Previous: Attributes, Up: Trees
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
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
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
COMPLEX_CST
__complex__
whose parts are constant nodes. The
TREE_REALPART
and TREE_IMAGPART
return the real and the
imaginary parts respectively.
VECTOR_CST
TREE_LIST
of the
constant nodes and is accessed through TREE_VECTOR_CST_ELTS
.
STRING_CST
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
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
NEGATE_EXPR
The behavior of this operation on signed arithmetic overflow is
controlled by the flag_wrapv
and flag_trapv
variables.
ABS_EXPR
abs
, labs
and llabs
builtins for
integer types, and the fabs
, fabsf
and fabsl
builtins for floating point types. The type of abs operation can
be determined by looking at the type of the expression.
This node is not used for complex types. To represent the modulus
or complex abs of a complex value, use the BUILT_IN_CABS
,
BUILT_IN_CABSF
or BUILT_IN_CABSL
builtins, as used
to implement the C99 cabs
, cabsf
and cabsl
built-in functions.
BIT_NOT_EXPR
TRUTH_NOT_EXPR
BOOLEAN_TYPE
or INTEGER_TYPE
.
PREDECREMENT_EXPR
PREINCREMENT_EXPR
POSTDECREMENT_EXPR
POSTINCREMENT_EXPR
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
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
FIX_TRUNC_EXPR
FLOAT_EXPR
FIXME: How is the operand supposed to be rounded? Is this dependent on
-mieee?
COMPLEX_EXPR
CONJ_EXPR
REALPART_EXPR
IMAGPART_EXPR
NON_LVALUE_EXPR
NOP_EXPR
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
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
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
BIT_IOR_EXPR
BIT_XOR_EXPR
BIT_AND_EXPR
TRUTH_ANDIF_EXPR
TRUTH_ORIF_EXPR
BOOLEAN_TYPE
or INTEGER_TYPE
.
TRUTH_AND_EXPR
TRUTH_OR_EXPR
TRUTH_XOR_EXPR
BOOLEAN_TYPE
or INTEGER_TYPE
.
PLUS_EXPR
MINUS_EXPR
MULT_EXPR
The behavior of these operations on signed arithmetic overflow is
controlled by the flag_wrapv
and flag_trapv
variables.
RDIV_EXPR
TRUNC_DIV_EXPR
FLOOR_DIV_EXPR
CEIL_DIV_EXPR
ROUND_DIV_EXPR
TRUNC_DIV_EXPR
rounds towards zero, FLOOR_DIV_EXPR
rounds towards negative infinity, CEIL_DIV_EXPR
rounds towards
positive infinity and ROUND_DIV_EXPR
rounds to the closest integer.
Integer division in C and C++ is truncating, i.e. TRUNC_DIV_EXPR
.
The behavior of these operations on signed arithmetic overflow, when
dividing the minimum signed integer by minus one, is controlled by the
flag_wrapv
and flag_trapv
variables.
TRUNC_MOD_EXPR
FLOOR_MOD_EXPR
CEIL_MOD_EXPR
ROUND_MOD_EXPR
a
and b
is
defined as a - (a/b)*b
where the division calculated using
the corresponding division operator. Hence for TRUNC_MOD_EXPR
this definition assumes division using truncation towards zero, i.e.
TRUNC_DIV_EXPR
. Integer remainder in C and C++ uses truncating
division, i.e. TRUNC_MOD_EXPR
.
EXACT_DIV_EXPR
EXACT_DIV_EXPR
code is used to represent integer divisions where
the numerator is known to be an exact multiple of the denominator. This
allows the backend to choose between the faster of TRUNC_DIV_EXPR
,
CEIL_DIV_EXPR
and FLOOR_DIV_EXPR
for the current target.
ARRAY_REF
array_ref_low_bound
and array_ref_element_size
instead.
ARRAY_RANGE_REF
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.
TARGET_MEM_REF
TMR_SYMBOL
and must be a VAR_DECL
of an object with
a fixed address. The second argument is TMR_BASE
and the
third one is TMR_INDEX
. The fourth argument is
TMR_STEP
and must be an INTEGER_CST
. The fifth
argument is TMR_OFFSET
and must be an INTEGER_CST
.
Any of the arguments may be NULL if the appropriate component
does not appear in the address. Address of the TARGET_MEM_REF
is determined in the following way.
&TMR_SYMBOL + TMR_BASE + TMR_INDEX * TMR_STEP + TMR_OFFSET
The sixth argument is the reference to the original memory access, which
is preserved for the purposes of the RTL alias analysis. The seventh
argument is a tag representing the results of tree level alias analysis.
LT_EXPR
LE_EXPR
GT_EXPR
GE_EXPR
EQ_EXPR
NE_EXPR
For floating point comparisons, if we honor IEEE NaNs and either operand
is NaN, then NE_EXPR
always returns true and the remaining operators
always return false. On some targets, comparisons against an IEEE NaN,
other than equality and inequality, may generate a floating point exception.
ORDERED_EXPR
UNORDERED_EXPR
UNLT_EXPR
UNLE_EXPR
UNGT_EXPR
UNGE_EXPR
UNEQ_EXPR
LTGT_EXPR
UNLT_EXPR
returns true if either operand is an IEEE
NaN or the first operand is less than the second. With the possible
exception of LTGT_EXPR
, all of these operations are guaranteed
not to generate a floating point exception. The result
type of these expressions will always be of integral or boolean type.
These operations return the result type's zero value for false,
and the result type's one value for true.
MODIFY_EXPR
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
MODIFY_EXPR
, but are used only when a
variable is initialized, rather than assigned to subsequently. This
means that we can assume that the target of the initialization is not
used in computing its own value; any reference to the lhs in computing
the rhs is undefined.
COMPONENT_REF
FIELD_DECL
for the data member. The third operand represents
the byte offset of the field, but should not be used directly; call
component_ref_field_offset
instead.
COMPOUND_EXPR
COND_EXPR
?:
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.
The second operand must have the same type as the entire expression,
unless it unconditionally throws an exception or calls a noreturn
function, in which case it should have void type. The same constraints
apply to the third operand. This allows array bounds checks to be
represented conveniently as (i >= 0 && i < 10) ? i : abort()
.
As a GNU extension, the C language front-ends allow the second
operand of the ?:
operator may be omitted in the source.
For example, x ? : 3
is equivalent to x ? x : 3
,
assuming that x
is an expression without side-effects.
In the tree representation, however, the second operand is always
present, possibly protected by SAVE_EXPR
if the first
argument does cause side-effects.
CALL_EXPR
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
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. The value of the expression is the value
of the last sub-statement in the body. More precisely, the value is the
value computed by the last statement nested inside BIND_EXPR
,
TRY_FINALLY_EXPR
, or TRY_CATCH_EXPR
. For example, in:
({ 3; })
the value is 3
while in:
({ if (x) { 3; } })
there is no value. If the STMT_EXPR
does not yield a value,
it's type will be void
.
BIND_EXPR
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
LOOP_EXPR_BODY
represents the body of the loop. It should be executed forever, unless
an EXIT_EXPR
is encountered.
EXIT_EXPR
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
CONSTRUCTOR
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.
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
or a RANGE_EXPR
of
two INTEGER_CST
s. A single INTEGER_CST
indicates which
element of the array (indexed from zero) is being assigned to. A
RANGE_EXPR
indicates an inclusive range of elements to
initialize. In both cases the TREE_VALUE
is the corresponding
initializer. It is re-evaluated for each element of a
RANGE_EXPR
. If the TREE_PURPOSE
is NULL_TREE
, then
the initializer is for the next available array element.
In the front end, you should not depend on the fields appearing in any
particular order. However, in the middle end, fields must appear in
declaration order. You should not assume that all fields will be
represented. Unrepresented fields will be set to zero.
COMPOUND_LITERAL_EXPR
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
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
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,
if non-void, copied (bitwise) into the temporary. If the initializer
is void, that means that it will perform the initialization itself.
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
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 a full-expression, or 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
.
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 either case, the expression is void.
VA_ARG_EXPR
va_arg (ap, type)
.
Its TREE_TYPE
yields the tree representation for type
and
its sole argument yields the representation for ap
.