HP OpenVMS Systems Documentation

HP OpenVMS Calling Standard

Body regions can do anything that does not invalidate the state of the stack frames and preserved registers as recorded for that region. A body region must obey the following restrictions:

• If its memory stack frame is fixed in size, a body region must not modify the SP register.
• If its memory stack frame is variable in size, a body region can modify SP at any point, but the unwind descriptors must indicate where a valid PSP value can be found at any point while the body region is executing.
• The unwind descriptors must indicate where a valid copy of the previous frame marker can be found at any point while the body region is executing. The body region code must not make a procedure call while the previous frame marker remains only in AR.PFS.
• The unwind descriptors must indicate where a valid copy of the return PC can be found at any point while the body region is executing. The body region code must not make a procedure call while the saved return PC remains only in B0.
• The unwind descriptors must indicate where a valid copy of each preserved register can be found at any point while the body region is executing.

At every point in a body region, the unwind descriptors identify a single location where a valid value for SP, PSP, AR.PFS, PC, and each preserved register can be found. The body region must not modify a register or memory location while the unwind descriptors indicate that one of these items (SP, PSP, AR.PFS, PC, preserved register) is stored there.

The locations of these saved values (SP, PSP, AR.PFS, PC, preserved registers) generally remain constant throughout the body region in locations specified in the prologue descriptor records. However, when this is not the case, the unwind descriptors described in Table A-13 can be used to mark changes in the unwind state within a body region. A body region can restore AR.PFS, RP, and any preserved registers.

A.3.4 Conventions for Epilogues

The memory stack pointer (SP) is typically restored just before executing a return branch. In a normal epilogue at the end of a body region, the instruction that restores the previous SP value can be anywhere within a few instructions of the end of the region; the unwind descriptor format provides a place to record the exact location of this instruction. If the procedure has a memory stack frame and has return instructions in the middle of the body, the procedure must be divided into separate body regions, each ending at the point of each return instruction.

The unwinder does not need a specific epilogue region that is distinct from the body region.

A.3.5 Conventions for the Spill Area in the Memory Stack Frame

The spill area for preserved general, floating-point, and branch registers is near the base of the stack frame, in a continuous range ending (by default) at the base of the stack frame plus 16 bytes (PSP+16). In other words, the 16-byte scratch area in the caller's stack frame is normally included in the spill area. If the scratch area is needed to save register parameters for a variable-argument list procedure, the spill area can be moved so that it ends at a lower address, but the ending address must be a fixed location relative to the base of the frame (PSP).

Locations in the spill area are reserved for each preserved general, floating-point, and branch register that is saved anywhere within the procedure (including shrink-wrapped regions). Locations are allocated, from low address to high, for (in order) general registers, then branch registers, and then floating-point registers. Registers are saved in numerical order, lower-numbered registers at lower addresses. The spill area must end at a 16-byte boundary, so that all the floating-point spill locations are 16-byte aligned.

It is not required that all registers preserved in the spill area be consecutive from each register file. If, for example, R4 and R7 are preserved, but R5 and R6 are not, space is allocated only for R4 and R7.

Code may need to spill scratch registers in addition to preserved registers. There are no conventions for spilling scratch registers, because they do not need to be recovered during a stack unwind. To make the best use of the User NaT collection register, general register spills should be adjacent to the preserved general register spill area.

Normally, the unwinder expects to find the NaT bits for the preserved registers in the User NaT collection register, AR.UNAT. If the total spill area for general registers (scratch and preserved registers combined) exceeds 64 quadwords, it is necessary to save the User NaT collection register in order to spill up to an additional 64 general registers. In this overflow situation, two or more NaT collections are managed by swapping them in and out of the single collection register. The NaT collection that contains the NaT bits for the preserved registers is called the primary UNaT collection, and the unwinder must know where to find these bits. In procedures where the NaT collection register is multiplexed, the location of the primary UNaT collection is recorded in the unwind information.

If the primary UNaT collection is saved, then the location of the primary UNaT value must be recorded, as well as when that value is restored. The only way to do the latter is by using one of the general unwind descriptors found in Section A.4.1.1.

The unwinder must take special note of the time at which the primary UNaT is restored. In the case of an unwind after the primary UNaT restore, the unwinder must not attempt to redundantly reperform any fills that preceded that restore because the applicable UNaT state will have been lost.

The condition handling mechanism uses the following data structures:

• A master unwind table, which allows the unwinder and dispatcher to associate a PC value with an image
• An unwind table for each image, which allows the dispatcher and unwinder to associate a PC value with a procedure and its unwind and exception handling information

Every procedure (except some leaf procedures) has one entry in this table. (If the compiler has generated more than one noncontiguous region of code for a procedure, there is one entry in this table for each region.) Each unwind table entry points to an information block that contains the following data structures:

• A set of unwind descriptors
• (Optional) A pointer to a condition handler
• (Optional) An operating system-specific data area
• (Optional) A language-specific data area for each procedure

Given a PC value, the dispatcher and unwinder both use the unwind table to locate an unwind entry for a procedure. The unwinder also uses the unwind descriptor list to unwind the stack from any point in the procedure.

The operating system-specific data area contains information about a routine as a whole that is not otherwise expressible using the unwind descriptors, independent of whether the routine has a condition handler.

The language-specific data area contains information specific to the condition handler that uses it. The address of the language-specific data area is passed to the condition handler whenever the condition handler is invoked by the dispatcher.

A.4.1 Unwind Table and Unwind Information Block

The unwind table is a sequence of sorted unwind table entries. Unwind table entries contain three fields, as illustrated in Figure A-1; each field is a 64-bit quadword. The first two fields define the starting and ending addresses of the region, respectively. The third field points to a variable-size information block that contains the unwind descriptor list and language-specific data area. The ending address is the address of the first bundle beyond the end of the procedure. Because these values are all segment-relative offsets rather than absolute addresses, they do not require run-time relocations. The unwind table entries are sorted by the region start address. The shaded area in the figure represents the language-specific data area.

Figure A-1 Unwind Table and Unwind Information Block

Note that a leaf procedure may have no unwind table entry (see Section A.5).

The unwind table and the unwind information block must each be aligned at an 8-byte boundary. Within the information block, the condition handler pointer must also be aligned at an 8-byte boundary.

The first quadword of the information block consists of the following fields:

• ULEN, a 32-bit longword field that contains the length in quadwords of the unwind descriptor area (zero is a legitimate value).
• F, a 16-bit flag field (see Table A-1). Four bits are set aside for operating system-specific use. Two of these bits are defined by the Itanium software conventions, and the remaining bits are reserved.
  In this version, OpenVMS uses only the two low-order bits of the four bits available for operating system-specific use. These OpenVMS-specific bits can be accessed using the following:

  #define UNW_IVMS_MODE(x) (((x) >> 44) & 0x3L)

  
  These two bits form an enumeration code, which is interpreted as shown in Table A-1.

  Note For OpenVMS I64, the value of UNW_IVMS_MODE field must be 2 or 3. Otherwise, exception handling behaviour is undefined.

  
  The EHANDLER flag is set if the condition handler must be called during search for an exception handler. The UHANDLER flag is set if this routine must be called during the second unwind. (Note that for OpenVMS I64, the EHANDLER and UHANDLER flags are both set or both not set.) If neither bit is set, there is no frame handler for this procedure, and the condition handler identifier must be omitted along with the entire language-specific data area.
• V, a 16-bit version number that identifies the version of the unwind descriptor format. For this specification, the version number is 1.

These fields may be accessed with the following macros:

The unwind descriptor area contains a contiguous sequence of records describing the unwind regions in the procedure. Each group of records begins with a region header record that identifies the type and length of the region. The region header record is followed by any number of descriptor records that supply additional unwind information about the region.

Unwind descriptor records are divided into three categories:

• Region header records
• Descriptor records for prologue regions
• Descriptor records for body regions

This section describes the record types in each of these categories, lists rules for using unwind descriptor records, and explains how the records must be processed.

The information is encoded in variable-length records with a record type and one or more additional fields. The length of each record is implicit from the record type and its fields. All records are an integral number of bytes in length. In the descriptor record tables in the next three sections, the third column lists the format of each record type. These record formats are described in Appendix B.

Because the unwind descriptor area must be a multiple of 8 bytes, the last unwind descriptor must be followed by zero bytes as necessary to pad the area to an 8-byte boundary. These zero bytes will be interpreted as prologue region header records, specifying a zero-length prologue region, and serve as no-ops.

A.4.1.2 Region Header Records

The region header records are listed in Table A-2.

The fields in these records are used as follows:

• RLEN---Contains the length of the region, measured in instruction slots (three slots per bundle, counting X-unit instructions as two slots).
• MASK---Indicates which registers are saved in the prologue. The PROLOGUE_GR region type is used for entry prologues that save one or more preserved registers in the local register area of the register stack frame. This field defines what combination of RP, AR.PFS, PSP, and the predicate registers are preserved in standard general registers in the local area of the register stack frame. This mask is four bits; see Appendix B for the allocation of these bits. Other registers may be preserved in the prologue, but additional descriptor records are required for registers other than these four.
• GRSAVE---Identifies the first general register used to save the preserved registers identified in the mask field. Normally, this identifies a register in the procedure's local stack frame (that is, it should be greater than or equal to 32). However, leaf procedures can choose to use any consecutive sequence of scratch registers.

The entry state for a region matches the exit state of the preceding region, except for body regions that contain a COPY_STATE descriptor record, which is described in Table A-12.

The exit state of a region is determined as follows:

• For prologue regions, and body regions with no epilogue code, the exit state is the logical combination of the entry state with the modifications described by the descriptor records for the region.
• For body regions with epilogue code, the exit state is the same as the entry state of the corresponding prologue region whose effect is being undone. When shrink-wrap regions are nested, it is possible to reverse the effects of multiple prologues at once.

This section lists the descriptor records that can be used to describe prologue regions. In addition, the descriptor records described in Section A.4.1.5 can also be used. In the absence of any descriptor records or information in the region header record, a prologue region is assumed to create no memory stack frame and save no registers. Descriptors need be supplied only to override these defaults.

Table A-3 describes the descriptor records that are used to record information about the stack frame and the state of the previous stack pointer (PSP).

The fields in these records are used as follows:

• T---Describes a time, T, when a particular action occurs within the prologue. The time is specified as an instruction slot number, counting three slots per bundle. The first instruction slot in the prologue is numbered zero.
  For procedures with a memory stack frame, the instruction that modifies SP (fixed-size frame) or that saves PSP (variable-size frame) must be identified with either a MEM_STACK_F or a MEM_STACK_V record.
  In all other cases, if the time is not specified, the unwinder can assume that both of the following are true:
  • The original contents of the register is valid through the end of the prologue region.
  • The saved copy of the register is valid by the end of the prologue region.
  
  In a zero-length prologue region, the time parameter is irrelevant, and must be specified as zero.
• SIZE---Contains the fixed size of the memory stack frame, measured in 16-byte units.
• GR---Identifies a general register, or the first in a consecutive group of general registers, that is used for preserving the value of another register (as implied by the record type). Typically, this field identifies a general register in the procedure's local stack frame. A leaf procedure, however, can choose to use scratch registers. (A non-leaf procedure can also use scratch registers through a body region that makes no calls, but then it must move any values saved in scratch registers to a more permanent save location prior to making any calls, and needs a second prologue region to describe this process.)
• SPOFF---Identifies a location in the memory stack where a register or group of registers are spilled to memory. This location is specified relative to the current stack pointer. See Appendix B for the encoding of this field.

Table A-4 describes the descriptor records that are used to record the state of the return pointer (RP).

The fields in these records are used as follows:

• BR---Identifies a branch register that contains the return link, when the return link is not either in B0 or saved to another location.
• PSPOFF---Identifies a location in the memory stack where a register or group of registers is spilled to memory. The location is specified relative to the previous stack pointer (which is equal to the current stack pointer plus the frame size). See Appendix B for the encoding of this field.

Table A-5 describes the descriptor records that are used to record the state of the previous function state register (AR.PFS).

Table A-6 describes the descriptor records that are used to record the state of the preserved predicate registers.

Table A-7 describes the descriptor records that are used to record the state of the preserved general registers, floating-point registers, and branch registers.