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Machine registers correspond directly to the processor registers in the target architecture. They are only needed when doing register allocation. The machine registers are divided into general-purpose and floating-point registers. Each of these groups is further divided into callee-saved and caller-saved registers. The values in callee-saved registers are preserved across procedures but you have to explicitly save the caller-saved registers at each call site where they are live. The different classes of machine registers are identified by the following constants:
CALLEE_GPRS
CALLER_GPRS
CALLEE_FPRS
CALLER_FPRS
The num_machine_regs array contains the number of machine
registers in each class. For example,
num_machine_regs[CALLER_GPRS] is equal to the number of
caller-saved general-purpose registers. Pointers to the actual
simple_reg structures for machine registers are stored in the
two-dimensional machine_regs array. The first array index
identifies the machine register class and the second index identifies a
particular register within the class. Note that the second array index
ranges from zero to num_machine_regs[class]; it is not
the register number.
The current library only handles machine registers for MIPS-based architectures. The general-purpose registers contain signed 32-bit variables, and the floating-point registers contain 64-bit floating-point variables. There is no way to access a floating-point register as a pair of 32-bit values. Support for other architectures may be added in the future.
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