This is about addressing modes.
), but a few machines are more restrictive in which constant addresses are supported.
CONSTANT_P accepts integer-values expressions whose values are
not explicitly known, such as
high expressions and
const arithmetic expressions, in
GO_IF_LEGITIMATE_ADDRESSwould ever accept.
;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_STRICT conditional 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
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
and an integer are stored inside a
const RTX to mark them as
constant. Therefore, there is no need to recognize such sums
specifically as legitimate addresses. Normally you would simply
const as legitimate.
PRINT_OPERAND_ADDRESS is not prepared to handle constant
sums that are not marked with
const. It assumes that a naked
plus indicates indexing. If so, then you must reject such
naked constant sums as illegitimate addresses, so that none of them will
be given to
On some machines, whether a symbolic address is legitimate depends on
the section that the address refers to. On these machines, define the
ENCODE_SECTION_INFO to 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_ref in order to determine the section. See Assembler Format.
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_LABELREF to remove and decode the added text and
output the name accordingly, and define
access the original name string.
You can check the information stored here into the
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_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.
It is always safe for this macro to not be defined. It exists so that alias analysis can understand machine-dependent addresses.
The typical use of this macro is to handle addresses containing
a label_ref or symbol_ref within an UNSPEC.
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
and oldx will be the operand that was given to that function to produce
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.
For example, on the i386, it is sometimes possible to use a single
reload register instead of two by reloading a sum of two pseudo
registers into a register. On the other hand, for number of RISC
processors offsets are limited so that often an intermediate address
needs to be generated in order to address a stack slot. By defining
LEGITIMIZE_RELOAD_ADDRESS appropriately, the intermediate addresses
generated for adjacent some stack slots can be made identical, and thus
Note: This macro should be used with caution. It is necessary to know something of how reload works in order to effectively use this, and it is quite easy to produce macros that build in too much knowledge of reload internals.
Note: This macro must be able to reload an address created by a previous invocation of this macro. If it fails to handle such addresses then the compiler may generate incorrect code or abort.
The macro definition should use
push_reload to indicate parts that
need reloading; opnum, type and ind_levels are usually
suitable to be passed unaltered to
The code generated by this macro must 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.
This also applies to parts that you change indirectly by calling
The macro definition may use
strict_memory_address_p to test if
the address has become legitimate.
If you want to change only a part of x, one standard way of doing
this is to use
copy_rtx. Note, however, that is unshares only a
single level of rtl. Thus, if the part to be changed is not at the
top level, you'll need to replace first the top level.
It is not necessary for this macro to come up with a legitimate
address; but often a machine-dependent strategy can generate better code.
;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,
1is a suitable definition for this macro on machines where anything