Addressing Modes (GNU Compiler Collection (GCC) Internals)

### 19.13 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 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.

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 `TARGET_LEGITIMATE_ADDRESS_P` would ever accept.

Target Hook: `bool` TARGET_LEGITIMATE_ADDRESS_P `(machine_mode mode, rtx x, bool strict, code_helper ch)` ¶

  A function that returns whether x (an RTX) is a legitimate memory address on the target machine for a memory operand of mode mode. If ch is not `ERROR_MARK`, it can be called from middle-end to determine if it is valid to use x as a memory operand for RTX insn which is generated for the given code_helper ch. For example, assuming the given ch is IFN_LEN_LOAD, on some target its underlying hardware instructions support fewer addressing modes than what are for the normal vector load and store, then with this ch target can know the actual use context and return more exact result.

  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 Defining the Output Assembler Language.

  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.

Macro: TARGET_MEM_CONSTRAINT ¶

  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.cc` can easily find the base term. This macro is used in only two places: `find_base_value` and `find_base_term` in `alias.cc`.

  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, 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 Emulating 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 `(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: `bool` TARGET_PRECOMPUTE_TLS_P `(machine_mode mode, rtx x)` ¶

  This hook returns true if x is a TLS operand on the target machine that should be pre-computed when used as the argument in a call. You can assume that x satisfies `CONSTANT_P`, so you need not check this.

  The default definition returns false.

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 `UNSPEC`s 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 `(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 `(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 `(tree fndecl)` ¶

  This hook should return the DECL of a function that implements the reciprocal of the machine-specific builtin function fndecl, or `NULL_TREE` if such a function is not available.