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 | Data type |
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 |
| N_REG_CLASSES | Macro |
The number of distinct register classes, defined as follows:
#define N_REG_CLASSES (int) LIM_REG_CLASSES
|
| REG_CLASS_NAMES | Macro |
| 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 | Macro |
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 & (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 |
| REGNO_REG_CLASS (regno) | Macro |
| 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 | Macro |
| 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. |
| MODE_BASE_REG_CLASS (mode) | Macro |
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.
|
| INDEX_REG_CLASS | Macro |
| 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). |
| CONSTRAINT_LEN (char, str) | Macro |
| For the constraint at the start of str, which starts with the letter c, return the length. This allows you to have register class / constant / extra constraints that are longer than a single letter; you don't need to define this macro if you can do with single-letter constraints only. The definition of this macro should use DEFAULT_CONSTRAINT_LEN for all the characters that you don't want to handle specially. There are some sanity checks in genoutput.c that check the constraint lengths for the md file, so you can also use this macro to help you while you are transitioning from a byzantine single-letter-constraint scheme: when you return a negative length for a constraint you want to re-use, genoutput will complain about every instance where it is used in the md file. |
| REG_CLASS_FROM_LETTER (char) | Macro |
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.
|
| REG_CLASS_FROM_CONSTRAINT (char, str) | Macro |
Like REG_CLASS_FROM_LETTER, but you also get the constraint string
passed in str, so that you can use suffixes to distinguish between
different variants.
|
| REGNO_OK_FOR_BASE_P (num) | Macro |
| 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_MODE_OK_FOR_BASE_P (num, mode) | Macro |
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.
|
| REGNO_OK_FOR_INDEX_P (num) | Macro |
|
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) | Macro |
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 One case where |
| PREFERRED_OUTPUT_RELOAD_CLASS (x, class) | Macro |
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) | Macro |
|
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 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) | Macro |
| SECONDARY_INPUT_RELOAD_CLASS (class, mode, x) | Macro |
| SECONDARY_OUTPUT_RELOAD_CLASS (class, mode, x) | Macro |
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 If copying a register class in mode to x requires an
intermediate or scratch register, The values returned by these macros are often If a scratch register is required (either with or without an
intermediate register), you should define patterns for
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 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 |
| SECONDARY_MEMORY_NEEDED (class1, class2, m) | Macro |
|
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. |
| SECONDARY_MEMORY_NEEDED_RTX (mode) | Macro |
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_MODE (mode) | Macro |
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 Do not define this macro if you do not define
|
| SMALL_REGISTER_CLASSES | Macro |
|
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 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. |
| CLASS_LIKELY_SPILLED_P (class) | Macro |
|
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 |
| CLASS_MAX_NREGS (class, mode) | Macro |
|
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 This macro helps control the handling of multiple-word values in the reload pass. |
| CANNOT_CHANGE_MODE_CLASS (from, to, class) | Macro |
|
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, #define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \
(GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
? reg_classes_intersect_p (FLOAT_REGS, (CLASS)) : 0)
|
Three other special macros describe which operands fit which constraint letters.
| CONST_OK_FOR_LETTER_P (value, c) | Macro |
A C expression that defines the machine-dependent operand constraint
letters (I, J, K, ... P) 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_OK_FOR_CONSTRAINT_P (value, c, str) | Macro |
Like CONST_OK_FOR_LETTER_P, but you also get the constraint
string passed in str, so that you can use suffixes to distinguish
between different variants.
|
| CONST_DOUBLE_OK_FOR_LETTER_P (value, c) | Macro |
A C expression that defines the machine-dependent operand constraint
letters that specify particular ranges of const_double values
(G or H).
If c is one of those letters, the expression should check that
value, an RTX of code
|
| CONST_DOUBLE_OK_FOR_CONSTRAINT_P (value, c, str) | Macro |
Like CONST_DOUBLE_OK_FOR_LETTER_P, but you also get the constraint
string passed in str, so that you can use suffixes to distinguish
between different variants.
|
| EXTRA_CONSTRAINT (value, c) | Macro |
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. Any letter that is not
elsewhere defined and not matched by REG_CLASS_FROM_LETTER /
REG_CLASS_FROM_CONSTRAINT
may be used. 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 |
| EXTRA_CONSTRAINT_STR (value, c, str) | Macro |
Like EXTRA_CONSTRAINT, but you also get the constraint string passed
in str, so that you can use suffixes to distinguish between different
variants.
|
| EXTRA_MEMORY_CONSTRAINT (c, str) | Macro |
A C expression that defines the optional machine-dependent constraint
letters, amongst those accepted by EXTRA_CONSTRAINT, that should
be treated like memory constraints by the reload pass.
It should return 1 if the operand type represented by the constraint at the start of str, the first letter of which is the letter c, comprises a subset of all memory references including all those whose address is simply a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of c, by copying its address into a base register. For example, on the S/390, some instructions do not accept arbitrary
memory references, but only those that do not make use of an index
register. The constraint letter |
| EXTRA_ADDRESS_CONSTRAINT (c, str) | Macro |
A C expression that defines the optional machine-dependent constraint
letters, amongst those accepted by EXTRA_CONSTRAINT /
EXTRA_CONSTRAINT_STR, that should
be treated like address constraints by the reload pass.
It should return 1 if the operand type represented by the constraint at the start of str, which starts with the letter c, comprises a subset of all memory addresses including all those that consist of just a base register. This allows the reload pass to reload an operand, if it does not directly correspond to the operand type of str, by copying it into a base register. Any constraint marked as |