This section discusses the macros that describe which kinds of values (specifically, which machine modes) each register can hold, and how many consecutive registers are needed for a given mode.
On a machine where all registers are exactly one word, a suitable definition of this macro is
#define HARD_REGNO_NREGS(REGNO, MODE) \ ((GET_MODE_SIZE (MODE) + UNITS_PER_WORD - 1) \ / UNITS_PER_WORD)
#define HARD_REGNO_MODE_OK(REGNO, MODE) 1
You need not include code to check for the numbers of fixed registers, because the allocation mechanism considers them to be always occupied.
On some machines, double-precision values must be kept in even/odd register pairs. You can implement that by defining this macro to reject odd register numbers for such modes.
The minimum requirement for a mode to be OK in a register is that the
instruction pattern support moves between the
register and other hard register in the same class and that moving a
value into the register and back out not alter it.
Since the same instruction used to move
word_mode will work for
all narrower integer modes, it is not necessary on any machine for
HARD_REGNO_MODE_OK to distinguish between these modes, provided
you define patterns
movhi, etc., to take advantage of this. This
is useful because of the interaction between
MODES_TIEABLE_P; it is very desirable for all integer modes
to be tieable.
Many machines have special registers for floating point arithmetic. Often people assume that floating point machine modes are allowed only in floating point registers. This is not true. Any registers that can hold integers can safely hold a floating point machine mode, whether or not floating arithmetic can be done on it in those registers. Integer move instructions can be used to move the values.
On some machines, though, the converse is true: fixed-point machine
modes may not go in floating registers. This is true if the floating
registers normalize any value stored in them, because storing a
non-floating value there would garble it. In this case,
HARD_REGNO_MODE_OK should reject fixed-point machine modes in
floating registers. But if the floating registers do not automatically
normalize, if you can store any bit pattern in one and retrieve it
unchanged without a trap, then any machine mode may go in a floating
register, so you can define this macro to say so.
The primary significance of special floating registers is rather that
they are the registers acceptable in floating point arithmetic
instructions. However, this is of no concern to
HARD_REGNO_MODE_OK. You handle it by writing the proper
constraints for those instructions.
On some machines, the floating registers are especially slow to access,
so that it is better to store a value in a stack frame than in such a
register if floating point arithmetic is not being done. As long as the
floating registers are not in class
GENERAL_REGS, they will not
be used unless some pattern's constraint asks for one.
) are always the same for
any r, then
should be nonzero. If they differ for any r, you should define
this macro to return zero unless some other mechanism ensures the
accessibility of the value in a narrower mode.
You should define this macro to return nonzero in as many cases as
possible since doing so will allow GCC to perform better register
CCmoderegisters. You should only define this macro if support for copying to/from