16.19.5 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:

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)))])