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(gcc-295.info)Insn Lengths

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Computing the Length of an Insn
-------------------------------

   For many machines, multiple types of branch instructions are
provided, each for different length branch displacements.  In most
cases, the assembler will choose the correct instruction to use.
However, when the assembler cannot do so, GCC can when a special
attribute, the `length' attribute, is defined.  This attribute must be
defined to have numeric values by specifying a null string in its
`define_attr'.

   In the case of the `length' attribute, two additional forms of
arithmetic terms are allowed in test expressions:

`(match_dup N)'
     This refers to the address of operand N of the current insn, which
     must be a `label_ref'.

`(pc)'
     This refers to the address of the *current* insn.  It might have
     been more consistent with other usage to make this the address of
     the *next* insn but this would be confusing because the length of
     the current insn is to be computed.

   For normal insns, the length will be determined by value of the
`length' attribute.  In the case of `addr_vec' and `addr_diff_vec' insn
patterns, the length is computed as the number of vectors multiplied by
the size of each vector.

   Lengths are measured in addressable storage units (bytes).

   The following macros can be used to refine the length computation:

`FIRST_INSN_ADDRESS'
     When the `length' insn attribute is used, this macro specifies the
     value to be assigned to the address of the first insn in a
     function.  If not specified, 0 is used.

`ADJUST_INSN_LENGTH (INSN, LENGTH)'
     If defined, modifies the length assigned to instruction INSN as a
     function of the context in which it is used.  LENGTH is an lvalue
     that contains the initially computed length of the insn and should
     be updated with the correct length of the insn.

     This macro will normally not be required.  A case in which it is
     required is the ROMP.  On this machine, the size of an `addr_vec'
     insn must be increased by two to compensate for the fact that
     alignment may be required.

   The routine that returns `get_attr_length' (the value of the
`length' attribute) can be used by the output routine to determine the
form of the branch instruction to be written, as the example below
illustrates.

   As an example of the specification of variable-length branches,
consider the IBM 360.  If we adopt the convention that a register will
be set to the starting address of a function, we can jump to labels
within 4k of the start using a four-byte instruction.  Otherwise, we
need a six-byte sequence to load the address from memory and then
branch to it.

   On such a machine, a pattern for a branch instruction might be
specified as follows:

     (define_insn "jump"
       [(set (pc)
             (label_ref (match_operand 0 "" "")))]
       ""
       "*
     {
        return (get_attr_length (insn) == 4
                ? \"b %l0\" : \"l r15,=a(%l0); br r15\");
     }"
       [(set (attr "length") (if_then_else (lt (match_dup 0) (const_int 4096))
                                           (const_int 4)
                                           (const_int 6)))])