User`s guide

How the MACRO Compiler Functions on Different Platforms

2.6 Declaring a Routine’s Register Use

The scratch argument also pertains to the compiler’s temporary register usage.

The compiler may use registers R13 and above as temporary registers if they are

unused in the routine source code. Since R13 through R15 must be preserved, if

modiﬁed, on OpenVMS Alpha and OpenVMS I64 systems, the compiler preserves

those registers if it uses them.

However, if they appear in the scratch register set declaration, the compiler

will not preserve them if it uses them as temporary registers. As a result, these

registers may be scratched at routine exit, even if they were not used in the

routine source but are in the scratch set. If the VAXREGS optimization is used

(Alpha systems only), this applies to registers R2 through R12, as well.

For .JSB32_ENTRY routines, since R2 through R12 are not preserved by default,

their inclusion in the scratch declaration is for documentation purposes only.

2.6.4 Preserve Argument for Entry Point Register Declaration

The preserve argument indicates those registers that should be preserved over

the routine call. This should include only those registers that are modiﬁed and

whose full 64-bit contents should be saved and restored.

The preserve argument causes registers to be preserved whether or not they

would have been preserved automatically by the compiler’s processing of a

.CALL_ENTRY or .JSB_ENTRY directive. This is also the only way in a .JSB32_

ENTRY routine to save and restore the full 64 bits of a register. Note that

because R0 and R1 are scratch registers, the compiler never saves and restores

them in any routine unless you specify them in the preserve argument at the

routine’s entry point.

This argument overrides the output and scratch arguments. If you specify

a register both in the preserve argument and in the output or scratch

arguments, the compiler will preserve the register but will report the following

warning:

%AMAC-W-REGDECCON, register declaration conflict in routine A

The preserve argument has no effect on the compiler’s temporary register

usage.

2.6.5 Help for Specifying Register Sets

When you invoke the compiler, specifying /FLAG=HINTS on the command line,

the compiler generates messages that can assist you in constructing the register

sets for routine entry points. Among the hints the compiler provides are the

following:

• Registers that might be used as input to the routine, which may not have

been your intention. If a register is read before being written, it will be

included in this set.

• Registers that might be used for output values. If a register is written but not

subsequently read, the compiler will include it in the list of possible output

values. Again, this may not have been your intention.

• Registers the compiler saves and restores that are not listed in the entry

point’s preserve argument.

How the MACRO Compiler Functions on Different Platforms 2–15