GNU Compiler Collection (GCC) Internals: Constant expressions

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10.6.1 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:

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 represented in an array of HOST_WIDE_INT. There are enough elements in the array to represent the value without taking extra elements for redundant 0s or -1. The number of elements used to represent e is available via TREE_INT_CST_NUNITS. Element i can be extracted by using TREE_INT_CST_ELT (e, i). TREE_INT_CST_LOW is a shorthand for TREE_INT_CST_ELT (e, 0).

The functions tree_fits_shwi_p and tree_fits_uhwi_p can be used to tell if the value is small enough to fit in a signed HOST_WIDE_INT or an unsigned HOST_WIDE_INT respectively. The value can then be extracted using tree_to_shwi and tree_to_uhwi.

REAL_CST

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

FIXED_CST

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.

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.

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