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14.13 Addressing Modes

This is about addressing modes.

— Macro: HAVE_PRE_INCREMENT
— Macro: HAVE_PRE_DECREMENT
— Macro: HAVE_POST_INCREMENT
— Macro: HAVE_POST_DECREMENT

A C expression that is nonzero if the machine supports pre-increment, pre-decrement, post-increment, or post-decrement addressing respectively.

— Macro: HAVE_PRE_MODIFY_DISP
— Macro: HAVE_POST_MODIFY_DISP

A C expression that is nonzero if the machine supports pre- or post-address side-effect generation involving constants other than the size of the memory operand.

— Macro: HAVE_PRE_MODIFY_REG
— Macro: HAVE_POST_MODIFY_REG

A C expression that is nonzero if the machine supports pre- or post-address side-effect generation involving a register displacement.

— Macro: CONSTANT_ADDRESS_P (x)

A C expression that is 1 if the RTX x is a constant which is a valid address. On most machines, this can be defined as CONSTANT_P (x), but a few machines are more restrictive in which constant addresses are supported.

— Macro: CONSTANT_P (x)

CONSTANT_P, which is defined by target-independent code, accepts integer-values expressions whose values are not explicitly known, such as symbol_ref, label_ref, and high expressions and const arithmetic expressions, in addition to const_int and const_double expressions.

— Macro: MAX_REGS_PER_ADDRESS

A number, the maximum number of registers that can appear in a valid memory address. Note that it is up to you to specify a value equal to the maximum number that GO_IF_LEGITIMATE_ADDRESS would ever accept.

— Macro: GO_IF_LEGITIMATE_ADDRESS (mode, x, label)

A C compound statement with a conditional 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_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 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_ref 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 recognize any const as legitimate.

Usually 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 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 target hook TARGET_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.

— Macro: REG_OK_FOR_BASE_P (x)

A C expression that is nonzero if x (assumed to be a reg RTX) 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_STRICT as described above. This usually requires two variant definitions, of which REG_OK_STRICT controls the one actually used.

— Macro: REG_MODE_OK_FOR_BASE_P (x, mode)

A C expression that is just like 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 REG_OK_FOR_BASE_P.

— Macro: REG_MODE_OK_FOR_REG_BASE_P (x, mode)

A C expression which is nonzero if x (assumed to be a reg RTX) 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.

— Macro: REG_OK_FOR_INDEX_P (x)

A C expression that is nonzero if x (assumed to be a reg RTX) 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.

— Macro: FIND_BASE_TERM (x)

A C expression to determine the base term of address x. This macro is used in only one place: `find_base_term' in alias.c.

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.

— Macro: LEGITIMIZE_ADDRESS (x, oldx, mode, win)

A C compound statement that attempts to replace x with a valid memory address for an operand of mode mode. win will be a C statement label elsewhere in the code; the macro definition may use

          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 to omit this macro. But often a machine-dependent strategy can generate better code.

— Macro: LEGITIMIZE_RELOAD_ADDRESS (x, mode, opnum, type, ind_levels, win)

A C compound statement that attempts to replace x, which is an address that needs reloading, with a valid memory address for an operand of mode mode. win will be a C statement label elsewhere in the code. It is not necessary to define this macro, but it might be useful for performance reasons.

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 be shared.

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 push_reload.

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 push_reload.

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.

— Macro: GO_IF_MODE_DEPENDENT_ADDRESS (addr, label)

A C statement or compound statement with a conditional 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.

— Macro: LEGITIMATE_CONSTANT_P (x)

A C expression that is nonzero if x is a legitimate constant for an immediate operand on the target machine. You can assume that x satisfies CONSTANT_P, so you need not check this. In fact, `1' is a suitable definition for this macro on machines where anything CONSTANT_P is valid.

— Target Hook: rtx TARGET_DELEGITIMIZE_ADDRESS (rtx x)

This hook is used to undo the possibly obfuscating effects of the LEGITIMIZE_ADDRESS and LEGITIMIZE_RELOAD_ADDRESS target macros. Some backend implementations of these macros wrap symbol references inside an UNSPEC rtx to represent PIC or similar addressing modes. This target hook allows GCC's optimizers to understand the semantics of these opaque UNSPECs by converting them back into their original form.

— Target Hook: bool TARGET_CANNOT_FORCE_CONST_MEM (rtx x)

This hook should return true if x is of a form that cannot (or should not) be spilled to the constant pool. The default version of this hook returns false.

The primary reason to define this hook is to prevent reload from deciding that a non-legitimate constant would be better reloaded from the constant pool instead of spilling and reloading a register holding the constant. This restriction is often true of addresses of TLS symbols for various targets.

— Target Hook: tree TARGET_VECTORIZE_BUILTIN_MASK_FOR_LOAD (void)

This hook should return the DECL of a function f that given an address addr as an argument returns a mask m that can be used to extract from two vectors the relevant data that resides in addr in case addr is not properly aligned.

The autovectrizer, when vectorizing a load operation from an address addr that may be unaligned, will generate two vector loads from the two aligned addresses around addr. It then generates a REALIGN_LOAD operation to extract the relevant data from the two loaded vectors. The first two arguments to REALIGN_LOAD, v1 and v2, are the two vectors, each of size VS, and the third argument, OFF, defines how the data will be extracted from these two vectors: if OFF is 0, then the returned vector is v2; otherwise, the returned vector is composed from the last VS-OFF elements of v1 concatenated to the first OFF elements of v2.

If this hook is defined, the autovectorizer will generate a call to f (using the DECL tree that this hook returns) and will use the return value of f as the argument OFF to REALIGN_LOAD. Therefore, the mask m returned by f should comply with the semantics expected by REALIGN_LOAD described above. If this hook is not defined, then addr will be used as the argument OFF to REALIGN_LOAD, in which case the low log2(VS)-1 bits of addr will be considered.