For each expression type `rtl.def' specifies the number of
contained objects and their kinds, with four possibilities: `e' for
expression (actually a pointer to an expression), `i' for integer,
`w' for wide integer, `s' for string, and `E' for vector
of expressions. The sequence of letters for an expression code is
called its format. Thus, the format of
A few other format characters are used occasionally:
There are macros to get the number of operands, the format, and the class of an expression code:
subregis not in this class.
EQ(which have class `<').
Operands of expressions are accessed using the macros
XSTR. Each of these macros takes
two arguments: an expression-pointer (RTX) and an operand number
(counting from zero). Thus,
XEXP (x, 2)
accesses operand 2 of expression x, as an expression.
XINT (x, 2)
accesses the same operand as an integer.
XSTR, used in the same
fashion, would access it as a string.
Any operand can be accessed as an integer, as an expression or as a string. You must choose the correct method of access for the kind of value actually stored in the operand. You would do this based on the expression code of the containing expression. That is also how you would know how many operands there are.
For example, if x is a
subreg expression, you know that it has
two operands which can be correctly accessed as
XEXP (x, 0)
XINT (x, 1). If you did
XINT (x, 0), you
would get the address of the expression operand but cast as an integer;
that might occasionally be useful, but it would be cleaner to write
(int) XEXP (x, 0).
XEXP (x, 1) would also
compile without error, and would return the second, integer operand cast as
an expression pointer, which would probably result in a crash when
accessed. Nothing stops you from writing
XEXP (x, 28) either,
but this will access memory past the end of the expression with
Access to operands which are vectors is more complicated. You can use the
XVEC to get the vector-pointer itself, or the macros
XVECLEN to access the elements and length of a
XVEC (exp, idx)
XVECLEN (exp, idx)
XVECEXP (exp, idx, eltnum)
XVECLEN (exp, idx).
All the macros defined in this section expand into lvalues and therefore can be used to assign the operands, lengths and vector elements as well as to access them.