On many machines, the numbered registers are not all equivalent. For example, certain registers may not be allowed for indexed addressing; certain registers may not be allowed in some instructions. These machine restrictions are described to the compiler using register classes.
You define a number of register classes, giving each one a name and saying which of the registers belong to it. Then you can specify register classes that are allowed as operands to particular instruction patterns.
In general, each register will belong to several classes. In fact, one
class must be named
ALL_REGS and contain all the registers. Another
class must be named
NO_REGS and contain no registers. Often the
union of two classes will be another class; however, this is not required.
One of the classes must be named
GENERAL_REGS. There is nothing
terribly special about the name, but the operand constraint letters
`r' and `g' specify this class. If
the same as
ALL_REGS, just define it as a macro which expands
Order the classes so that if class x is contained in class y then x has a lower class number than y.
The way classes other than
GENERAL_REGS are specified in operand
constraints is through machine-dependent operand constraint letters.
You can define such letters to correspond to various classes, then use
them in operand constraints.
You should define a class for the union of two classes whenever some
instruction allows both classes. For example, if an instruction allows
either a floating point (coprocessor) register or a general register for a
certain operand, you should define a class
which includes both of them. Otherwise you will get suboptimal code.
You must also specify certain redundant information about the register classes: for each class, which classes contain it and which ones are contained in it; for each pair of classes, the largest class contained in their union.
When a value occupying several consecutive registers is expected in a
certain class, all the registers used must belong to that class.
Therefore, register classes cannot be used to enforce a requirement for
a register pair to start with an even-numbered register. The way to
specify this requirement is with
Register classes used for input-operands of bitwise-and or shift
instructions have a special requirement: each such class must have, for
each fixed-point machine mode, a subclass whose registers can transfer that
mode to or from memory. For example, on some machines, the operations for
single-byte values (
QImode) are limited to certain registers. When
this is so, each register class that is used in a bitwise-and or shift
instruction must have a subclass consisting of registers from which
single-byte values can be loaded or stored. This is so that
PREFERRED_RELOAD_CLASS can always have a possible value to return.
NO_REGSmust be first.
ALL_REGSmust be the last register class, followed by one more enumeral value,
LIM_REG_CLASSES, which is not a register class but rather tells how many classes there are. Each register class has a number, which is the value of casting the class name to type
int. The number serves as an index in many of the tables described below.
#define N_REG_CLASSES (int) LIM_REG_CLASSES
mask & (1 << r)is 1. When the machine has more than 32 registers, an integer does not suffice. Then the integers are replaced by sub-initializers, braced groupings containing several integers. Each sub-initializer must be suitable as an initializer for the type
HARD_REG_SETwhich is defined in `hard-reg-set.h'.
NO_REGS. The register letter `r', corresponding to class
GENERAL_REGS, will not be passed to this macro; you do not need to handle it.
REGNO_MODE_OK_FOR_BASE_P (num, mode)
REGNO_OK_FOR_BASE_P, except that that expression may examine the mode of the memory reference in mode. You should define this macro if the mode of the memory reference affects whether a register may be used as a base register. If you define this macro, the compiler will use it instead of
PREFERRED_RELOAD_CLASS (x, class)
#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASSSometimes returning a more restrictive class makes better code. For example, on the 68000, when x is an integer constant that is in range for a `moveq' instruction, the value of this macro is always
DATA_REGSas long as class includes the data registers. Requiring a data register guarantees that a `moveq' will be used. If x is a
const_double, by returning
NO_REGSyou can force x into a memory constant. This is useful on certain machines where immediate floating values cannot be loaded into certain kinds of registers.
PREFERRED_OUTPUT_RELOAD_CLASS (x, class)
PREFERRED_RELOAD_CLASS, but for output reloads instead of input reloads. If you don't define this macro, the default is to use class, unchanged.
LIMIT_RELOAD_CLASS (mode, class)
PREFERRED_RELOAD_CLASS, this macro should be used when there are certain modes that simply can't go in certain reload classes. The value is a register class; perhaps class, or perhaps another, smaller class. Don't define this macro unless the target machine has limitations which require the macro to do something nontrivial.
SECONDARY_RELOAD_CLASS (class, mode, x)
SECONDARY_INPUT_RELOAD_CLASS (class, mode, x)
SECONDARY_OUTPUT_RELOAD_CLASS (class, mode, x)
SECONDARY_INPUT_RELOAD_CLASSto return the largest register class all of whose registers can be used as intermediate registers or scratch registers. If copying a register class in mode to x requires an intermediate or scratch register,
SECONDARY_OUTPUT_RELOAD_CLASSshould be defined to return the largest register class required. If the requirements for input and output reloads are the same, the macro
SECONDARY_RELOAD_CLASSshould be used instead of defining both macros identically. The values returned by these macros are often
NO_REGSif no spare register is needed; i.e., if x can be directly copied to or from a register of class in mode without requiring a scratch register. Do not define this macro if it would always return
NO_REGS. If a scratch register is required (either with or without an intermediate register), you should define patterns for `reload_inm' or `reload_outm', as required (see section Standard Pattern Names For Generation. These patterns, which will normally be implemented with a
define_expand, should be similar to the `movm' patterns, except that operand 2 is the scratch register. Define constraints for the reload register and scratch register that contain a single register class. If the original reload register (whose class is class) can meet the constraint given in the pattern, the value returned by these macros is used for the class of the scratch register. Otherwise, two additional reload registers are required. Their classes are obtained from the constraints in the insn pattern. x might be a pseudo-register or a
subregof a pseudo-register, which could either be in a hard register or in memory. Use
true_regnumto find out; it will return -1 if the pseudo is in memory and the hard register number if it is in a register. These macros should not be used in the case where a particular class of registers can only be copied to memory and not to another class of registers. In that case, secondary reload registers are not needed and would not be helpful. Instead, a stack location must be used to perform the copy and the
movmpattern should use memory as a intermediate storage. This case often occurs between floating-point and general registers.
SECONDARY_MEMORY_NEEDED (class1, class2, m)
SECONDARY_MEMORY_NEEDEDis defined, the compiler allocates a stack slot for a memory location needed for register copies. If this macro is defined, the compiler instead uses the memory location defined by this macro. Do not define this macro if you do not define
BITS_PER_WORDbits and performs the store and load operations in a mode that many bits wide and whose class is the same as that of mode. This is right thing to do on most machines because it ensures that all bits of the register are copied and prevents accesses to the registers in a narrower mode, which some machines prohibit for floating-point registers. However, this default behavior is not correct on some machines, such as the DEC Alpha, that store short integers in floating-point registers differently than in integer registers. On those machines, the default widening will not work correctly and you must define this macro to suppress that widening in some cases. See the file `alpha.h' for details. Do not define this macro if you do not define
SECONDARY_MEMORY_NEEDEDor if widening mode to a mode that is
BITS_PER_WORDbits wide is correct for your machine.
SMALL_REGISTER_CLASSESto be an expression with a non-zero value on these machines. When this macro has a non-zero value, the compiler allows registers explicitly used in the rtl to be used as spill registers but avoids extending the lifetime of these registers. It is always safe to define this macro with a non-zero value, but if you unnecessarily define it, you will reduce the amount of optimizations that can be performed in some cases. If you do not define this macro with a non-zero value when it is required, the compiler will run out of spill registers and print a fatal error message. For most machines, you should not define this macro at all.
CLASS_MAX_NREGS (class, mode)
HARD_REGNO_NREGS. In fact, the value of the macro
CLASS_MAX_NREGS (class, mode)should be the maximum value of
HARD_REGNO_NREGS (regno, mode)for all regno values in the class class. This macro helps control the handling of multiple-word values in the reload pass.
Three other special macros describe which operands fit which constraint letters.
CONST_OK_FOR_LETTER_P (value, c)
CONST_DOUBLE_OK_FOR_LETTER_P (value, c)
const_doublevalues (`G' or `H'). If c is one of those letters, the expression should check that value, an RTX of code
const_double, is in the appropriate range and return 1 if so, 0 otherwise. If c is not one of those letters, the value should be 0 regardless of value.
const_doubleis used for all floating-point constants and for
DImodefixed-point constants. A given letter can accept either or both kinds of values. It can use
GET_MODEto distinguish between these kinds.
EXTRA_CONSTRAINT (value, c)