Using and Porting GNU CC

14.2: Access to Operands

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 subreg is `ei'.

A few other format characters are used occasionally:

u

`u' is equivalent to `e' except that it is printed differently in debugging dumps. It is used for pointers to insns.

n

`n' is equivalent to `i' except that it is printed differently in debugging dumps. It is used for the line number or code number of a note insn.

S

`S' indicates a string which is optional. In the RTL objects in core, `S' is equivalent to `s', but when the object is read, from an `md' file, the string value of this operand may be omitted. An omitted string is taken to be the null string.

V

`V' indicates a vector which is optional. In the RTL objects in core, `V' is equivalent to `E', but when the object is read from an `md' file, the vector value of this operand may be omitted. An omitted vector is effectively the same as a vector of no elements.

0

`0' means a slot whose contents do not fit any normal category. `0' slots are not printed at all in dumps, and are often used in special ways by small parts of the compiler.

There are macros to get the number of operands, the format, and the class of an expression code:

GET_RTX_LENGTH (code)

Number of operands of an RTX of code code.

GET_RTX_FORMAT (code)

The format of an RTX of code code, as a C string.

GET_RTX_CLASS (code)

A single character representing the type of RTX operation that code code performs.

The following classes are defined:

o

An RTX code that represents an actual object, such as reg or mem. subreg is not in this class.

<

An RTX code for a comparison. The codes in this class are NE, EQ, LE, LT, GE, GT, LEU, LTU, GEU, GTU.

1

An RTX code for a unary arithmetic operation, such as neg.

c

An RTX code for a commutative binary operation, other than NE and EQ (which have class `<').

2

An RTX code for a noncommutative binary operation, such as MINUS.

b

An RTX code for a bitfield operation, either ZERO_EXTRACT or SIGN_EXTRACT.

3

An RTX code for other three input operations, such as IF_THEN_ELSE.

i

An RTX code for a machine insn (INSN, JUMP_INSN, and CALL_INSN).

m

An RTX code for something that matches in insns, such as MATCH_DUP.

x

All other RTX codes.

Operands of expressions are accessed using the macros XEXP, XINT, XWINT and 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) and 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 unpredictable results.

Access to operands which are vectors is more complicated. You can use the macro XVEC to get the vector-pointer itself, or the macros XVECEXP and XVECLEN to access the elements and length of a vector.

XVEC (exp, idx)

Access the vector-pointer which is operand number idx in exp.

XVECLEN (exp, idx)

Access the length (number of elements) in the vector which is in operand number idx in exp. This value is an int.

XVECEXP (exp, idx, eltnum)

Access element number eltnum in the vector which is in operand number idx in exp. This value is an RTX.

It is up to you to make sure that eltnum is not negative and is less than 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.