### 15.8 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.

— Data type: enum reg_class

> An enumerated type that must be defined with all the register class names as enumerated values. `NO_REGS` must be first. `ALL_REGS` must be the last register class, followed by one more enumerated 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.

— Macro: N_REG_CLASSES

> The number of distinct register classes, defined as follows:

>               #define N_REG_CLASSES (int) LIM_REG_CLASSES

— Macro: 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.

— Macro: 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`. In this situation, the first integer in each sub-initializer corresponds to registers 0 through 31, the second integer to registers 32 through 63, and so on.

— Macro: 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.

— Macro: 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.

— Macro: MODE_BASE_REG_CLASS (mode)

> This is a variation of the `BASE_REG_CLASS` macro which allows the selection of a base register in a mode dependent manner. If mode is VOIDmode then it should return the same value as `BASE_REG_CLASS`.

— Macro: MODE_BASE_REG_REG_CLASS (mode)

> A C expression whose value is the register class to which a valid base register must belong in order to be used in a base plus index register address. You should define this macro if base plus index addresses have different requirements than other base register uses.

— Macro: MODE_CODE_BASE_REG_CLASS (mode, outer_code, index_code)

> A C expression whose value is the register class to which a valid base register must belong. outer_code and index_code define the context in which the base register occurs. outer_code is the code of the immediately enclosing expression (`MEM` for the top level of an address, `ADDRESS` for something that occurs in an `address_operand`). index_code is the code of the corresponding index expression if outer_code is `PLUS`; `SCRATCH` otherwise.

— Macro: 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).

— Macro: MODE_INDEX_REG_CLASS (mode)

> This is a variation of the `INDEX_REG_CLASS` macro which allows the selection of an index register in a mode dependent manner. It can return `NO_REGS` for modes that do not support any form of index register. If mode is `VOIDmode` then the macro should return a class of registers that is suitable for all addresses in which an index register of some form is allowed.

— Macro: 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.

— Macro: REGNO_MODE_OK_FOR_BASE_P (num, mode)

> A C expression that is just like `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 `REGNO_OK_FOR_BASE_P`. The mode may be `VOIDmode` for addresses that appear outside a `MEM`, i.e. as an `address_operand`.

— Macro: REGNO_MODE_OK_FOR_REG_BASE_P (num, mode)

> A C expression which is nonzero if register number num is suitable for use as a base register in base plus index operand addresses, accessing memory in mode mode. It may be either a suitable hard register or a pseudo register that has been allocated such a hard register. You should define this macro if base plus index addresses have different requirements than other base register uses.

> Use of this macro is deprecated; please use the more general `REGNO_MODE_CODE_OK_FOR_BASE_P`.

— Macro: REGNO_MODE_CODE_OK_FOR_BASE_P (num, mode, outer_code, index_code)

> A C expression that is just like `REGNO_MODE_OK_FOR_BASE_P`, except that that expression may examine the context in which the register appears in the memory reference. outer_code is the code of the immediately enclosing expression (`MEM` if at the top level of the address, `ADDRESS` for something that occurs in an `address_operand`). index_code is the code of the corresponding index expression if outer_code is `PLUS`; `SCRATCH` otherwise. The mode may be `VOIDmode` for addresses that appear outside a `MEM`, i.e. as an `address_operand`.

— Macro: 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.

— Macro: REGNO_MODE_OK_FOR_INDEX_P (num, mode)

> A C expression that is just like `REGNO_OK_FOR_INDEX_P`, except that the expression may examine the mode of the memory reference in mode. If mode is `VOIDmode`, the macro should return true if x is suitable for all modes in which some form of index register is allowed.

— Macro: 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.

> One case where `PREFERRED_RELOAD_CLASS` must not return class is if x is a legitimate constant which cannot be loaded into some register class. By returning `NO_REGS` you can force x into a memory location. For example, rs6000 can load immediate values into general-purpose registers, but does not have an instruction for loading an immediate value into a floating-point register, so `PREFERRED_RELOAD_CLASS` returns `NO_REGS` when x is a floating-point constant. If the constant can't be loaded into any kind of register, code generation will be better if `LEGITIMATE_CONSTANT_P` makes the constant illegitimate instead of using `PREFERRED_RELOAD_CLASS`.

> If an insn has pseudos in it after register allocation, reload will go through the alternatives and call repeatedly `PREFERRED_RELOAD_CLASS` to find the best one. Returning `NO_REGS`, in this case, makes reload add a `!` in front of the constraint: the x86 back-end uses this feature to discourage usage of 387 registers when math is done in the SSE registers (and vice versa).

— Macro: 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.

> You can also use `PREFERRED_OUTPUT_RELOAD_CLASS` to discourage reload from using some alternatives, like `PREFERRED_RELOAD_CLASS`.

— Macro: 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.

— Target Hook: enum reg_class TARGET_SECONDARY_RELOAD (bool in_p, rtx x, enum reg_class reload_class, enum machine_mode reload_mode, secondary_reload_info *sri)

> 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. Below, we shall be using the term 'intermediate register' when a move operation cannot be performed directly, but has to be done by copying the source into the intermediate register first, and then copying the intermediate register to the destination. An intermediate register always has the same mode as source and destination. Since it holds the actual value being copied, reload might apply optimizations to re-use an intermediate register and eliding the copy from the source when it can determine that the intermediate register still holds the required value.

> Another kind of secondary reload is required on some machines which 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). Scratch registers need not have the same mode as the value being copied, and usually hold a different value that that being copied. Special patterns in the md file are needed to describe how the copy is performed with the help of the scratch register; these patterns also describe the number, register class(es) and mode(s) of the scratch register(s).

> In some cases, both an intermediate and a scratch register are required.

> For input reloads, this target hook is called with nonzero in_p, and x is an rtx that needs to be copied to a register in of class reload_class in reload_mode. For output reloads, this target hook is called with zero in_p, and a register of class reload_mode needs to be copied to rtx x in reload_mode.

> If copying a register of reload_class from/to x requires an intermediate register, the hook `secondary_reload` should return the register class required for this intermediate register. If no intermediate register is required, it should return NO_REGS. If more than one intermediate register is required, describe the one that is closest in the copy chain to the reload register.

> If scratch registers are needed, you also have to describe how to perform the copy from/to the reload register to/from this closest intermediate register. Or if no intermediate register is required, but still a scratch register is needed, describe the copy from/to the reload register to/from the reload operand x.

> You do this by setting `sri->icode` to the instruction code of a pattern in the md file which performs the move. Operands 0 and 1 are the output and input of this copy, respectively. Operands from operand 2 onward are for scratch operands. These scratch operands must have a mode, and a single-register-class output constraint.

> When an intermediate register is used, the `secondary_reload` hook will be called again to determine how to copy the intermediate register to/from the reload operand x, so your hook must also have code to handle the register class of the intermediate operand.

> 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.

> Scratch operands in memory (constraint `"=m"` / `"=&m"`) are currently not supported. For the time being, you will have to continue to use `SECONDARY_MEMORY_NEEDED` for that purpose.

> `copy_cost` also uses this target hook to find out how values are copied. If you want it to include some extra cost for the need to allocate (a) scratch register(s), set `sri->extra_cost` to the additional cost. Or if two dependent moves are supposed to have a lower cost than the sum of the individual moves due to expected fortuitous scheduling and/or special forwarding logic, you can set `sri->extra_cost` to a negative amount.

— Macro: SECONDARY_RELOAD_CLASS (class, mode, x)
— Macro: SECONDARY_INPUT_RELOAD_CLASS (class, mode, x)
— Macro: SECONDARY_OUTPUT_RELOAD_CLASS (class, mode, x)

> These macros are obsolete, new ports should use the target hook `TARGET_SECONDARY_RELOAD` instead.

> These are obsolete macros, replaced by the `TARGET_SECONDARY_RELOAD` target hook. Older ports still 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 were supposed to 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` was supposed to be defined be defined to return the largest register class required. If the requirements for input and output reloads were the same, the macro `SECONDARY_RELOAD_CLASS` should have been 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 were supposed to define patterns for ‘`reload_inm`’ or ‘`reload_outm`’, as required (see Standard Names. These patterns, which were normally implemented with a `define_expand`, should be similar to the ‘`movm`’ patterns, except that operand 2 is the scratch register.

> These patterns need 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 `mov` m pattern should use memory as an intermediate storage. This case often occurs between floating-point and general registers.

— Macro: 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 nonzero 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.

— Macro: 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`.

— Macro: 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.

— Macro: SMALL_REGISTER_CLASSES

> On some machines, it is risky to let hard registers live across arbitrary insns. Typically, these machines have instructions that require values to be in specific registers (like an accumulator), and reload will fail if the required hard register is used for another purpose across such an insn.

> Define `SMALL_REGISTER_CLASSES` to be an expression with a nonzero value on these machines. When this macro has a nonzero value, the compiler will try to minimize the lifetime of hard registers.

> It is always safe to define this macro with a nonzero 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 nonzero 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.

— 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 pseudos allocated by `local-alloc.c` end up in memory because their hard registers were needed for spill registers. 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.

— Macro: 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.

— Macro: CANNOT_CHANGE_MODE_CLASS (from, to, class)

> If defined, a C expression that returns nonzero for a class for which a change from mode from to mode to is invalid.

> For the example, loading 32-bit integer or floating-point objects into floating-point registers on the Alpha extends them to 64 bits. Therefore loading a 64-bit object and then storing it as a 32-bit object does not store the low-order 32 bits, as would be the case for a normal register. Therefore, `alpha.h` defines `CANNOT_CHANGE_MODE_CLASS` as below:

>               #define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \
>                 (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
>                  ? reg_classes_intersect_p (FLOAT_REGS, (CLASS)) : 0)