Next: Type Layout, Previous: Per-Function Data, Up: Target Macros [Contents][Index]
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.
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.
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.
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.
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.
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.
Number of bits in a word. If you do not define this macro, the default
is BITS_PER_UNIT * UNITS_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.
Number of storage units in a word; normally the size of a general-purpose register, a power of two from 1 or 8.
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.
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.
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.
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.
Return a value, with the same meaning as the C99 macro FLT_EVAL_METHOD that describes which excess precision should be applied. type is either EXCESS_PRECISION_TYPE_IMPLICIT, EXCESS_PRECISION_TYPE_FAST, or EXCESS_PRECISION_TYPE_STANDARD. 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 and EXCESS_PRECISION_TYPE_FAST, the target should return the explicit excess precision that should be added depending on the value set for -fexcess-precision=[standard|fast]. Note that unpredictable explicit excess precision does not make sense, so a target should never return FLT_EVAL_METHOD_UNPREDICTABLE when type is EXCESS_PRECISION_TYPE_STANDARD or EXCESS_PRECISION_TYPE_FAST.
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.
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.
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.
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.
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.
Alignment required for a function entry point, in bits.
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.
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.
Alignment, in bits, a C conformant malloc implementation has to
provide. If not defined, the default value is BITS_PER_WORD.
Alignment used by the __attribute__ ((aligned)) construct. If
not defined, the default value is BIGGEST_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.
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.
An expression for the alignment of a structure field field of
type type 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. Note that field
may be NULL_TREE in case we just query for the minimum alignment
of a field of type type in structure context.
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.
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 config/elfos.h) is the largest supported 32-bit ELF section alignment representable on a 32-bit host e.g. ‘(((uint64_t) 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.
This hook returns the preferred alignment in bits for a statically-allocated rtx, such as a constant pool entry. mode is the mode of the rtx. The default implementation returns ‘GET_MODE_ALIGNMENT (mode)’.
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.
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.
This hook returns the alignment in bits of 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 default definition just returns 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Like PCC_BITFIELD_TYPE_MATTERS except that its effect is limited
to aligning a bit-field within the structure.
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.
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.
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.
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
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.
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 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.
If defined, an expression of type 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.
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.
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.
Return machine mode to be used for _Unwind_Word type.
The default is to use word_mode.
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.
Returns true if the target supports decimal floating point.
Returns true if the target supports fixed-point arithmetic.
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.
This hook allows the backend to perform additional instantiations on rtl that are not actually in any insns yet, but will be later.
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.
Next: Type Layout, Previous: Per-Function Data, Up: Target Macros [Contents][Index]