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11.5 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; GCC fundamentally assumes that the order of words in memory is the same as the order in registers. This macro need not be a constant.

— Macro: LIBGCC2_WORDS_BIG_ENDIAN

Define this macro if WORDS_BIG_ENDIAN is not constant. This must be a constant value with the same meaning as WORDS_BIG_ENDIAN, which will be used only when compiling libgcc2.c. Typically the value will be set based on preprocessor defines.

— 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_UNIT

Define this macro to be the number of bits in an addressable storage unit (byte). If you do not define this macro the default is 8.

— 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 4.

— 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 whose value is greater than zero if pointers that need to be extended from being POINTER_SIZE bits wide to Pmode are to be zero-extended and zero if they are to be sign-extended. If the value is less then zero then there must be an "ptr_extend" instruction that extends a pointer from POINTER_SIZE to Pmode.

You need not define this macro if the POINTER_SIZE is equal to the width of Pmode.

— 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: bool TARGET_PROMOTE_FUNCTION_ARGS (tree fntype)

This target hook should return true if the promotion described by PROMOTE_MODE should also be done for outgoing function arguments.

— Target Hook: bool TARGET_PROMOTE_FUNCTION_RETURN (tree fntype)

This target hook should return true if the promotion described by PROMOTE_MODE should also be done for the return value of functions.

If this target hook returns true, FUNCTION_VALUE must perform the same promotions done by PROMOTE_MODE.

— Macro: PROMOTE_FOR_CALL_ONLY

Define this macro if the promotion described by PROMOTE_MODE should only be performed for outgoing function arguments or function return values, as specified by TARGET_PROMOTE_FUNCTION_ARGS and TARGET_PROMOTE_FUNCTION_RETURN, respectively.

— 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: FORCE_PREFERRED_STACK_BOUNDARY_IN_MAIN

A C expression that evaluates true if PREFERRED_STACK_BOUNDARY is not guaranteed by the runtime and we should emit code to align the stack at the beginning of main.

If PUSH_ROUNDING is not defined, the stack will always be aligned to the specified boundary. If PUSH_ROUNDING is defined and specifies a less strict alignment than PREFERRED_STACK_BOUNDARY, the stack may be momentarily unaligned while pushing arguments.

— 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.

— 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_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.

— 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: 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.

— 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.

— Macro: MEMBER_TYPE_FORCES_BLK (field, mode)

Return 1 if a structure 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. See the file c4x.h for an example of how to use this macro to prevent a structure having a floating point field from being accessed in an integer mode.

— 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: VECTOR_MODE_SUPPORTED_P (mode)

Define this macro to be nonzero if the port is prepared to handle insns involving vector mode mode. At the very least, it must have move patterns for this mode.

— 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.

— Macro: TARGET_FLOAT_FORMAT

A code distinguishing the floating point format of the target machine. There are four defined values:

IEEE_FLOAT_FORMAT
This code indicates IEEE floating point. It is the default; there is no need to define TARGET_FLOAT_FORMAT when the format is IEEE.
VAX_FLOAT_FORMAT
This code indicates the “F float” (for float) and “D float” or “G float” formats (for double) used on the VAX and PDP-11.
IBM_FLOAT_FORMAT
This code indicates the format used on the IBM System/370.
C4X_FLOAT_FORMAT
This code indicates the format used on the TMS320C3x/C4x.

If your target uses a floating point format other than these, you must define a new name_FLOAT_FORMAT code for it, and add support for it to real.c.

The ordering of the component words of floating point values stored in memory is controlled by FLOAT_WORDS_BIG_ENDIAN.

— Macro: MODE_HAS_NANS (mode)

When defined, this macro should be true if mode has a NaN representation. The compiler assumes that NaNs are not equal to anything (including themselves) and that addition, subtraction, multiplication and division all return NaNs when one operand is NaN.

By default, this macro is true if mode is a floating-point mode and the target floating-point format is IEEE.

— Macro: MODE_HAS_INFINITIES (mode)

This macro should be true if mode can represent infinity. At present, the compiler uses this macro to decide whether `x - x' is always defined. By default, the macro is true when mode is a floating-point mode and the target format is IEEE.

— Macro: MODE_HAS_SIGNED_ZEROS (mode)

True if mode distinguishes between positive and negative zero. The rules are expected to follow the IEEE standard:

The default definition is true if mode is a floating-point mode and the target format is IEEE.

— Macro: MODE_HAS_SIGN_DEPENDENT_ROUNDING (mode)

If defined, this macro should be true for mode if it has at least one rounding mode in which `x' and `-x' can be rounded to numbers of different magnitude. Two such modes are towards −infinity and towards +infinity.

The default definition of this macro is true if mode is a floating-point mode and the target format is IEEE.

— Macro: ROUND_TOWARDS_ZERO

If defined, this macro should be true if the prevailing rounding mode is towards zero. A true value has the following effects:

The macro does not affect the parsing of string literals. When the primary rounding mode is towards zero, library functions like strtod might still round towards nearest, and the compiler's parser should behave like the target's strtod where possible.

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 to true for size causes MODE_HAS_NANS and MODE_HAS_INFINITIES to be false for size-bit modes. It also affects the way libgcc.a and real.c emulate floating-point arithmetic.

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

— Target Hook: bool TARGET_VECTOR_OPAQUE_P (tree type)

This target hook should return true a vector is opaque. That is, if no cast is needed when copying a vector value of type type into another vector lvalue of the same size. Vector opaque types cannot be initialized. The default is that there are no such types.

— Target Hook: bool TARGET_MS_BITFIELD_LAYOUT_P (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: const char * TARGET_MANGLE_FUNDAMENTAL_TYPE (tree type)

If your target defines any fundamental types, 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 `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 `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.

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