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   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:: shrink-wrapping separate components
   7
   8 .. _shrink-wrapping-separate-components:
   9
  10 Shrink-wrapping separate components
  11 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  12
  13 The prologue may perform a variety of target dependent tasks such as
  14 saving callee-saved registers, saving the return address, aligning the
  15 stack, creating a stack frame, initializing the PIC register, setting
  16 up the static chain, etc.
  17
  18 On some targets some of these tasks may be independent of others and
  19 thus may be shrink-wrapped separately.  These independent tasks are
  20 referred to as components and are handled generically by the target
  21 independent parts of GCC.
  22
  23 Using the following hooks those prologue or epilogue components can be
  24 shrink-wrapped separately, so that the initialization (and possibly
  25 teardown) those components do is not done as frequently on execution
  26 paths where this would unnecessary.
  27
  28 What exactly those components are is up to the target code; the generic
  29 code treats them abstractly, as a bit in an ``sbitmap``.  These
  30 ``sbitmap`` s are allocated by the ``shrink_wrap.get_separate_components``
  31 and ``shrink_wrap.components_for_bb`` hooks, and deallocated by the
  32 generic code.
  33
  34 .. function:: sbitmap TARGET_SHRINK_WRAP_GET_SEPARATE_COMPONENTS (void)
  35
  36   .. hook-start:TARGET_SHRINK_WRAP_GET_SEPARATE_COMPONENTS
  37
  38   This hook should return an ``sbitmap`` with the bits set for those
  39   components that can be separately shrink-wrapped in the current function.
  40   Return ``NULL`` if the current function should not get any separate
  41   shrink-wrapping.
  42   Don't define this hook if it would always return ``NULL``.
  43   If it is defined, the other hooks in this group have to be defined as well.
  44
  45 .. hook-end
  46
  47 .. function:: sbitmap TARGET_SHRINK_WRAP_COMPONENTS_FOR_BB (basic_block)
  48
  49   .. hook-start:TARGET_SHRINK_WRAP_COMPONENTS_FOR_BB
  50
  51   This hook should return an ``sbitmap`` with the bits set for those
  52   components where either the prologue component has to be executed before
  53   the ``basic_block``, or the epilogue component after it, or both.
  54
  55 .. hook-end
  56
  57 .. function:: void TARGET_SHRINK_WRAP_DISQUALIFY_COMPONENTS (sbitmap components, edge e, sbitmap edge_components, bool is_prologue)
  58
  59   .. hook-start:TARGET_SHRINK_WRAP_DISQUALIFY_COMPONENTS
  60
  61   This hook should clear the bits in the :samp:`{components}` bitmap for those
  62   components in :samp:`{edge_components}` that the target cannot handle on edge
  63   :samp:`{e}`, where :samp:`{is_prologue}` says if this is for a prologue or an
  64   epilogue instead.
  65
  66 .. hook-end
  67
  68 .. function:: void TARGET_SHRINK_WRAP_EMIT_PROLOGUE_COMPONENTS (sbitmap)
  69
  70   .. hook-start:TARGET_SHRINK_WRAP_EMIT_PROLOGUE_COMPONENTS
  71
  72   Emit prologue insns for the components indicated by the parameter.
  73
  74 .. hook-end
  75
  76 .. function:: void TARGET_SHRINK_WRAP_EMIT_EPILOGUE_COMPONENTS (sbitmap)
  77
  78   .. hook-start:TARGET_SHRINK_WRAP_EMIT_EPILOGUE_COMPONENTS
  79
  80   Emit epilogue insns for the components indicated by the parameter.
  81
  82 .. hook-end
  83
  84 .. function:: void TARGET_SHRINK_WRAP_SET_HANDLED_COMPONENTS (sbitmap)
  85
  86   .. hook-start:TARGET_SHRINK_WRAP_SET_HANDLED_COMPONENTS
  87
  88   Mark the components in the parameter as handled, so that the
  89   ``prologue`` and ``epilogue`` named patterns know to ignore those
  90   components.  The target code should not hang on to the ``sbitmap``, it
  91   will be deleted after this call.
  92
  93 .. hook-end