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.