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 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.

`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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