(gcc.info.gz) Accessors

 
 Access to Operands
 ==================
 
    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.
Info Catalog
(gcc.info.gz) RTL Classes
(gcc.info.gz) RTL
(gcc.info.gz) Flags
automatically generated byinfo2html