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17.13 Addressing Modes

This is about addressing modes.


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


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.


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


A C expression that is 1 if the RTX x is a constant which is a valid address. On most machines the default definition of (CONSTANT_P (x) && GET_CODE (x) != CONST_DOUBLE) is acceptable, but a few machines are more restrictive as to 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.


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 TARGET_LEGITIMATE_ADDRESS_P would ever accept.

— Target Hook: bool TARGET_LEGITIMATE_ADDRESS_P (enum machine_mode mode, rtx x, bool strict)

A function that returns whether x (an RTX) is a legitimate memory address on the target machine for a memory operand of mode mode.

Legitimate addresses are defined in two variants: a strict variant and a non-strict one. The strict parameter chooses which variant is desired by the caller.

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. This is because in contexts where some kind of register is required, a pseudo-register with no hard register must be rejected. For non-hard registers, the strict variant should look up the reg_renumber array; it should then proceed using the hard register number in the array, or treat the pseudo as a memory reference if the array holds -1.

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.

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.

Some ports are still using a deprecated legacy substitute for this hook, the GO_IF_LEGITIMATE_ADDRESS macro. This macro has this syntax:

          #define GO_IF_LEGITIMATE_ADDRESS (mode, x, label)

and should goto label if the address x is a valid address on the target machine for a memory operand of mode mode.

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.

Using the hook is usually simpler because it limits the number of files that are recompiled when changes are made.


A single character to be used instead of the default 'm' character for general memory addresses. This defines the constraint letter which matches the memory addresses accepted by TARGET_LEGITIMATE_ADDRESS_P. Define this macro if you want to support new address formats in your back end without changing the semantics of the 'm' constraint. This is necessary in order to preserve functionality of inline assembly constructs using the 'm' constraint.

— Macro: FIND_BASE_TERM (x)

A C expression to determine the base term of address x, or to provide a simplified version of x from which alias.c can easily find the base term. This macro is used in only two places: find_base_value and 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.

— Target Hook: rtx TARGET_LEGITIMIZE_ADDRESS (rtx x, rtx oldx, enum machine_mode mode)

This hook is given an invalid memory address x for an operand of mode mode and should try to return a valid memory address.

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 of the hook should not alter the substructure of x. If it transforms x into a more legitimate form, it should return the new x.

It is not necessary for this hook to come up with a legitimate address, with the exception of native TLS addresses (see Emulated TLS). The compiler has standard ways of doing so in all cases. In fact, if the target supports only emulated TLS, it is safe to omit this hook or make it return x if it cannot find a valid way to legitimize the address. 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 it 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.

— Target Hook: bool TARGET_MODE_DEPENDENT_ADDRESS_P (const_rtx addr, addr_space_t addrspace)

This hook returns true if memory address addr in address space addrspace 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.

The default version of this hook returns false.

— Target Hook: bool TARGET_LEGITIMATE_CONSTANT_P (enum machine_mode mode, rtx x)

This hook returns true if x is a legitimate constant for a mode-mode immediate operand on the target machine. You can assume that x satisfies CONSTANT_P, so you need not check this.

The default definition returns true.

— 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_CONST_NOT_OK_FOR_DEBUG_P (rtx x)

This hook should return true if x should not be emitted into debug sections.

— Target Hook: bool TARGET_CANNOT_FORCE_CONST_MEM (enum machine_mode mode, rtx x)

This hook should return true if x is of a form that cannot (or should not) be spilled to the constant pool. mode is the mode of x.

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: bool TARGET_USE_BLOCKS_FOR_CONSTANT_P (enum machine_mode mode, const_rtx x)

This hook should return true if pool entries for constant x can be placed in an object_block structure. mode is the mode of x.

The default version returns false for all constants.

— Target Hook: bool TARGET_USE_BLOCKS_FOR_DECL_P (const_tree decl)

This hook should return true if pool entries for decl should be placed in an object_block structure.

The default version returns true for all decls.

— Target Hook: tree TARGET_BUILTIN_RECIPROCAL (unsigned fn, bool md_fn, bool sqrt)

This hook should return the DECL of a function that implements reciprocal of the builtin function with builtin function code fn, or NULL_TREE if such a function is not available. md_fn is true when fn is a code of a machine-dependent builtin function. When sqrt is true, additional optimizations that apply only to the reciprocal of a square root function are performed, and only reciprocals of sqrt function are valid.


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 autovectorizer, 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.

— Target Hook: int TARGET_VECTORIZE_BUILTIN_VECTORIZATION_COST (enum vect_cost_for_stmt type_of_cost, tree vectype, int misalign)

Returns cost of different scalar or vector statements for vectorization cost model. For vector memory operations the cost may depend on type (vectype) and misalignment value (misalign).

— Target Hook: bool TARGET_VECTORIZE_VECTOR_ALIGNMENT_REACHABLE (const_tree type, bool is_packed)

Return true if vector alignment is reachable (by peeling N iterations) for the given type.

— Target Hook: bool TARGET_VECTORIZE_VEC_PERM_CONST_OK (enum machine_mode, const unsigned char *sel)

Return true if a vector created for vec_perm_const is valid.

— Target Hook: tree TARGET_VECTORIZE_BUILTIN_CONVERSION (unsigned code, tree dest_type, tree src_type)

This hook should return the DECL of a function that implements conversion of the input vector of type src_type to type dest_type. The value of code is one of the enumerators in enum tree_code and specifies how the conversion is to be applied (truncation, rounding, etc.).

If this hook is defined, the autovectorizer will use the TARGET_VECTORIZE_BUILTIN_CONVERSION target hook when vectorizing conversion. Otherwise, it will return NULL_TREE.

— Target Hook: tree TARGET_VECTORIZE_BUILTIN_VECTORIZED_FUNCTION (tree fndecl, tree vec_type_out, tree vec_type_in)

This hook should return the decl of a function that implements the vectorized variant of the builtin function with builtin function code code or NULL_TREE if such a function is not available. The value of fndecl is the builtin function declaration. The return type of the vectorized function shall be of vector type vec_type_out and the argument types should be vec_type_in.

— Target Hook: bool TARGET_VECTORIZE_SUPPORT_VECTOR_MISALIGNMENT (enum machine_mode mode, const_tree type, int misalignment, bool is_packed)

This hook should return true if the target supports misaligned vector store/load of a specific factor denoted in the misalignment parameter. The vector store/load should be of machine mode mode and the elements in the vectors should be of type type. is_packed parameter is true if the memory access is defined in a packed struct.

— Target Hook: enum machine_mode TARGET_VECTORIZE_PREFERRED_SIMD_MODE (enum machine_mode mode)

This hook should return the preferred mode for vectorizing scalar mode mode. The default is equal to word_mode, because the vectorizer can do some transformations even in absence of specialized SIMD hardware.


This hook should return a mask of sizes that should be iterated over after trying to autovectorize using the vector size derived from the mode returned by TARGET_VECTORIZE_PREFERRED_SIMD_MODE. The default is zero which means to not iterate over other vector sizes.

— Target Hook: void * TARGET_VECTORIZE_INIT_COST (struct loop *loop_info)

This hook should initialize target-specific data structures in preparation for modeling the costs of vectorizing a loop or basic block. The default allocates three unsigned integers for accumulating costs for the prologue, body, and epilogue of the loop or basic block. If loop_info is non-NULL, it identifies the loop being vectorized; otherwise a single block is being vectorized.

— Target Hook: unsigned TARGET_VECTORIZE_ADD_STMT_COST (void *data, int count, enum vect_cost_for_stmt kind, struct _stmt_vec_info *stmt_info, int misalign, enum vect_cost_model_location where)

This hook should update the target-specific data in response to adding count copies of the given kind of statement to a loop or basic block. The default adds the builtin vectorizer cost for the copies of the statement to the accumulator specified by where, (the prologue, body, or epilogue) and returns the amount added. The return value should be viewed as a tentative cost that may later be revised.

— Target Hook: void TARGET_VECTORIZE_FINISH_COST (void *data, unsigned *prologue_cost, unsigned *body_cost, unsigned *epilogue_cost)

This hook should complete calculations of the cost of vectorizing a loop or basic block based on data, and return the prologue, body, and epilogue costs as unsigned integers. The default returns the value of the three accumulators.

— Target Hook: void TARGET_VECTORIZE_DESTROY_COST_DATA (void *data)

This hook should release data and any related data structures allocated by TARGET_VECTORIZE_INIT_COST. The default releases the accumulator.


This hook should return the built-in decl needed to load a vector of the given type within a transaction.


This hook should return the built-in decl needed to store a vector of the given type within a transaction.

— Target Hook: tree TARGET_VECTORIZE_BUILTIN_GATHER (const_tree mem_vectype, const_tree index_type, int scale)

Target builtin that implements vector gather operation. mem_vectype is the vector type of the load and index_type is scalar type of the index, scaled by scale. The default is NULL_TREE which means to not vectorize gather loads.

— Target Hook: int TARGET_SIMD_CLONE_COMPUTE_VECSIZE_AND_SIMDLEN (struct cgraph_node *, struct cgraph_simd_clone *, tree, int)

This hook should set vecsize_mangle, vecsize_int, vecsize_float fields in simd_clone structure pointed by clone_info argument and also simdlen field if it was previously 0. The hook should return 0 if SIMD clones shouldn't be emitted, or number of vecsize_mangle variants that should be emitted.

— Target Hook: void TARGET_SIMD_CLONE_ADJUST (struct cgraph_node *)

This hook should add implicit attribute(target("...")) attribute to SIMD clone node if needed.

— Target Hook: int TARGET_SIMD_CLONE_USABLE (struct cgraph_node *)

This hook should return -1 if SIMD clone node shouldn't be used in vectorized loops in current function, or non-negative number if it is usable. In that case, the smaller the number is, the more desirable it is to use it.