13.23.3.4 Records of type: jump to label–offset

NOTE This record type is currently not used. Is it really needed? Can its use allow operations not possible with records of type “offset in object”?

These records are composed of 3 words: the second word contains a fixnum which, unpacked to iksword_t, represents a byte offset; the third word contains the tagged pointer to a subordinate Scheme code object. When these records are processed: a 32-bit byte offset is computed and stored in the target word in the data area of the subject code object:

   meta data         data area
  |.........|..........................|

s_code   p_data
  v         v
  |---------|--------------|---|-------| code object
            |..............|...|target word
      ------->displacement   ^
     |                       |
     |    -----+-------------
     |   |     |
     |  off   obj
  |----|----|----| jump label offset record
   1st  2nd  3rd

Let’s imagine the following scenario in Assembly language, in which a jmp instruction is used to jump directly to another entry point by specifying the target address as an immediate value:

   ...
   jmp L_target
   ...

   ...
L_target:
   ...

such operation can be implemented also as:

   ...
   jmp-pc-relative (L_target - L_after_jmp)
L_after_jmp:
   ...

   ...
L_target:
   ...

in which the jump is to the address at a computable offset from the jmp-pc-relative instruction; the target address is computed at run–time, by the CPU executing the jmp-pc-relative instruction, by adding the constant offset L_target - L_after_jmp to the value of the Instruction Pointer Register (or Program Counter Register).

The “jump to label–offset” records are used to implement this scenario, in which: the jmp-pc-relative instruction is in the subject code object; the target label is in the subordinate code object. The third word in the record contains the tagged pointer to the subordinate Scheme code object. The second word in the record contains a fixnum which, unpacked to iksword_t, represents the constant to add to a tagged pointer to the subordinate code object to obtain the offset of the target label:

 meta data        machine code
|---------|--------------|---|-----| subject code object
                             ^ L_after_jmp
          |..............|...|
            displacement  32-bit


 meta data     machine code
|---------|------------|-----------| subordinate code object
                       ^ L_target
|......................|
 offset - code_primary_tag

At present, the code handles only 32-bit offsets.