HP OpenVMS Systems Documentation

HP OpenVMS Calling Standard

Indirect procedure calls follow nearly the same sequence as direct calls (see Section 4.7.3.1), except that the branch target is established indirectly. This sequence is illustrated in Figure 4-4.

Figure 4-4 Indirect Procedure Calls

• Caller: Function Pointer. A function pointer is always the address of a function descriptor for the target procedure (see Section 4.3). An indirect call loads the GP value into the GP register before branching to the entry point address.
  In order to guarantee the uniqueness of a function pointer, and because its value is determined at program invocation time, code must materialize function pointers only by loading a pointer from the data segment.
• Caller: Prepare call. Indirect calls are made by first loading the function pointer into a general register, loading the entry point address and the new GP value, and using the Move to Branch Register operation to move the address of the procedure entry point into the branch register to be used for the call.
  Values in scratch registers that must be kept live across the call must be saved. They can be saved by copying them into local stacked registers, or by saving them on the memory stack. If the NaT bits associated with any live scratch registers must be saved, the compiler should use ST8.SPILL or STF.SPILL instructions. The User NaT collection register itself is preserved by the call, so the NaT bits need no further treatment at this point.
  Unless the call is known (at compile time) to be within the same image, the GP register must be saved before the new GP value is loaded.
  The parameters must be set up in registers and memory as described in Section 4.7.4
• Caller: Call. All indirect calls are made with the indirect form of the BR.CALL instruction, specifying B0 for the return link.
  The BR.CALL instruction saves the return link in the return branch register, saves the current frame marker in the AR.PFS register, and sets the base of the new register stack frame to the beginning of the output region of the old frame. Because the indirect call sequence obtains the entry point address and new GP value from the function descriptor, control flows directly to the target procedure, without the need for any intervening stubs.
• Callee: Entry; Exit. The remainder of the calling sequence is the same as for direct calls ( Section 4.7.3.1).

Parameters are passed in a combination of general registers, floating-point registers, and memory, as described below, and as illustrated in Figure 4-5.

The parameter list is formed by placing each individual parameter into fixed-size elements of the parameter list, referred to as parameter slots. Each parameter slot is 64 bits wide; parameters larger than 64 bits are placed in as many consecutive parameter slots as are needed to contain the entire parameter. The rules for allocation and alignment of parameter slots are described in Section 4.7.5.1.

The contents of the first eight parameter slots are always passed in registers, while the remaining parameters are always passed on the memory stack, beginning at the caller's stack pointer plus 16 bytes. The caller uses up to eight of the registers in the output region of its register stack for integer and VAX floating-point parameters, and up to eight floating-point registers for IEEE floating-point parameters. The maximum number of registers used is eight.

Figure 4-5 Parameter Passing in Registers and Memory

To accommodate variable argument lists in the C language, there is a fixed correspondence between parameter slots; the first parameter slot is always in either the first general output register or the first floating-point register (never both), the second parameter slot is always in the second general output register or the second floating-point register (never both), and so on. This allows a procedure to spill its register parameters easily to memory to form the argument home area before stepping through the parameter list with a pointer. The Argument Information register (AI) makes this possible, as explained in Section 4.7.5.3.

A procedure can assume that the NaT bits on its incoming general register arguments are clear, and that the incoming floating-point register arguments are not NaTVals. A procedure making a call must ensure only that registers containing actual parameters are clear of NaT bits or NaTVals; registers not used for actual parameters are undefined.

4.7.5 Parameter Passing Mechanisms

This OpenVMS calling standard defines three classes of argument items according to the mechanism used to pass the argument:

• Immediate value
• Reference
• Descriptor

Argument items are not self-defining; interpretation of each argument item depends on agreement between the calling and called procedures.

This standard does not dictate which passing mechanism must be used by a given language compiler. Language semantics and interoperability considerations might require different mechanisms in different situations.

Immediate value

An immediate value argument item contains the value of the data item. The argument item, or the value contained in it, is directly associated with the parameter.

Reference

A reference argument item contains the address of a data item such as a scalar, string, array, record, or procedure. This data item is associated with the parameter.

Descriptor

A descriptor argument item contains the address of a descriptor, which contains structural information about the argument's type (such as array bounds) and the address of a data item. This data item is associated with the parameter.

Requirements for using the argument passing mechanisms follow:

• By immediate value. An argument may be passed by immediate value only if the argument is one of the following:
  • One of the noncomplex scalar data types with a size known (at compile time) to be <= 64 bits
  • Either single or double precision complex
  • A record with a known size (at compile time)
  • A set, implemented as a bit vector, with a size known (at compile time) to be <= 64 bits
  
  No form of string or array data type may be passed by immediate value in a standard call.
  Unused high-order bits must be zero or sign extended, as appropriate depending on the date type, to fill all bits of each argument list item (as specified in Table 4-10).
  A single-precision or double-precision complex value is passed as two single- or double-precision floating-point values, respectively. Note that the argument count reflects that two argument positions are used rather than just one actual argument.
  A record value, which may be larger than 64 bits, is passed by immediate value as follows:
  • Allocate as many fully occupied argument item positions to the argument value as are needed to represent the argument.
  • If the final argument position is only partially occupied by the argument, the contents of the remaining bits are undefined.
  • If an argument position is passed in one of the registers, it can only be passed in an integer register (never in a floating-point register).
  
  Other argument values that are larger than 64 bits can be passed by immediate value using nonstandard conventions, typically using a method similar to those for passing records. Thus, for example, a 26-byte string can be passed by value in four integer registers.
• By reference. Nonparametric arguments (arguments for which associated information such as string size and array bounds are not required) can be passed by reference in a standard call. This includes extended precision floating and extended precision complex values.
• By descriptor. Parametric arguments (arguments for which associated information such as string size and array bounds must be passed to the caller) are passed by a single descriptor in a standard call.

Note that extended floating values are not passed using the immediate value mechanism; rather, they are passed using the by reference mechanism. (However, when by value semantics is required, it may be necessary to make a copy of the actual parameter and pass a reference to that copy in order to avoid improper alias effects.)

Also note that when a record is passed by immediate value, the component types are not material to how the argument is aligned; the record will always be quadword aligned.

4.7.5.1 Allocation of Parameter Slots

Parameter slots are allocated for each parameter, based on the parameter passing mechanism, type, and size, treating each parameter in sequence, from left to right. The rules for allocating parameter slots and placing the contents within the slot are given in Table 4-9. The allocation column of the table indicates how parameter slots are allocated to each type of parameter.

OpenVMS does not support passing the Itanium double-precision extended floating-point type (__float80), although that type may be used from time to time in code generation sequences.

This placement policy does not ensure that parameters greater than 64 bits in size will fall on a natural alignment boundary if passed in memory. Such parameters may need to be copied by the called procedure into an aligned temporary prior to use, or accessed in a way that does not depend on natural alignment.

4.7.5.2 Normal Register Parameters

The first eight parameter slots (64 bytes) are passed in registers, according to the rules in this section.

• These eight argument slots are associated, one-to-one, with the stacked output general registers, as shown in Figure 4-5.
• Integral scalar parameters, (including addresses and pointers), VAX floating-point parameters, and aggregate parameters in these slots are passed only in the corresponding output general registers.
• Aggregate parameters in these slots are passed by value only in the corresponding output general registers. The aggregate is treated as a sequence of 64-bit integral values, with each value allocated into the next available slot in aggregate memory address order. If the size of the aggregate is not an even multiple of 64 bits, then the unused bits in the last slot are undefined.
• If an aggregate or VAX floating-point complex parameter straddles the boundary between slot 7 and slot 8, the part that lies within the first eight slots is passed in general registers, and the remainder is passed in memory, as described in Table 4-10.
  Complex values (other than IEEE quad-precision floating-point complex), in those languages that include complex types, are passed as a pair of floating-point values (either single-precision or double-precision as appropriate). It is possible for the first of the two floating-point values in a complex value to occupy the last output register slot; in this case, the second floating-point value is passed in memory. IEEE quad-precision floating-point complex values are passed by reference.
• IEEE single-precision and double-precision floating-point scalar parameters are passed in the corresponding floating-point register slot. IEEE quad-precision floating point scalar parameters are passed by reference in the corresponding output general registers.

When IEEE floating-point parameters are passed in floating-point registers, they are passed in the register format, rounded to the appropriate precision. They are never passed in the general registers unless part of an aggregate, in which case they are passed in the aggregate memory format. When VAX floating-point parameters are passed in general registers, they are passed in memory format.

Parameters allocated beyond the eighth parameter slot are never passed in registers.

Unsigned integral (except unsigned 32-bit), set, and VAX floating-point values passed in registers are zero-filled; signed integral values as well as unsigned 32-bit integral values are sign-extended to 64 bits. For all other types passed in the general registers, unused bits are undefined.

The rules contained in this section are summarized in Tables 4-10 and 4-11.

Table 4-11 contains the defined meanings for the memory extension type symbols used in Table 4-10.