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.
BITS_BIG_ENDIAN
This macro does not affect the way structure fields are packed into bytes or words; that is controlled by BYTES_BIG_ENDIAN .
BYTES_BIG_ENDIAN
WORDS_BIG_ENDIAN
LIBGCC2_WORDS_BIG_ENDIAN
FLOAT_WORDS_BIG_ENDIAN 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.
BITS_PER_UNIT
BITS_PER_WORD
MAX_BITS_PER_WORD BITS_PER_WORD . Otherwise, it is the constant value that is the largest value that BITS_PER_WORD can have at run-time.
UNITS_PER_WORD
MIN_UNITS_PER_WORD UNITS_PER_WORD . Otherwise, it is the constant value that is the smallest value that UNITS_PER_WORD can have at run-time.
POINTER_SIZE Pmode . If it is not equal to the width of Pmode , you must define POINTERS_EXTEND_UNSIGNED .
POINTERS_EXTEND_UNSIGNED POINTER_SIZE bits wide to Pmode are sign-extended and zero if they are zero-extended.
You need not define this macro if the POINTER_SIZE is equal to the width of Pmode .
PROMOTE_MODE (m, unsignedp, 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.
PROMOTE_FUNCTION_ARGS PROMOTE_MODE should also be done for outgoing function arguments.
PROMOTE_FUNCTION_RETURN PROMOTE_MODE should also be done for the return value of functions.
If this macro is defined, FUNCTION_VALUE must perform the same promotions done by PROMOTE_MODE .
PROMOTE_FOR_CALL_ONLY PROMOTE_MODE should only be performed for outgoing function arguments or function return values, as specified by PROMOTE_FUNCTION_ARGS and PROMOTE_FUNCTION_RETURN , respectively.
PARM_BOUNDARY
STACK_BOUNDARY
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 STACK_BOUNDARY , the stack may be momentarily unaligned while pushing arguments.
FUNCTION_BOUNDARY
BIGGEST_ALIGNMENT
BIGGEST_FIELD_ALIGNMENT BIGGEST_ALIGNMENT for structure fields only.
MAX_OFILE_ALIGNMENT __attribute__ ((aligned (n))) construct. If not defined, the default value is BIGGEST_ALIGNMENT .
DATA_ALIGNMENT (type, basic-align) 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.
CONSTANT_ALIGNMENT (constant, basic-align) 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.
EMPTY_FIELD_BOUNDARY int : 0; .
Note that PCC_BITFIELD_TYPE_MATTERS also affects the alignment that results from an empty field.
STRUCTURE_SIZE_BOUNDARY
If you do not define this macro, the default is the same as BITS_PER_UNIT .
STRICT_ALIGNMENT
PCC_BITFIELD_TYPE_MATTERS
The behavior is that the type written for a bitfield (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 bitfield 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 bitfield 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.)
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 bitfields 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 bitfields work is to define STRUCTURE_SIZE_BOUNDARY as large as BIGGEST_ALIGNMENT . Then every structure can be accessed with fullwords.
Unless the machine has bitfield 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 GNU CC use the same conventions for laying out bitfields 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 .
BITFIELD_NBYTES_LIMITED
ROUND_TYPE_SIZE (struct, size, align) The default is to round size up to a multiple of align.
ROUND_TYPE_ALIGN (struct, computed, specified)
The default is to use specified if it is larger; otherwise, use the smaller of computed and BIGGEST_ALIGNMENT
MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (DImode) is assumed.
CHECK_FLOAT_VALUE (mode, value, overflow) double ) for mode mode. This means that you check whether value fits within the possible range of values for mode mode on this target machine. The mode mode is always a mode of class MODE_FLOAT . overflow is nonzero if the value is already known to be out of range. If value is not valid or if overflow is nonzero, you should set overflow to 1 and then assign some valid value to value. Allowing an invalid value to go through the compiler can produce incorrect assembler code which may even cause Unix assemblers to crash.
This macro need not be defined if there is no work for it to do.
TARGET_FLOAT_FORMAT
IEEE_FLOAT_FORMAT
VAX_FLOAT_FORMAT
UNKNOWN_FLOAT_FORMAT
The value of this macro is compared with HOST_FLOAT_FORMAT (see Config) to determine whether the target machine has the same format as the host machine. If any other formats are actually in use on supported machines, new codes should be defined for them.
The ordering of the component words of floating point values stored in memory is controlled by FLOAT_WORDS_BIG_ENDIAN for the target machine and HOST_FLOAT_WORDS_BIG_ENDIAN for the host.