GNU Compiler Collection (GCC) Internals: Storage Layout

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### 17.5 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 Run-time Target.

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`.

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.

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.

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.

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.

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`.

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.

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.

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.

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`.

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.

Macro: PROMOTE_MODE (m, unsignedp, type)

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

  On most RISC machines, which only have operations that operate on a full register, define this macro to set m to `word_mode` if 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 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 unsignedp according to which kind of extension is more efficient.

  Do not define this macro if it would never modify m.

Target Hook: enum machine_mode TARGET_PROMOTE_FUNCTION_MODE (const_tree type, enum machine_mode mode, int *punsignedp, const_tree funtype, int for_return)

  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.

  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.

  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`.

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.

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`.

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`.

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`.

Macro: FUNCTION_BOUNDARY

  Alignment required for a function entry point, in bits.

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.

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`.

Macro: ATTRIBUTE_ALIGNED_VALUE

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

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.

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.

Macro: ADJUST_FIELD_ALIGN (field, computed)

  An expression for the alignment of a structure field field if the alignment computed in the usual way (including applying of `BIGGEST_ALIGNMENT` and `BIGGEST_FIELD_ALIGNMENT` to the alignment) is computed. It overrides alignment only if the field alignment has not been set by the `__attribute__ ((aligned (n)))` construct.

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`.

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

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

Macro: DATA_ALIGNMENT (type, basic-align)

  If defined, a C expression to compute the alignment for a variable in the static store. type is the data type, and 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 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. Another is to cause character arrays to be word-aligned so that `strcpy` calls that copy constants to character arrays can be done inline.

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 basic-align is used.

Macro: CONSTANT_ALIGNMENT (constant, basic-align)

  If defined, a C expression to compute the alignment given to a constant that is being placed in memory. constant is the constant and 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 basic-align is used.

  The typical use of this macro is to increase alignment for string constants to be word aligned so that `strcpy` calls that copy constants can be done inline.

Macro: LOCAL_ALIGNMENT (type, basic-align)

  If defined, a C expression to compute the alignment for a variable in the local store. type is the data type, and 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 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.

Target Hook: HOST_WIDE_INT TARGET_VECTOR_ALIGNMENT (const_tree type)

  This hook can be used to define the alignment for a vector of type 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.

Macro: STACK_SLOT_ALIGNMENT (type, mode, basic-align)

  If defined, a C expression to compute the alignment for stack slot. type is the data type, mode is the widest mode available, and 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 basic-align is used when 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.

Macro: LOCAL_DECL_ALIGNMENT (decl)

  If defined, a C expression to compute the alignment for a local variable 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.

Macro: MINIMUM_ALIGNMENT (exp, mode, align)

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

  If this macro is not defined, then align will be used.

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.

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`.

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.

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 ‘`insv`’, ‘`extv`’, and ‘`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:

      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`.

Macro: BITFIELD_NBYTES_LIMITED

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

Target Hook: bool TARGET_ALIGN_ANON_BITFIELD (void)

  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.

Target Hook: bool TARGET_NARROW_VOLATILE_BITFIELD (void)

  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`.

Target Hook: bool TARGET_MEMBER_TYPE_FORCES_BLK (const_tree field, enum machine_mode mode)

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

  If field is the only field in the structure, mode is its mode, otherwise mode is VOIDmode. 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.

Macro: ROUND_TYPE_ALIGN (type, computed, specified)

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

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

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.

Macro: STACK_SAVEAREA_MODE (save_level)

  If defined, an expression of type `enum machine_mode` that specifies the mode of the save area operand of a `save_stack_level` named pattern (see Standard Names). 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.

Macro: STACK_SIZE_MODE

  If defined, an expression of type `enum machine_mode` that specifies the mode of the size increment operand of an `allocate_stack` named pattern (see 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.

Target Hook: enum machine_mode TARGET_LIBGCC_CMP_RETURN_MODE (void)

  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.

Target Hook: enum machine_mode TARGET_LIBGCC_SHIFT_COUNT_MODE (void)

  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.

Target Hook: enum machine_mode TARGET_UNWIND_WORD_MODE (void)

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

Macro: ROUND_TOWARDS_ZERO

  If defined, this macro should be true if the prevailing rounding mode is towards zero.

  Defining this macro only affects the way `libgcc.a` emulates floating-point arithmetic.

  Not defining this macro is equivalent to returning zero.

Macro: LARGEST_EXPONENT_IS_NORMAL (size)

  This macro should return true if floats with size bits do not have a NaN or infinity representation, but use the largest exponent for normal numbers instead.

  Defining this macro only affects the way `libgcc.a` emulates floating-point arithmetic.

  The default definition of this macro returns false for all sizes.

Target Hook: bool TARGET_MS_BITFIELD_LAYOUT_P (const_tree record_type)

  This target hook returns `true` if bit-fields in the given 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 ‘`__attribute__((packed))`’ are used, the latter will take precedence. If ‘`__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.

Target Hook: bool TARGET_DECIMAL_FLOAT_SUPPORTED_P (void)

  Returns true if the target supports decimal floating point.

Target Hook: bool TARGET_FIXED_POINT_SUPPORTED_P (void)

  Returns true if the target supports fixed-point arithmetic.

Target Hook: void TARGET_EXPAND_TO_RTL_HOOK (void)

  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.

Target Hook: void TARGET_INSTANTIATE_DECLS (void)

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

Target Hook: const char * TARGET_MANGLE_TYPE (const_tree 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 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 ‘`u n name`’, where name is the name used for the type in source code, and n is the length of name in decimal. Encode qualified versions of ordinary types as ‘`U n name code`’, where name is the name used for the type qualifier in source code, n is the length of name as above, and code is the code used to represent the unqualified version of this type. (See `write_builtin_type` in `cp/mangle.c` 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.

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