User`s guide

How the MACRO Compiler Functions on Different Platforms

2.6 Declaring a Routine’s Register Use

It is recommended that the .CALL_ENTRY, .JSB_ENTRY, and .JSB32_ENTRY

text header. Wherever possible, you should declare input, output, scratch,

and preserve register arguments for all routines. You only need to provide the

argument when there are registers to be declared (for instance, input=<> is not

necessary).

2.7 Branching Between Local Routines

The compiler allows a branch from the body of one routine into the body of

another routine in the same module and psect. However, because this may result

in additional overhead in both routines, the compiler reports an information-level

message.

Note

The compiler does not recognize a call to $EXIT as terminating a routine.

Add an extra RET or RSB, whichever is applicable, after $EXIT to

terminate the routine.

If a CALL routine branches into a code path that executes an RSB, an error

message is reported. Such a CALL routine, if not corrected, will fail at run time.

If a JSB routine branches into a code path that executes a RET instruction, and

the JSB routine preserves any registers, an informational message is issued. This

construct will work at run time, but the registers saved by the JSB routine will

not be restored.

If routines that share a code path have different register declarations, the register

restores will be done conditionally. That is, the registers saved at routine entry

will be the same for both routines, but whether or not the register is restored will

depend upon which entry point was invoked.

For example:

rout1: .jsb_entry output=r3

movl r1, r3 ! R3 is output, not preserved

movl r2, r4 ! R4 should be preserved

blss lab1

rsb

rout2: .jsb_entry ! R3 is not output, and

movl #4, r3 ! should be auto-preserved

movl r0, r4 ! R4 should be preserved

lab1: clrl r0

rsb

For both routines, R3 will be included in the registers saved at entry. However,

at exit, a mask (also saved at entry) will be tested before restoring R3. The mask

will not be tested before R4 is restored, because R4 should be restored for both

entry points.

Note that declaring registers that are destroyed in two routines that share code

as scratch in one but not the other is actually more expensive than letting them

be saved and restored. In this case, declare them as scratch in both, or if one

routine requires that they be preserved, as preserve in both.

2–16 How the MACRO Compiler Functions on Different Platforms