Using and Porting GNU CC

16.6: Register Classes

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 GENERAL_REGS is the same as ALL_REGS, just define it as a macro which expands to ALL_REGS.

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 FLOAT_OR_GENERAL_REGS 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 HARD_REGNO_MODE_OK.

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.

enum reg_class

An enumeral type that must be defined with all the register class names as enumeral values. NO_REGS must be first. ALL_REGS must 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.

N_REG_CLASSES

The number of distinct register classes, defined as follows:

#define N_REG_CLASSES (int) LIM_REG_CLASSES

REG_CLASS_NAMES

An initializer containing the names of the register classes as C string constants. These names are used in writing some of the debugging dumps.

REG_CLASS_CONTENTS

An initializer containing the contents of the register classes, as integers which are bit masks. The nth integer specifies the contents of class n. The way the integer mask is interpreted is that register r is in the class if 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_SET which is defined in `hard-reg-set.h'.

REGNO_REG_CLASS (regno)

A C expression whose value is a register class containing hard register regno. In general there is more than one such class; choose a class which is minimal, meaning that no smaller class also contains the register.

BASE_REG_CLASS

A macro whose definition is the name of the class to which a valid base register must belong. A base register is one used in an address which is the register value plus a displacement.

INDEX_REG_CLASS

A macro whose definition is the name of the class to which a valid index register must belong. An index register is one used in an address where its value is either multiplied by a scale factor or added to another register (as well as added to a displacement).

REG_CLASS_FROM_LETTER (char)

A C expression which defines the machine-dependent operand constraint letters for register classes. If char is such a letter, the value should be the register class corresponding to it. Otherwise, the value should be 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_OK_FOR_BASE_P (num)

A C expression which is nonzero if register number num is suitable for use as a base register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register.

REGNO_OK_FOR_INDEX_P (num)

A C expression which is nonzero if register number num is suitable for use as an index register in operand addresses. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register.

The difference between an index register and a base register is that the index register may be scaled. If an address involves the sum of two registers, neither one of them scaled, then either one may be labeled the ``base'' and the other the ``index''; but whichever labeling is used must fit the machine's constraints of which registers may serve in each capacity. The compiler will try both labelings, looking for one that is valid, and will reload one or both registers only if neither labeling works.

PREFERRED_RELOAD_CLASS (x, class)

A C expression that places additional restrictions on the register class to use when it is necessary to copy value x into a register in class class. The value is a register class; perhaps class, or perhaps another, smaller class. On many machines, the following definition is safe:

#define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS

Sometimes 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_REGS as 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_REGS you 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)

Like 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)

A C expression that places additional restrictions on the register class to use when it is necessary to be able to hold a value of mode mode in a reload register for which class class would ordinarily be used.

Unlike 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)

Many machines have some registers that cannot be copied directly to or from memory or even from other types of registers. An example is the `MQ' register, which on most machines, can only be copied to or from general registers, but not memory. Some machines allow copying all registers to and from memory, but require a scratch register for stores to some memory locations (e.g., those with symbolic address on the RT, and those with certain symbolic address on the Sparc when compiling PIC). In some cases, both an intermediate and a scratch register are required.

You should define these macros to indicate to the reload phase that it may need to allocate at least one register for a reload in addition to the register to contain the data. Specifically, if copying x to a register class in mode requires an intermediate register, you should define SECONDARY_INPUT_RELOAD_CLASS to 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_CLASS should be defined to return the largest register class required. If the requirements for input and output reloads are the same, the macro SECONDARY_RELOAD_CLASS should be used instead of defining both macros identically.

The values returned by these macros are often GENERAL_REGS. Return NO_REGS if 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 Standard Names. 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 subreg of a pseudo-register, which could either be in a hard register or in memory. Use true_regnum to 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 movm pattern should use memory as a intermediate storage. This case often occurs between floating-point and general registers.

SECONDARY_MEMORY_NEEDED (class1, class2, m)

Certain machines have the property that some registers cannot be copied to some other registers without using memory. Define this macro on those machines to be a C expression that is non-zero if objects of mode m in registers of class1 can only be copied to registers of class class2 by storing a register of class1 into memory and loading that memory location into a register of class2.

Do not define this macro if its value would always be zero.

SECONDARY_MEMORY_NEEDED_RTX (mode)

Normally when SECONDARY_MEMORY_NEEDED is 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 SECONDARY_MEMORY_NEEDED.

SECONDARY_MEMORY_NEEDED_MODE (mode)

When the compiler needs a secondary memory location to copy between two registers of mode mode, it normally allocates sufficient memory to hold a quantity of BITS_PER_WORD bits 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_NEEDED or if widening mode to a mode that is BITS_PER_WORD bits wide is correct for your machine.

SMALL_REGISTER_CLASSES

Normally the compiler avoids choosing registers that have been explicitly mentioned in the rtl as spill registers (these registers are normally those used to pass parameters and return values). However, some machines have so few registers of certain classes that there would not be enough registers to use as spill registers if this were done.

Define SMALL_REGISTER_CLASSES on these machines. When it is defined, 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, 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 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.

CLASS_LIKELY_SPILLED_P (class)

A C expression whose value is nonzero if pseudos that have been assigned to registers of class class would likely be spilled because registers of class are needed for spill registers.

The default value of this macro returns 1 if class has exactly one register and zero otherwise. On most machines, this default should be used. Only define this macro to some other expression if pseudo allocated by `local-alloc.c' end up in memory because their hard registers were needed for spill regisers. If this macro returns nonzero for those classes, those pseudos will only be allocated by `global.c', which knows how to reallocate the pseudo to another register. If there would not be another register available for reallocation, you should not change the definition of this macro since the only effect of such a definition would be to slow down register allocation.

CLASS_MAX_NREGS (class, mode)

A C expression for the maximum number of consecutive registers of class class needed to hold a value of mode mode.

This is closely related to the macro 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)

A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of integer values. If c is one of those letters, the expression should check that value, an integer, 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_DOUBLE_OK_FOR_LETTER_P (value, c)

A C expression that defines the machine-dependent operand constraint letters that specify particular ranges of const_double values.

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_double is used for all floating-point constants and for DImode fixed-point constants. A given letter can accept either or both kinds of values. It can use GET_MODE to distinguish between these kinds.

EXTRA_CONSTRAINT (value, c)

A C expression that defines the optional machine-dependent constraint letters that can be used to segregate specific types of operands, usually memory references, for the target machine. Normally this macro will not be defined. If it is required for a particular target machine, it should return 1 if value corresponds to the operand type represented by the constraint letter c. If c is not defined as an extra constraint, the value returned should be 0 regardless of value.

For example, on the ROMP, load instructions cannot have their output in r0 if the memory reference contains a symbolic address. Constraint letter `Q' is defined as representing a memory address that does not contain a symbolic address. An alternative is specified with a `Q' constraint on the input and `r' on the output. The next alternative specifies `m' on the input and a register class that does not include r0 on the output.