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13.24 Defining coprocessor specifics for MIPS targets.
The MIPS specification allows MIPS implementations to have as many as 4 coprocessors, each with as many as 32 private registers. GCC supports accessing these registers and transferring values between the registers and memory using asm-ized variables. For example:
register unsigned int cp0count asm ("c0r1");
unsigned int d;
d = cp0count + 3;
(“c0r1” is the default name of register 1 in coprocessor 0; alternate
names may be added as described below, or the default names may be
overridden entirely in SUBTARGET_CONDITIONAL_REGISTER_USAGE.)
Coprocessor registers are assumed to be epilogue-used; sets to them will be preserved even if it does not appear that the register is used again later in the function.
Another note: according to the MIPS spec, coprocessor 1 (if present) is the FPU. One accesses COP1 registers through standard mips floating-point support; they are not included in this mechanism.
There is one macro used in defining the MIPS coprocessor interface which you may want to override in subtargets; it is described below.