPR' is always rounded towards zero.
     The `TRUNC_MOD_EXPR' of two operands `a' and `b' is always `a -
     a/b' where the division is as if computed by a `TRUNC_DIV_EXPR'.

`ARRAY_REF'
     These nodes represent array accesses.  The first operand is the
     array; the second is the index.  To calculate the address of the
     memory accessed, you must scale the index by the size of the type
     of the array elements.  The type of these expressions must be the
     type of a component of the array.

`ARRAY_RANGE_REF'
     These nodes represent access to a range (or "slice") of an array.
     The operands are the same as that for `ARRAY_REF' and have the same
     meanings.  The type of these expressions must be an array whose
     component type is the same as that of the first operand.  The
     range of that array type determines the amount of data these
     expressions access.

`EXACT_DIV_EXPR'
     Document.

`LT_EXPR'
`LE_EXPR'
`GT_EXPR'
`GE_EXPR'
`EQ_EXPR'
`NE_EXPR'
     These nodes represent the less than, less than or equal to, greater
     than, greater than or equal to, equal, and not equal comparison
     operators.  The first and second operand with either be both of
     integral type or both of floating type.  The result type of these
     expressions will always be of integral or boolean type.

`MODIFY_EXPR'
     These nodes represent assignment.  The left-hand side is the first
     operand; the right-hand side is the second operand.  The left-hand
     side will be a `VAR_DECL', `INDIRECT_REF', `COMPONENT_REF', or
     other lvalue.

     These nodes are used to represent not only assignment with `=' but
     also compound assignments (like `+='), by reduction to `='
     assignment.  In other words, the representation for `i += 3' looks
     just like that for `i = i + 3'.

`INIT_EXPR'
     These nodes are just like `MODIFY_EXPR', but are used only when a
     variable is initialized, rather than assigned to subsequently.

`COMPONENT_REF'
     These nodes represent non-static data member accesses.  The first
     operand is the object (rather than a pointer to it); the second
     operand is the `FIELD_DECL' for the data member.

`COMPOUND_EXPR'
     These nodes represent comma-expressions.  The first operand is an
     expression whose value is computed and thrown away prior to the
     evaluation of the second operand.  The value of the entire
     expression is the value of the second operand.

`COND_EXPR'
     These nodes represent `?:' expressions.  The first operand is of
     boolean or integral type.  If it evaluates to a nonzero value, the
     second operand should be evaluated, and returned as the value of
     the expression.  Otherwise, the third operand is evaluated, and
     returned as the value of the expression.  As a GNU extension, the
     middle operand of the `?:' operator may be omitted in the source,
     like this:

          x ? : 3

     which is equivalent to

          x ? x : 3

     assuming that `x' is an expression without side-effects.  However,
     in the case that the first operation causes side effects, the
     side-effects occur only once.  Consumers of the internal
     representation do not need to worry about this oddity; the second
     operand will be always be present in the internal representation.

`CALL_EXPR'
     These nodes are used to represent calls to functions, including
     non-static member functions.  The first operand is a pointer to the
     function to call; it is always an expression whose type is a
     `POINTER_TYPE'.  The second argument is a `TREE_LIST'.  The
     arguments to the call appear left-to-right in the list.  The
     `TREE_VALUE' of each list node contains the expression
     corresponding to that argument.  (The value of `TREE_PURPOSE' for
     these nodes is unspecified, and should be ignored.)  For non-static
     member functions, there will be an operand corresponding to the
     `this' pointer.  There will always be expressions corresponding to
     all of the arguments, even if the function is declared with default
     arguments and some arguments are not explicitly provided at the
     call sites.

`STMT_EXPR'
     These nodes are used to represent GCC's statement-expression
     extension.  The statement-expression extension allows code like
     this:
          int f() { return ({ int j; j = 3; j + 7; }); }
     In other words, an sequence of statements may occur where a single
     expression would normally appear.  The `STMT_EXPR' node represents
     such an expression.  The `STMT_EXPR_STMT' gives the statement
     contained in the expression; this is always a `COMPOUND_STMT'.  The
     value of the expression is the value of the last sub-statement in
     the `COMPOUND_STMT'.  More precisely, the value is the value
     computed by the last `EXPR_STMT' in the outermost scope of the
     `COMPOUND_STMT'.  For example, in:
          ({ 3; })
     the value is `3' while in:
          ({ if (x) { 3; } })
     (represented by a nested `COMPOUND_STMT'), there is no value.  If
     the `STMT_EXPR' does not yield a value, it's type will be `void'.

`BIND_EXPR'
     These nodes represent local blocks.  The first operand is a list of
     temporary variables, connected via their `TREE_CHAIN' field.  These
     will never require cleanups.  The scope of these variables is just
     the body of the `BIND_EXPR'.  The body of the `BIND_EXPR' is the
     second operand.

`LOOP_EXPR'
     These nodes represent "infinite" loops.  The `LOOP_EXPR_BODY'
     represents the body of the loop.  It should be executed forever,
     unless an `EXIT_EXPR' is encountered.

`EXIT_EXPR'
     These nodes represent conditional exits from the nearest enclosing
     `LOOP_EXPR'.  The single operand is the condition; if it is
     nonzero, then the loop should be exited.  An `EXIT_EXPR' will only
     appear within a `LOOP_EXPR'.

`CLEANUP_POINT_EXPR'
     These nodes represent full-expressions.  The single operand is an
     expression to evaluate.  Any destructor calls engendered by the
     creation of temporaries during the evaluation of that expression
     should be performed immediately after the expression is evaluated.

`CONSTRUCTOR'
     These nodes represent the brace-enclosed initializers for a
     structure or array.  The first operand is reserved for use by the
     back end.  The second operand is a `TREE_LIST'.  If the
     `TREE_TYPE' of the `CONSTRUCTOR' is a `RECORD_TYPE' or
     `UNION_TYPE', then the `TREE_PURPOSE' of each node in the
     `TREE_LIST' will be a `FIELD_DECL' and the `TREE_VALUE' of each
     node will be the expression used to initialize that field.  You
     should not depend on the fields appearing in any particular order,
     nor should you assume that all fields will be represented.
     Unrepresented fields may be assigned any value.

     If the `TREE_TYPE' of the `CONSTRUCTOR' is an `ARRAY_TYPE', then
     the `TREE_PURPOSE' of each element in the `TREE_LIST' will be an
     `INTEGER_CST'.  This constant indicates which element of the array
     (indexed from zero) is being assigned to; again, the `TREE_VALUE'
     is the corresponding initializer.  If the `TREE_PURPOSE' is
     `NULL_TREE', then the initializer is for the next available array
     element.

     Conceptually, before any initialization is done, the entire area of
     storage is initialized to zero.

`COMPOUND_LITERAL_EXPR'
     These nodes represent ISO C99 compound literals.  The
     `COMPOUND_LITERAL_EXPR_DECL_STMT' is a `DECL_STMT' containing an
     anonymous `VAR_DECL' for the unnamed object represented by the
     compound literal; the `DECL_INITIAL' of that `VAR_DECL' is a
     `CONSTRUCTOR' representing the brace-enclosed list of initializers
     in the compound literal.  That anonymous `VAR_DECL' can also be
     accessed directly by the `COMPOUND_LITERAL_EXPR_DECL' macro.

`SAVE_EXPR'
     A `SAVE_EXPR' represents an expression (possibly involving
     side-effects) that is used more than once.  The side-effects should
     occur only the first time the expression is evaluated.  Subsequent
     uses should just reuse the computed value.  The first operand to
     the `SAVE_EXPR' is the expression to evaluate.  The side-effects
     should be executed where the `SAVE_EXPR' is first encountered in a
     depth-first preorder traversal of the expression tree.

`TARGET_EXPR'
     A `TARGET_EXPR' represents a temporary object.  The first operand
     is a `VAR_DECL' for the temporary variable.  The second operand is
     the initializer for the temporary.  The initializer is evaluated,
     and copied (bitwise) into the temporary.

     Often, a `TARGET_EXPR' occurs on the right-hand side of an
     assignment, or as the second operand to a comma-expression which is
     itself the right-hand side of an assignment, etc.  In this case,
     we say that the `TARGET_EXPR' is "normal"; otherwise, we say it is
     "orphaned".  For a normal `TARGET_EXPR' the temporary variable
     should be treated as an alias for the left-hand side of the
     assignment, rather than as a new temporary variable.

     The third operand to the `TARGET_EXPR', if present, is a
     cleanup-expression (i.e., destructor call) for the temporary.  If
     this expression is orphaned, then this expression must be executed
     when the statement containing this expression is complete.  These
     cleanups must always be executed in the order opposite to that in
     which they were encountered.  Note that if a temporary is created
     on one branch of a conditional operator (i.e., in the second or
     third operand to a `COND_EXPR'), the cleanup must be run only if
     that branch is actually executed.

     See `STMT_IS_FULL_EXPR_P' for more information about running these
     cleanups.

`AGGR_INIT_EXPR'
     An `AGGR_INIT_EXPR' represents the initialization as the return
     value of a function call, or as the result of a constructor.  An
     `AGGR_INIT_EXPR' will only appear as the second operand of a
     `TARGET_EXPR'.  The first operand to the `AGGR_INIT_EXPR' is the
     address of a function to call, just as in a `CALL_EXPR'.  The
     second operand are the arguments to pass that function, as a
     `TREE_LIST', again in a manner similar to that of a `CALL_EXPR'.
     The value of the expression is that returned by the function.

     If `AGGR_INIT_VIA_CTOR_P' holds of the `AGGR_INIT_EXPR', then the
     initialization is via a constructor call.  The address of the third
     operand of the `AGGR_INIT_EXPR', which is always a `VAR_DECL', is
     taken, and this value replaces the first argument in the argument
     list.  In this case, the value of the expression is the `VAR_DECL'
     given by the third operand to the `AGGR_INIT_EXPR'; constructors do
     not return a value.

`VTABLE_REF'
     A `VTABLE_REF' indicates that the interior expression computes a
     value that is a vtable entry.  It is used with `-fvtable-gc' to
     track the reference through to front end to the middle end, at
     which point we transform this to a `REG_VTABLE_REF' note, which
     survives the balance of code generation.

     The first operand is the expression that computes the vtable
     reference.  The second operand is the `VAR_DECL' of the vtable.
     The third operand is an `INTEGER_CST' of the byte offset into the
     vtable.


File: gccint.info,  Node: RTL,  Next: Machine Desc,  Prev: Trees,  Up: Top

RTL Representation
******************

   Most of the work of the compiler is done on an intermediate
representation called register transfer language.  In this language,
the instructions to be output are described, pretty much one by one, in
an algebraic form that describes what the instruction does.

   RTL is inspired by Lisp lists.  It has both an internal form, made
up of structures that point at other structures, and a textual form
that is used in the machine description and in printed debugging dumps.
The textual form uses nested parentheses to indicate the pointers in
the internal form.

* Menu:

* RTL Objects::       Expressions vs vectors vs strings vs integers.
* RTL Classes::       Categories of RTL expression objects, and their structure.
* Accessors::         Macros to access expression operands or vector elts.
* Flags::             Other flags in an RTL expression.
* Machine Modes::     Describing the size and format of a datum.
* Constants::         Expressions with constant values.
* Regs and Memory::   Expressions representing register contents or memory.
* Arithmetic::        Expressions representing arithmetic on other expressions.
* Comparisons::       Expressions representing comparison of expressions.
* Bit-Fields::        Expressions representing bit-fields in memory or reg.
* Vector Operations:: Expressions involving vector datatypes.
* Conversions::       Extending, truncating, floating or fixing.
* RTL Declarations::  Declaring volatility, constancy, etc.
* Side Effects::      Expressions for storing in registers, etc.
* Incdec::            Embedded side-effects for autoincrement addressing.
* Assembler::         Representing `asm' with operands.
* Insns::             Expression types for entire insns.
* Calls::             RTL representation of function call insns.
* Sharing::           Some expressions are unique; others *must* be copied.
* Reading RTL::       Reading textual RTL from a file.


File: gccint.info,  Node: RTL Objects,  Next: RTL Classes,  Up: RTL

RTL Object Types
================

   RTL uses five kinds of objects: expressions, integers, wide integers,
strings and vectors.  Expressions are the most important ones.  An RTL
expression ("RTX", for short) is a C structure, but it is usually
referred to with a pointer; a type that is given the typedef name `rtx'.

   An integer is simply an `int'; their written form uses decimal
digits.  A wide integer is an integral object whose type is
`HOST_WIDE_INT'; their written form uses decimal digits.

   A string is a sequence of characters.  In core it is represented as a
`char *' in usual C fashion, and it is written in C syntax as well.
However, strings in RTL may never be null.  If you write an empty
string in a machine description, it is represented in core as a null
pointer rather than as a pointer to a null character.  In certain
contexts, these null pointers instead of strings are valid.  Within RTL
code, strings are most commonly found inside `symbol_ref' expressions,
but they appear in other contexts in the RTL expressions that make up
machine descriptions.

   In a machine description, strings are normally written with double
quotes, as you would in C.  However, strings in machine descriptions may
extend over many lines, which is invalid C, and adjacent string
constants are not concatenated as they are in C.  Any string constant
may be surrounded with a single set of parentheses.  Sometimes this
makes the machine description easier to read.

   There is also a special syntax for strings, which can be useful when
C code is embedded in a machine description.  Wherever a string can
appear, it is also valid to write a C-style brace block.  The entire
brace block, including the outermost pair of braces, is considered to be
the string constant.  Double quote characters inside the braces are not
special.  Therefore, if you write string constants in the C code, you
need not escape each quote character with a backslash.

   A vector contains an arbitrary number of pointers to expressions.
The number of elements in the vector is explicitly present in the
vector.  The written form of a vector consists of square brackets
(`[...]') surrounding the elements, in sequence and with whitespace
separating them.  Vectors of length zero are not created; null pointers
are used instead.

   Expressions are classified by "expression codes" (also called RTX
codes).  The expression code is a name defined in `rtl.def', which is
also (in upper case) a C enumeration constant.  The possible expression
codes and their meanings are machine-independent.  The code of an RTX
can be extracted with the macro `GET_CODE (X)' and altered with
`PUT_CODE (X, NEWCODE)'.

   The expression code determines how many operands the expression
contains, and what kinds of objects they are.  In RTL, unlike Lisp, you
cannot tell by looking at an operand what kind of object it is.
Instead, you must know from its context--from the expression code of
the containing expression.  For example, in an expression of code
`subreg', the first operand is to be regarded as an expression and the
second operand as an integer.  In an expression of code `plus', there
are two operands, both of which are to be regarded as expressions.  In
a `symbol_ref' expression, there is one operand, which is to be
regarded as a string.

   Expressions are written as parentheses containing the name of the
expression type, its flags and machine mode if any, and then the
operands of the expression (separated by spaces).

   Expression code names in the `md' file are written in lower case,
but when they appear in C code they are written in upper case.  In this
manual, they are shown as follows: `const_int'.

   In a few contexts a null pointer is valid where an expression is
normally wanted.  The written form of this is `(nil)'.

