gcc/doc/gccint/target-macros/storage-layout.rst

..
  Copyright 1988-2022 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the copyright.rst file.

.. index:: storage layout

.. _storage-layout:

Storage Layout
**************

Note that the definitions of the macros in this table which are sizes or
alignments measured in bits do not need to be constant.  They can be C
expressions that refer to static variables, such as the ``target_flags``.
See :ref:`run-time-target`.

.. c:macro:: BITS_BIG_ENDIAN

  Define this macro to have the value 1 if the most significant bit in a
  byte has the lowest number; otherwise define it to have the value zero.
  This means that bit-field instructions count from the most significant
  bit.  If the machine has no bit-field instructions, then this must still
  be defined, but it doesn't matter which value it is defined to.  This
  macro need not be a constant.

  This macro does not affect the way structure fields are packed into
  bytes or words; that is controlled by ``BYTES_BIG_ENDIAN``.

.. c:macro:: BYTES_BIG_ENDIAN

  Define this macro to have the value 1 if the most significant byte in a
  word has the lowest number.  This macro need not be a constant.

.. c:macro:: WORDS_BIG_ENDIAN

  Define this macro to have the value 1 if, in a multiword object, the
  most significant word has the lowest number.  This applies to both
  memory locations and registers; see ``REG_WORDS_BIG_ENDIAN`` if the
  order of words in memory is not the same as the order in registers.  This
  macro need not be a constant.

.. c:macro:: REG_WORDS_BIG_ENDIAN

  On some machines, the order of words in a multiword object differs between
  registers in memory.  In such a situation, define this macro to describe
  the order of words in a register.  The macro ``WORDS_BIG_ENDIAN`` controls
  the order of words in memory.

.. c:macro:: FLOAT_WORDS_BIG_ENDIAN

  Define this macro to have the value 1 if ``DFmode``, ``XFmode`` or
  ``TFmode`` floating point numbers are stored in memory with the word
  containing the sign bit at the lowest address; otherwise define it to
  have the value 0.  This macro need not be a constant.

  You need not define this macro if the ordering is the same as for
  multi-word integers.

.. c:macro:: BITS_PER_WORD

  Number of bits in a word.  If you do not define this macro, the default
  is ``BITS_PER_UNIT * UNITS_PER_WORD``.

.. c:macro:: MAX_BITS_PER_WORD

  Maximum number of bits in a word.  If this is undefined, the default is
  ``BITS_PER_WORD``.  Otherwise, it is the constant value that is the
  largest value that ``BITS_PER_WORD`` can have at run-time.

.. c:macro:: UNITS_PER_WORD

  Number of storage units in a word; normally the size of a general-purpose
  register, a power of two from 1 or 8.

.. c:macro:: MIN_UNITS_PER_WORD

  Minimum number of units in a word.  If this is undefined, the default is
  ``UNITS_PER_WORD``.  Otherwise, it is the constant value that is the
  smallest value that ``UNITS_PER_WORD`` can have at run-time.

.. c:macro:: POINTER_SIZE

  Width of a pointer, in bits.  You must specify a value no wider than the
  width of ``Pmode``.  If it is not equal to the width of ``Pmode``,
  you must define ``POINTERS_EXTEND_UNSIGNED``.  If you do not specify
  a value the default is ``BITS_PER_WORD``.

.. c:macro:: POINTERS_EXTEND_UNSIGNED

  A C expression that determines how pointers should be extended from
  ``ptr_mode`` to either ``Pmode`` or ``word_mode``.  It is
  greater than zero if pointers should be zero-extended, zero if they
  should be sign-extended, and negative if some other sort of conversion
  is needed.  In the last case, the extension is done by the target's
  ``ptr_extend`` instruction.

  You need not define this macro if the ``ptr_mode``, ``Pmode``
  and ``word_mode`` are all the same width.

.. c:macro:: PROMOTE_MODE (m, unsignedp, type)

  A macro to update :samp:`{m}` and :samp:`{unsignedp}` when an object whose type
  is :samp:`{type}` and which has the specified mode and signedness is to be
  stored in a register.  This macro is only called when :samp:`{type}` is a
  scalar type.

  On most RISC machines, which only have operations that operate on a full
  register, define this macro to set :samp:`{m}` to ``word_mode`` if
  :samp:`{m}` is an integer mode narrower than ``BITS_PER_WORD``.  In most
  cases, only integer modes should be widened because wider-precision
  floating-point operations are usually more expensive than their narrower
  counterparts.

  For most machines, the macro definition does not change :samp:`{unsignedp}`.
  However, some machines, have instructions that preferentially handle
  either signed or unsigned quantities of certain modes.  For example, on
  the DEC Alpha, 32-bit loads from memory and 32-bit add instructions
  sign-extend the result to 64 bits.  On such machines, set
  :samp:`{unsignedp}` according to which kind of extension is more efficient.

  Do not define this macro if it would never modify :samp:`{m}`.

.. function:: enum flt_eval_method TARGET_C_EXCESS_PRECISION (enum excess_precision_type type)

  .. hook-start:TARGET_C_EXCESS_PRECISION

  Return a value, with the same meaning as the C99 macro
  ``FLT_EVAL_METHOD`` that describes which excess precision should be
  applied.  :samp:`{type}` is either ``EXCESS_PRECISION_TYPE_IMPLICIT``,
  ``EXCESS_PRECISION_TYPE_FAST``,
  ``EXCESS_PRECISION_TYPE_STANDARD``, or
  ``EXCESS_PRECISION_TYPE_FLOAT16``.  For
  ``EXCESS_PRECISION_TYPE_IMPLICIT``, the target should return which
  precision and range operations will be implictly evaluated in regardless
  of the excess precision explicitly added.  For
  ``EXCESS_PRECISION_TYPE_STANDARD``,
  ``EXCESS_PRECISION_TYPE_FLOAT16``, and
  ``EXCESS_PRECISION_TYPE_FAST``, the target should return the
  explicit excess precision that should be added depending on the
  value set for :option:`-fexcess-precision=[standard|fast|16]`.
  Note that unpredictable explicit excess precision does not make sense,
  so a target should never return ``FLT_EVAL_METHOD_UNPREDICTABLE``
  when :samp:`{type}` is ``EXCESS_PRECISION_TYPE_STANDARD``,
  ``EXCESS_PRECISION_TYPE_FLOAT16`` or
  ``EXCESS_PRECISION_TYPE_FAST``.

Return a value, with the same meaning as the C99 macro
``FLT_EVAL_METHOD`` that describes which excess precision should be
applied.

.. hook-end

.. function:: machine_mode TARGET_PROMOTE_FUNCTION_MODE (const_tree type, machine_mode mode, int *punsignedp, const_tree funtype, int for_return)

  .. hook-start:TARGET_PROMOTE_FUNCTION_MODE

  Like ``PROMOTE_MODE``, but it is applied to outgoing function arguments or
  function return values.  The target hook should return the new mode
  and possibly change ``*punsignedp`` if the promotion should
  change signedness.  This function is called only for scalar *or
  pointer* types.

  :samp:`{for_return}` allows to distinguish the promotion of arguments and
  return values.  If it is ``1``, a return value is being promoted and
  ``TARGET_FUNCTION_VALUE`` must perform the same promotions done here.
  If it is ``2``, the returned mode should be that of the register in
  which an incoming parameter is copied, or the outgoing result is computed;
  then the hook should return the same mode as ``promote_mode``, though
  the signedness may be different.

  :samp:`{type}` can be NULL when promoting function arguments of libcalls.

  The default is to not promote arguments and return values.  You can
  also define the hook to ``default_promote_function_mode_always_promote``
  if you would like to apply the same rules given by ``PROMOTE_MODE``.

.. hook-end

.. c:macro:: PARM_BOUNDARY

  Normal alignment required for function parameters on the stack, in
  bits.  All stack parameters receive at least this much alignment
  regardless of data type.  On most machines, this is the same as the
  size of an integer.

.. c:macro:: STACK_BOUNDARY

  Define this macro to the minimum alignment enforced by hardware for the
  stack pointer on this machine.  The definition is a C expression for the
  desired alignment (measured in bits).  This value is used as a default
  if ``PREFERRED_STACK_BOUNDARY`` is not defined.  On most machines,
  this should be the same as ``PARM_BOUNDARY``.

.. c:macro:: PREFERRED_STACK_BOUNDARY

  Define this macro if you wish to preserve a certain alignment for the
  stack pointer, greater than what the hardware enforces.  The definition
  is a C expression for the desired alignment (measured in bits).  This
  macro must evaluate to a value equal to or larger than
  ``STACK_BOUNDARY``.

.. c:macro:: INCOMING_STACK_BOUNDARY

  Define this macro if the incoming stack boundary may be different
  from ``PREFERRED_STACK_BOUNDARY``.  This macro must evaluate
  to a value equal to or larger than ``STACK_BOUNDARY``.

.. c:macro:: FUNCTION_BOUNDARY

  Alignment required for a function entry point, in bits.

.. c:macro:: BIGGEST_ALIGNMENT

  Biggest alignment that any data type can require on this machine, in
  bits.  Note that this is not the biggest alignment that is supported,
  just the biggest alignment that, when violated, may cause a fault.

.. c:var:: HOST_WIDE_INT TARGET_ABSOLUTE_BIGGEST_ALIGNMENT

  .. hook-start:TARGET_ABSOLUTE_BIGGEST_ALIGNMENT

  If defined, this target hook specifies the absolute biggest alignment
  that a type or variable can have on this machine, otherwise,
  ``BIGGEST_ALIGNMENT`` is used.

.. hook-end

.. c:macro:: MALLOC_ABI_ALIGNMENT

  Alignment, in bits, a C conformant malloc implementation has to
  provide.  If not defined, the default value is ``BITS_PER_WORD``.

.. c:macro:: ATTRIBUTE_ALIGNED_VALUE

  Alignment used by the ``__attribute__ ((aligned))`` construct.  If
  not defined, the default value is ``BIGGEST_ALIGNMENT``.

.. c:macro:: MINIMUM_ATOMIC_ALIGNMENT

  If defined, the smallest alignment, in bits, that can be given to an
  object that can be referenced in one operation, without disturbing any
  nearby object.  Normally, this is ``BITS_PER_UNIT``, but may be larger
  on machines that don't have byte or half-word store operations.

.. c:macro:: BIGGEST_FIELD_ALIGNMENT

  Biggest alignment that any structure or union field can require on this
  machine, in bits.  If defined, this overrides ``BIGGEST_ALIGNMENT`` for
  structure and union fields only, unless the field alignment has been set
  by the ``__attribute__ ((aligned (n)))`` construct.

.. c:macro:: ADJUST_FIELD_ALIGN (field, type, computed)

  An expression for the alignment of a structure field :samp:`{field}` of
  type :samp:`{type}` if the alignment computed in the usual way (including
  applying of ``BIGGEST_ALIGNMENT`` and ``BIGGEST_FIELD_ALIGNMENT`` to the
  alignment) is :samp:`{computed}`.  It overrides alignment only if the
  field alignment has not been set by the
  ``__attribute__ ((aligned (n)))`` construct.  Note that :samp:`{field}`
  may be ``NULL_TREE`` in case we just query for the minimum alignment
  of a field of type :samp:`{type}` in structure context.

.. c:macro:: MAX_STACK_ALIGNMENT

  Biggest stack alignment guaranteed by the backend.  Use this macro
  to specify the maximum alignment of a variable on stack.

  If not defined, the default value is ``STACK_BOUNDARY``.

  .. todo:: The default should be @code{PREFERRED_STACK_BOUNDARY}.
    But the fix for PR 32893 indicates that we can only guarantee
    maximum stack alignment on stack up to @code{STACK_BOUNDARY}, not
    @code{PREFERRED_STACK_BOUNDARY}, if stack alignment isn't supported.

.. c:macro:: MAX_OFILE_ALIGNMENT

  Biggest alignment supported by the object file format of this machine.
  Use this macro to limit the alignment which can be specified using the
  ``__attribute__ ((aligned (n)))`` construct for functions and
  objects with static storage duration.  The alignment of automatic
  objects may exceed the object file format maximum up to the maximum
  supported by GCC.  If not defined, the default value is
  ``BIGGEST_ALIGNMENT``.

  On systems that use ELF, the default (in :samp:`config/elfos.h`) is
  the largest supported 32-bit ELF section alignment representable on
  a 32-bit host e.g. :samp:`(((uint64_t) 1 << 28) * 8)`.
  On 32-bit ELF the largest supported section alignment in bits is
  :samp:`(0x80000000 * 8)`, but this is not representable on 32-bit hosts.

.. function:: void TARGET_LOWER_LOCAL_DECL_ALIGNMENT (tree decl)

  .. hook-start:TARGET_LOWER_LOCAL_DECL_ALIGNMENT

  Define this hook to lower alignment of local, parm or result
  decl :samp:`({decl})`.

.. hook-end

.. function:: HOST_WIDE_INT TARGET_STATIC_RTX_ALIGNMENT (machine_mode mode)

  .. hook-start:TARGET_STATIC_RTX_ALIGNMENT

  This hook returns the preferred alignment in bits for a
  statically-allocated rtx, such as a constant pool entry.  :samp:`{mode}`
  is the mode of the rtx.  The default implementation returns
  :samp:`GET_MODE_ALIGNMENT ({mode})`.

.. hook-end

.. c:macro:: DATA_ALIGNMENT (type, basic_align)

  If defined, a C expression to compute the alignment for a variable in
  the static store.  :samp:`{type}` is the data type, and :samp:`{basic_align}` is
  the alignment that the object would ordinarily have.  The value of this
  macro is used instead of that alignment to align the object.

  If this macro is not defined, then :samp:`{basic_align}` is used.

  .. index:: strcpy

  One use of this macro is to increase alignment of medium-size data to
  make it all fit in fewer cache lines.  Another is to cause character
  arrays to be word-aligned so that ``strcpy`` calls that copy
  constants to character arrays can be done inline.

.. c:macro:: DATA_ABI_ALIGNMENT (type, basic_align)

  Similar to ``DATA_ALIGNMENT``, but for the cases where the ABI mandates
  some alignment increase, instead of optimization only purposes.  E.g.AMD x86-64 psABI says that variables with array type larger than 15 bytes
  must be aligned to 16 byte boundaries.

  If this macro is not defined, then :samp:`{basic_align}` is used.

.. function:: HOST_WIDE_INT TARGET_CONSTANT_ALIGNMENT (const_tree constant, HOST_WIDE_INT basic_align)

  .. hook-start:TARGET_CONSTANT_ALIGNMENT

  This hook returns the alignment in bits of a constant that is being
  placed in memory.  :samp:`{constant}` is the constant and :samp:`{basic_align}`
  is the alignment that the object would ordinarily have.

  The default definition just returns :samp:`{basic_align}`.

  The typical use of this hook is to increase alignment for string
  constants to be word aligned so that ``strcpy`` calls that copy
  constants can be done inline.  The function
  ``constant_alignment_word_strings`` provides such a definition.

.. hook-end

.. c:macro:: LOCAL_ALIGNMENT (type, basic_align)

  If defined, a C expression to compute the alignment for a variable in
  the local store.  :samp:`{type}` is the data type, and :samp:`{basic_align}` is
  the alignment that the object would ordinarily have.  The value of this
  macro is used instead of that alignment to align the object.

  If this macro is not defined, then :samp:`{basic_align}` is used.

  One use of this macro is to increase alignment of medium-size data to
  make it all fit in fewer cache lines.

  If the value of this macro has a type, it should be an unsigned type.

.. function:: HOST_WIDE_INT TARGET_VECTOR_ALIGNMENT (const_tree type)

  .. hook-start:TARGET_VECTOR_ALIGNMENT

  This hook can be used to define the alignment for a vector of type
  :samp:`{type}`, in order to comply with a platform ABI.  The default is to
  require natural alignment for vector types.  The alignment returned by
  this hook must be a power-of-two multiple of the default alignment of
  the vector element type.

.. hook-end

.. c:macro:: STACK_SLOT_ALIGNMENT (type, mode, basic_align)

  If defined, a C expression to compute the alignment for stack slot.
  :samp:`{type}` is the data type, :samp:`{mode}` is the widest mode available,
  and :samp:`{basic_align}` is the alignment that the slot would ordinarily
  have.  The value of this macro is used instead of that alignment to
  align the slot.

  If this macro is not defined, then :samp:`{basic_align}` is used when
  :samp:`{type}` is ``NULL``.  Otherwise, ``LOCAL_ALIGNMENT`` will
  be used.

  This macro is to set alignment of stack slot to the maximum alignment
  of all possible modes which the slot may have.

  If the value of this macro has a type, it should be an unsigned type.

.. c:macro:: LOCAL_DECL_ALIGNMENT (decl)

  If defined, a C expression to compute the alignment for a local
  variable :samp:`{decl}`.

  If this macro is not defined, then
  ``LOCAL_ALIGNMENT (TREE_TYPE (decl), DECL_ALIGN (decl))``
  is used.

  One use of this macro is to increase alignment of medium-size data to
  make it all fit in fewer cache lines.

  If the value of this macro has a type, it should be an unsigned type.

.. c:macro:: MINIMUM_ALIGNMENT (exp, mode, align)

  If defined, a C expression to compute the minimum required alignment
  for dynamic stack realignment purposes for :samp:`{exp}` (a type or decl),
  :samp:`{mode}`, assuming normal alignment :samp:`{align}`.

  If this macro is not defined, then :samp:`{align}` will be used.

.. c:macro:: EMPTY_FIELD_BOUNDARY

  Alignment in bits to be given to a structure bit-field that follows an
  empty field such as ``int : 0;``.

  If ``PCC_BITFIELD_TYPE_MATTERS`` is true, it overrides this macro.

.. c:macro:: STRUCTURE_SIZE_BOUNDARY

  Number of bits which any structure or union's size must be a multiple of.
  Each structure or union's size is rounded up to a multiple of this.

  If you do not define this macro, the default is the same as
  ``BITS_PER_UNIT``.

.. c:macro:: STRICT_ALIGNMENT

  Define this macro to be the value 1 if instructions will fail to work
  if given data not on the nominal alignment.  If instructions will merely
  go slower in that case, define this macro as 0.

.. c:macro:: PCC_BITFIELD_TYPE_MATTERS

  Define this if you wish to imitate the way many other C compilers handle
  alignment of bit-fields and the structures that contain them.

  The behavior is that the type written for a named bit-field (``int``,
  ``short``, or other integer type) imposes an alignment for the entire
  structure, as if the structure really did contain an ordinary field of
  that type.  In addition, the bit-field is placed within the structure so
  that it would fit within such a field, not crossing a boundary for it.

  Thus, on most machines, a named bit-field whose type is written as
  ``int`` would not cross a four-byte boundary, and would force
  four-byte alignment for the whole structure.  (The alignment used may
  not be four bytes; it is controlled by the other alignment parameters.)

  An unnamed bit-field will not affect the alignment of the containing
  structure.

  If the macro is defined, its definition should be a C expression;
  a nonzero value for the expression enables this behavior.

  Note that if this macro is not defined, or its value is zero, some
  bit-fields may cross more than one alignment boundary.  The compiler can
  support such references if there are :samp:`insv`, :samp:`extv`, and
  :samp:`extzv` insns that can directly reference memory.

  The other known way of making bit-fields work is to define
  ``STRUCTURE_SIZE_BOUNDARY`` as large as ``BIGGEST_ALIGNMENT``.
  Then every structure can be accessed with fullwords.

  Unless the machine has bit-field instructions or you define
  ``STRUCTURE_SIZE_BOUNDARY`` that way, you must define
  ``PCC_BITFIELD_TYPE_MATTERS`` to have a nonzero value.

  If your aim is to make GCC use the same conventions for laying out
  bit-fields as are used by another compiler, here is how to investigate
  what the other compiler does.  Compile and run this program:

  .. code-block:: c++

    struct foo1
    {
      char x;
      char :0;
      char y;
    };

    struct foo2
    {
      char x;
      int :0;
      char y;
    };

    main ()
    {
      printf ("Size of foo1 is %d\n",
              sizeof (struct foo1));
      printf ("Size of foo2 is %d\n",
              sizeof (struct foo2));
      exit (0);
    }

  If this prints 2 and 5, then the compiler's behavior is what you would
  get from ``PCC_BITFIELD_TYPE_MATTERS``.

.. c:macro:: BITFIELD_NBYTES_LIMITED

  Like ``PCC_BITFIELD_TYPE_MATTERS`` except that its effect is limited
  to aligning a bit-field within the structure.

.. function:: bool TARGET_ALIGN_ANON_BITFIELD (void)

  .. hook-start:TARGET_ALIGN_ANON_BITFIELD

  When ``PCC_BITFIELD_TYPE_MATTERS`` is true this hook will determine
  whether unnamed bitfields affect the alignment of the containing
  structure.  The hook should return true if the structure should inherit
  the alignment requirements of an unnamed bitfield's type.

.. hook-end

.. function:: bool TARGET_NARROW_VOLATILE_BITFIELD (void)

  .. hook-start:TARGET_NARROW_VOLATILE_BITFIELD

  This target hook should return ``true`` if accesses to volatile bitfields
  should use the narrowest mode possible.  It should return ``false`` if
  these accesses should use the bitfield container type.

  The default is ``false``.

.. hook-end

.. function:: bool TARGET_MEMBER_TYPE_FORCES_BLK (const_tree field, machine_mode mode)

  .. hook-start:TARGET_MEMBER_TYPE_FORCES_BLK

  Return true if a structure, union or array containing :samp:`{field}` should
  be accessed using ``BLKMODE``.

  If :samp:`{field}` is the only field in the structure, :samp:`{mode}` is its
  mode, otherwise :samp:`{mode}` is VOIDmode.  :samp:`{mode}` is provided in the
  case where structures of one field would require the structure's mode to
  retain the field's mode.

  Normally, this is not needed.

.. hook-end

.. c:macro:: ROUND_TYPE_ALIGN (type, computed, specified)

  Define this macro as an expression for the alignment of a type (given
  by :samp:`{type}` as a tree node) if the alignment computed in the usual
  way is :samp:`{computed}` and the alignment explicitly specified was
  :samp:`{specified}`.

  The default is to use :samp:`{specified}` if it is larger; otherwise, use
  the smaller of :samp:`{computed}` and ``BIGGEST_ALIGNMENT``

.. c:macro:: MAX_FIXED_MODE_SIZE

  An integer expression for the size in bits of the largest integer
  machine mode that should actually be used.  All integer machine modes of
  this size or smaller can be used for structures and unions with the
  appropriate sizes.  If this macro is undefined, ``GET_MODE_BITSIZE
  (DImode)`` is assumed.

.. c:macro:: STACK_SAVEAREA_MODE (save_level)

  If defined, an expression of type ``machine_mode`` that
  specifies the mode of the save area operand of a
  ``save_stack_level`` named pattern (see :ref:`standard-names`).
  :samp:`{save_level}` is one of ``SAVE_BLOCK``, ``SAVE_FUNCTION``, or
  ``SAVE_NONLOCAL`` and selects which of the three named patterns is
  having its mode specified.

  You need not define this macro if it always returns ``Pmode``.  You
  would most commonly define this macro if the
  ``save_stack_level`` patterns need to support both a 32- and a
  64-bit mode.

.. c:macro:: STACK_SIZE_MODE

  If defined, an expression of type ``machine_mode`` that
  specifies the mode of the size increment operand of an
  ``allocate_stack`` named pattern (see :ref:`standard-names`).

  You need not define this macro if it always returns ``word_mode``.
  You would most commonly define this macro if the ``allocate_stack``
  pattern needs to support both a 32- and a 64-bit mode.

.. function:: scalar_int_mode TARGET_LIBGCC_CMP_RETURN_MODE (void)

  .. hook-start:TARGET_LIBGCC_CMP_RETURN_MODE

  This target hook should return the mode to be used for the return value
  of compare instructions expanded to libgcc calls.  If not defined
  ``word_mode`` is returned which is the right choice for a majority of
  targets.

.. hook-end

.. function:: scalar_int_mode TARGET_LIBGCC_SHIFT_COUNT_MODE (void)

  .. hook-start:TARGET_LIBGCC_SHIFT_COUNT_MODE

  This target hook should return the mode to be used for the shift count operand
  of shift instructions expanded to libgcc calls.  If not defined
  ``word_mode`` is returned which is the right choice for a majority of
  targets.

.. hook-end

.. function:: scalar_int_mode TARGET_UNWIND_WORD_MODE (void)

  .. hook-start:TARGET_UNWIND_WORD_MODE

  Return machine mode to be used for ``_Unwind_Word`` type.
  The default is to use ``word_mode``.

.. hook-end

.. function:: bool TARGET_MS_BITFIELD_LAYOUT_P (const_tree record_type)

  .. hook-start:TARGET_MS_BITFIELD_LAYOUT_P

  This target hook returns ``true`` if bit-fields in the given
  :samp:`{record_type}` are to be laid out following the rules of Microsoft
  Visual C/C++, namely: (i) a bit-field won't share the same storage
  unit with the previous bit-field if their underlying types have
  different sizes, and the bit-field will be aligned to the highest
  alignment of the underlying types of itself and of the previous
  bit-field; (ii) a zero-sized bit-field will affect the alignment of
  the whole enclosing structure, even if it is unnamed; except that
  (iii) a zero-sized bit-field will be disregarded unless it follows
  another bit-field of nonzero size.  If this hook returns ``true``,
  other macros that control bit-field layout are ignored.

  When a bit-field is inserted into a packed record, the whole size
  of the underlying type is used by one or more same-size adjacent
  bit-fields (that is, if its long:3, 32 bits is used in the record,
  and any additional adjacent long bit-fields are packed into the same
  chunk of 32 bits.  However, if the size changes, a new field of that
  size is allocated).  In an unpacked record, this is the same as using
  alignment, but not equivalent when packing.

  If both MS bit-fields and :samp:`__attribute__((packed))` are used,
  the latter will take precedence.  If :samp:`__attribute__((packed))` is
  used on a single field when MS bit-fields are in use, it will take
  precedence for that field, but the alignment of the rest of the structure
  may affect its placement.

.. hook-end

.. function:: bool TARGET_DECIMAL_FLOAT_SUPPORTED_P (void)

  .. hook-start:TARGET_DECIMAL_FLOAT_SUPPORTED_P

  Returns true if the target supports decimal floating point.

.. hook-end

.. function:: bool TARGET_FIXED_POINT_SUPPORTED_P (void)

  .. hook-start:TARGET_FIXED_POINT_SUPPORTED_P

  Returns true if the target supports fixed-point arithmetic.

.. hook-end

.. function:: void TARGET_EXPAND_TO_RTL_HOOK (void)

  .. hook-start:TARGET_EXPAND_TO_RTL_HOOK

  This hook is called just before expansion into rtl, allowing the target
  to perform additional initializations or analysis before the expansion.
  For example, the rs6000 port uses it to allocate a scratch stack slot
  for use in copying SDmode values between memory and floating point
  registers whenever the function being expanded has any SDmode
  usage.

.. hook-end

.. function:: void TARGET_INSTANTIATE_DECLS (void)

  .. hook-start:TARGET_INSTANTIATE_DECLS

  This hook allows the backend to perform additional instantiations on rtl
  that are not actually in any insns yet, but will be later.

.. hook-end

.. function:: const char * TARGET_MANGLE_TYPE (const_tree type)

  .. hook-start:TARGET_MANGLE_TYPE

  If your target defines any fundamental types, or any types your target
  uses should be mangled differently from the default, define this hook
  to return the appropriate encoding for these types as part of a C++
  mangled name.  The :samp:`{type}` argument is the tree structure representing
  the type to be mangled.  The hook may be applied to trees which are
  not target-specific fundamental types; it should return ``NULL``
  for all such types, as well as arguments it does not recognize.  If the
  return value is not ``NULL``, it must point to a statically-allocated
  string constant.

  Target-specific fundamental types might be new fundamental types or
  qualified versions of ordinary fundamental types.  Encode new
  fundamental types as :samp:`u {n}{name}`, where :samp:`{name}`
  is the name used for the type in source code, and :samp:`{n}` is the
  length of :samp:`{name}` in decimal.  Encode qualified versions of
  ordinary types as :samp:`U{n}{name}{code}`, where
  :samp:`{name}` is the name used for the type qualifier in source code,
  :samp:`{n}` is the length of :samp:`{name}` as above, and :samp:`{code}` is the
  code used to represent the unqualified version of this type.  (See
  ``write_builtin_type`` in :samp:`cp/mangle.cc` for the list of
  codes.)  In both cases the spaces are for clarity; do not include any
  spaces in your string.

  This hook is applied to types prior to typedef resolution.  If the mangled
  name for a particular type depends only on that type's main variant, you
  can perform typedef resolution yourself using ``TYPE_MAIN_VARIANT``
  before mangling.

  The default version of this hook always returns ``NULL``, which is
  appropriate for a target that does not define any new fundamental
  types.

.. hook-end