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18.7.1 Basic Characteristics of Registers

Registers have various characteristics.


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.


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.


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.


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). This macro is optional. If not specified, it defaults to the value of CALL_USED_REGISTERS.

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

This hook should return true if regno is partly call-saved and partly call-clobbered, and if a value of mode mode would be partly clobbered by a call. For example, if the low 32 bits of regno are preserved across a call but higher bits are clobbered, this hook should return true for a 64-bit mode but false for a 32-bit mode.

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


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_constraints 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.)


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.


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.


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

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