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11.6.1 Constant expressions

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

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_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.

FIXME: Talk about how to obtain representations of this constant, do comparisons, and so forth.
These nodes represent fixed-point constants. The type of these constants is obtained with TREE_TYPE. TREE_FIXED_CST_PTR points to a struct fixed_value; TREE_FIXED_CST returns the structure itself. struct fixed_value contains data with the size of two HOST_BITS_PER_WIDE_INT and mode as the associated fixed-point machine mode for data.
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.
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.
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.