This is about addressing modes.
CONSTANT_P (x), but a few machines are more restrictive in which constant addresses are supported.
CONSTANT_Paccepts integer-values expressions whose values are not explicitly known, such as
constarithmetic expressions, in addition to
GO_IF_LEGITIMATE_ADDRESSwould ever accept.
GO_IF_LEGITIMATE_ADDRESS (mode, x, label)
goto label;executed if x (an RTX) is a legitimate memory address on the target machine for a memory operand of mode mode. It usually pays to define several simpler macros to serve as subroutines for this one. Otherwise it may be too complicated to understand. This macro must exist in two variants: a strict variant and a non-strict one. The strict variant is used in the reload pass. It must be defined so that any pseudo-register that has not been allocated a hard register is considered a memory reference. In contexts where some kind of register is required, a pseudo-register with no hard register must be rejected. The non-strict variant is used in other passes. It must be defined to accept all pseudo-registers in every context where some kind of register is required. Compiler source files that want to use the strict variant of this macro define the macro
REG_OK_STRICT. You should use an
#ifdef REG_OK_STRICTconditional to define the strict variant in that case and the non-strict variant otherwise. Subroutines to check for acceptable registers for various purposes (one for base registers, one for index registers, and so on) are typically among the subroutines used to define
GO_IF_LEGITIMATE_ADDRESS. Then only these subroutine macros need have two variants; the higher levels of macros may be the same whether strict or not. Normally, constant addresses which are the sum of a
symbol_refand an integer are stored inside a
constRTX to mark them as constant. Therefore, there is no need to recognize such sums specifically as legitimate addresses. Normally you would simply recognize any
constas legitimate. Usually
PRINT_OPERAND_ADDRESSis not prepared to handle constant sums that are not marked with
const. It assumes that a naked
plusindicates indexing. If so, then you must reject such naked constant sums as illegitimate addresses, so that none of them will be given to
PRINT_OPERAND_ADDRESS. On some machines, whether a symbolic address is legitimate depends on the section that the address refers to. On these machines, define the macro
ENCODE_SECTION_INFOto store the information into the
symbol_ref, and then check for it here. When you see a
const, you will have to look inside it to find the
symbol_refin order to determine the section. See section Defining the Output Assembler Language. The best way to modify the name string is by adding text to the beginning, with suitable punctuation to prevent any ambiguity. Allocate the new name in
saveable_obstack. You will have to modify
ASM_OUTPUT_LABELREFto remove and decode the added text and output the name accordingly, and define
STRIP_NAME_ENCODINGto access the original name string. You can check the information stored here into the
symbol_refin the definitions of the macros
regRTX) is valid for use as a base register. For hard registers, it should always accept those which the hardware permits and reject the others. Whether the macro accepts or rejects pseudo registers must be controlled by
REG_OK_STRICTas described above. This usually requires two variant definitions, of which
REG_OK_STRICTcontrols the one actually used.
REG_MODE_OK_FOR_BASE_P (x, mode)
REG_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
regRTX) is valid for use as an index 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.
LEGITIMIZE_ADDRESS (x, oldx, mode, win)
GO_IF_LEGITIMATE_ADDRESS (mode, x, win);to avoid further processing if the address has become legitimate. x will always be the result of a call to
break_out_memory_refs, and oldx will be the operand that was given to that function to produce x. The code generated by this macro should not alter the substructure of x. If it transforms x into a more legitimate form, it should assign x (which will always be a C variable) a new value. It is not necessary for this macro to come up with a legitimate address. The compiler has standard ways of doing so in all cases. In fact, it is safe for this macro to do nothing. But often a machine-dependent strategy can generate better code.
GO_IF_MODE_DEPENDENT_ADDRESS (addr, label)
goto label;executed if memory address x (an RTX) can have different meanings depending on the machine mode of the memory reference it is used for or if the address is valid for some modes but not others. Autoincrement and autodecrement addresses typically have mode-dependent effects because the amount of the increment or decrement is the size of the operand being addressed. Some machines have other mode-dependent addresses. Many RISC machines have no mode-dependent addresses. You may assume that addr is a valid address for the machine.
CONSTANT_P, so you need not check this. In fact, `1' is a suitable definition for this macro on machines where anything
REG_EQUALnote x should not be promoted to a
REG_EQUIVnote. Define this macro if note refers to a constant that must be accepted by
LEGITIMATE_CONSTANT_P, but must not appear as an immediate operand. Most machine descriptions do not need to define this macro.
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