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#### 18.7.1 Basic Characteristics of Registers

Registers have various characteristics.

— Macro: FIRST_PSEUDO_REGISTER

> Number of hardware registers known to the compiler. They receive numbers 0 through `FIRST_PSEUDO_REGISTER-1`; thus, the first pseudo register's number really is assigned the number `FIRST_PSEUDO_REGISTER`.

— Macro: FIXED_REGISTERS

> An initializer that says which registers are used for fixed purposes all throughout the compiled code and are therefore not available for general allocation. These would include the stack pointer, the frame pointer (except on machines where that can be used as a general register when no frame pointer is needed), the program counter on machines where that is considered one of the addressable registers, and any other numbered register with a standard use.

> This information is expressed as a sequence of numbers, separated by commas and surrounded by braces. The nth number is 1 if register n is fixed, 0 otherwise.

> The table initialized from this macro, and the table initialized by the following one, may be overridden at run time either automatically, by the actions of the macro `CONDITIONAL_REGISTER_USAGE`, or by the user with the command options `-ffixed-reg`, `-fcall-used-reg` and `-fcall-saved-reg`.

— Macro: CALL_USED_REGISTERS

> Like `FIXED_REGISTERS` but has 1 for each register that is clobbered (in general) by function calls as well as for fixed registers. This macro therefore identifies the registers that are not available for general allocation of values that must live across function calls.

> If a register has 0 in `CALL_USED_REGISTERS`, the compiler automatically saves it on function entry and restores it on function exit, if the register is used within the function.

> Exactly one of `CALL_USED_REGISTERS` and `CALL_REALLY_USED_REGISTERS` must be defined. Modern ports should define `CALL_REALLY_USED_REGISTERS`.

— Macro: CALL_REALLY_USED_REGISTERS

> Like `CALL_USED_REGISTERS` except this macro doesn't require that the entire set of `FIXED_REGISTERS` be included. (`CALL_USED_REGISTERS` must be a superset of `FIXED_REGISTERS`).

> Exactly one of `CALL_USED_REGISTERS` and `CALL_REALLY_USED_REGISTERS` must be defined. Modern ports should define `CALL_REALLY_USED_REGISTERS`.

— Target Hook: const predefined_function_abi & TARGET_FNTYPE_ABI (const_tree type)

> Return the ABI used by a function with type type; see the definition of `predefined_function_abi` for details of the ABI descriptor. Targets only need to define this hook if they support interoperability between several ABIs in the same translation unit.

— Target Hook: const predefined_function_abi & TARGET_INSN_CALLEE_ABI (const rtx_insn *insn)

> This hook returns a description of the ABI used by the target of call instruction insn; see the definition of `predefined_function_abi` for details of the ABI descriptor. Only the global function `insn_callee_abi` should call this hook directly.

> Targets only need to define this hook if they support interoperability between several ABIs in the same translation unit.

— Target Hook: bool TARGET_HARD_REGNO_CALL_PART_CLOBBERED (unsigned int abi_id, unsigned int regno, machine_mode mode)

> ABIs usually specify that calls must preserve the full contents of a particular register, or that calls can alter any part of a particular register. This information is captured by the target macro `CALL_REALLY_USED_REGISTERS`. However, some ABIs specify that calls must preserve certain bits of a particular register but can alter others. This hook should return true if this applies to at least one of the registers in ``(reg:mode regno)`', and if as a result the call would alter part of the mode value. For example, if a call preserves the low 32 bits of a 64-bit hard register regno but can clobber the upper 32 bits, this hook should return true for a 64-bit mode but false for a 32-bit mode.

> The value of abi_id comes from the `predefined_function_abi` structure that describes the ABI of the call; see the definition of the structure for more details. If (as is usual) the target uses the same ABI for all functions in a translation unit, abi_id is always 0.

> The default implementation returns false, which is correct for targets that don't have partly call-clobbered registers.

— Target Hook: const char * TARGET_GET_MULTILIB_ABI_NAME (void)

> This hook returns name of multilib ABI name.

— Target Hook: void TARGET_CONDITIONAL_REGISTER_USAGE (void)

> This hook may conditionally modify five variables `fixed_regs`, `call_used_regs`, `global_regs`, `reg_names`, and `reg_class_contents`, to take into account any dependence of these register sets on target flags. The first three of these are of type `char []` (interpreted as boolean vectors). `global_regs` is a `const char *[]`, and `reg_class_contents` is a `HARD_REG_SET`. Before the macro is called, `fixed_regs`, `call_used_regs`, `reg_class_contents`, and `reg_names` have been initialized from `FIXED_REGISTERS`, `CALL_USED_REGISTERS`, `REG_CLASS_CONTENTS`, and `REGISTER_NAMES`, respectively. `global_regs` has been cleared, and any `-ffixed-reg`, `-fcall-used-reg` and `-fcall-saved-reg` command options have been applied.

> If the usage of an entire class of registers depends on the target flags, you may indicate this to GCC by using this macro to modify `fixed_regs` and `call_used_regs` to 1 for each of the registers in the classes which should not be used by GCC. Also make `define_register_constraint`s return `NO_REGS` for constraints that shouldn't be used.

> (However, if this class is not included in `GENERAL_REGS` and all of the insn patterns whose constraints permit this class are controlled by target switches, then GCC will automatically avoid using these registers when the target switches are opposed to them.)

— Macro: INCOMING_REGNO (out)

> Define this macro if the target machine has register windows. This C expression returns the register number as seen by the called function corresponding to the register number out as seen by the calling function. Return out if register number out is not an outbound register.

— Macro: OUTGOING_REGNO (in)

> Define this macro if the target machine has register windows. This C expression returns the register number as seen by the calling function corresponding to the register number in as seen by the called function. Return in if register number in is not an inbound register.

— Macro: LOCAL_REGNO (regno)

> Define this macro if the target machine has register windows. This C expression returns true if the register is call-saved but is in the register window. Unlike most call-saved registers, such registers need not be explicitly restored on function exit or during non-local gotos.

— Macro: PC_REGNUM

> If the program counter has a register number, define this as that register number. Otherwise, do not define it.