Register usage

Table 5 shows the registers available on the sparc. For floating point registers, double precision only uses even numbered registers, with the next odd numbered register holding the second half of the 64-bit value. Single precision instructions can't use %f32-%f62. However, one trick to keep in mind is anytime you need to load two consecutive single precision values, these can be loaded to an even and next odd register with one ldd, which is more efficient than two ld.

Table 5: Sparc register summary

		CALLEE
REGISTER	USAGE	SAVE
Global Registers
%g0 (%r0)	Always zero	NO
%g1-%g4 (%r1-%r4)	global regs	NO
%g5-%g7 (%r5-%r7)	system regs - no use	NO USE
Out Registers
%o0 (%r8)	Return value from callee	NO
	outgoing arg 1 to caller
%o1-%o5 (%r9-%r13)	outgoing arg 2-6 to caller	NO
%o6/%sp (%r14)	Stack pointer	YES
%o7 (%r15)	scratch/@ of CALL inst.	NO
Local Registers
%l0-%l7 (%r16-%r23)	scratch registers	YES
In Registers
%i0-%i5 (%r24-%r29)	incoming arg 1-6	YES
%i6/%fp (%r30)	frame pointer	YES
%i7 (%r31)	return address - 8	YES
Floating Point Registers
%f0-%f31	single prec regs	NO
	even #'s double prec
%f32-%f62	double prec regs	NO

The sparc has only eight globally visable integer registers, the Global registers. Applications are not allowed to use the system registers (%g5-%g7), not even if they are saved and restored. Note that in the 64-bit variants of the ABI, %g5 can be used, and need not be saved for v9, but must be saved for v8plus. The other 24 registers (In, Out, Local) are in a moving register window, which is moved by the save and restore commands. Note that %g0 can be the target of an operation, but it's value will not be effected (eg., its value will remain 0, even if you were to subcc %i2, 8, %g0).

Note that as long as you use the save statement, all callee-saved registers are implicitly saved by the register window.

The caller's Out registers are the callee's In registers.