gcc/doc/gccint/target-macros/position-independent-code.rst

   1 ..
   2   Copyright 1988-2022 Free Software Foundation, Inc.
   3   This is part of the GCC manual.
   4   For copying conditions, see the copyright.rst file.
   5
   6 .. index:: position independent code, PIC
   7
   8 .. _pic:
   9
  10 Position Independent Code
  11 *************************
  12
  13 This section describes macros that help implement generation of position
  14 independent code.  Simply defining these macros is not enough to
  15 generate valid PIC; you must also add support to the hook
  16 ``TARGET_LEGITIMATE_ADDRESS_P`` and to the macro
  17 ``PRINT_OPERAND_ADDRESS``, as well as ``LEGITIMIZE_ADDRESS``.  You
  18 must modify the definition of :samp:`movsi` to do something appropriate
  19 when the source operand contains a symbolic address.  You may also
  20 need to alter the handling of switch statements so that they use
  21 relative addresses.
  22
  23 .. i rearranged the order of the macros above to try to force one of
  24
  25 .. them to the next line, to eliminate an overfull hbox. -mew 10feb93
  26
  27 .. c:macro:: PIC_OFFSET_TABLE_REGNUM
  28
  29   The register number of the register used to address a table of static
  30   data addresses in memory.  In some cases this register is defined by a
  31   processor's 'application binary interface' (ABI).  When this macro
  32   is defined, RTL is generated for this register once, as with the stack
  33   pointer and frame pointer registers.  If this macro is not defined, it
  34   is up to the machine-dependent files to allocate such a register (if
  35   necessary).  Note that this register must be fixed when in use (e.g.
  36   when ``flag_pic`` is true).
  37
  38 .. c:macro:: PIC_OFFSET_TABLE_REG_CALL_CLOBBERED
  39
  40   A C expression that is nonzero if the register defined by
  41   ``PIC_OFFSET_TABLE_REGNUM`` is clobbered by calls.  If not defined,
  42   the default is zero.  Do not define
  43   this macro if ``PIC_OFFSET_TABLE_REGNUM`` is not defined.
  44
  45 .. c:macro:: LEGITIMATE_PIC_OPERAND_P (x)
  46
  47   A C expression that is nonzero if :samp:`{x}` is a legitimate immediate
  48   operand on the target machine when generating position independent code.
  49   You can assume that :samp:`{x}` satisfies ``CONSTANT_P``, so you need not
  50   check this.  You can also assume :samp:`{flag_pic}` is true, so you need not
  51   check it either.  You need not define this macro if all constants
  52   (including ``SYMBOL_REF``) can be immediate operands when generating
  53   position independent code.