HP OpenVMS Systems Documentation

Operator commands perform arithmetic on operands popped from the stack. The linker evaluates expressions in Post Fix Polish form. If the structure level EMBH$B_STRLVL is EOBJ$C_STRLVL64, all operations are performed using signed 64-bit two's complement integers. Otherwise, the low-order longwords of operands are used and operations are performed using signed 32-bit two's complement arithmetic. There is no provision for floating-point, string, or quadword computation. Attempts to divide by zero produce a zero result and a nonfatal warning message. Upon completion of the operation, the result is pushed back on the stack. When longword arithmetic is used, the contents of the high-order 32 bits of the result are not predictable. Therefore, the result should be popped from the stack with a longword store command (ETIR$C_STO_LW) rather than a quadword store command (ETIR$C_STO_QW).

The operator commands check the context saved by previous stack or operator commands. If one of the operands is defined by a shareable image, then the other operand must be a constant and the operation must be addition or subtraction. The context saved for the result is the context of the relocatable operand. In those cases where both operands are relocatable, neither is allowed to be external, so the saved context for the result simply indicates that it is relocatable and not external.

Table B-10 lists the operator commands and related values, together with a brief description of each command.

Control commands manipulate the linker's location counter. Table B-11 lists the control commands and related values, together with a brief description of each command.

There are two types of conditional store commands. The first type declares conditional linkage, and assigns the linkage a unique index. The second type enables the linker to replace an instruction in the code stream. If an instruction is replaced, the linker is able to identify the conditional linkage that the original instruction relied on and eliminate the relocation that would otherwise have been generated for it. This is called linkage retirement. Linkage retirement leaves the storage allocated for the linkage intact but eliminates the relocations. Linkage retirement is not supported for structure level 2; the linker unconditionally generates relocations for all linkages.

B.4.5.1 Defining Conditional Linkage with Address-Related Commands

Table B-12 lists the commands for allocating conditional linkage with the arguments that follow the size field.

The ETIR$C_STC_LP_PSB command is used to pass signature information associated with the procedure to the linker. The linker propagates this information to the fix-up section with a special type of fix-up when the /NONATIVE_ONLY switch is specified. The signature information is used by the image activator to build jackets at activation time if the routine resides in a translated image. The signature is opaque to the linker. For more information about the format of OpenVMS procedure signatures, see the OpenVMS Calling Standard.

The linkage index argument common to all of these commands is a positive integer value that must be unique to the module. Each index identifies a quadword of linkage. The commands used to declare a conditional linkage pair (ETIR$C_STC_LP and ETIR$C_STC_LP_PSB) reserve two index values. For example, if the index value 3 is declared, the index value 4 is implicitly declared. The linkage index is used by subsequent instruction replacement commands to identify the linkage on which the original instruction was dependent.

Linkage that is declared with a conditional linkage command and a nonzero linkage index can be used only by instructions that are identified by one of the instruction-related conditional store commands listed in Section B.4.5.2. Linkage that is declared with a conditional linkage command and a zero linkage index is not conditional. Improper use of conditional linkage cannot be detected by the linker and will result in incorrect program execution.

The linker assumes that its image location pointer points to the linkage being declared when the conditional store command is processed. If the linkage is needed, the linker will fill it with different values depending on the command as indicated in Tables B-13 and B-14. (Note that for structure level 2, the linker always fills in and relocates linkages.) The term symbol vector offset designates an offset into the symbol vector of the image that defined the symbol. The type of relocation or fix-up applied to the value in the linkage pair is specified in parentheses.

B.4.5.2 Optimizing Instructions with Instruction-Related Commands

Figure B-11 shows the offsets used by the linker to test for replacement and the replacement instructions that are used. The following commands might be used to replace a sequence of instructions used to call procedure B from inside procedure A:

• ETIR$C_STC_NOP
• ETIR$C_STC_LDA
• ETIR$C_STC_BSR

The ETIR$C_STC_NOP passes the addresses used to calculate the offset D2. If D2 can be expressed as a signed 21-bit integer, the linker replaces the first LDQ instruction with the instruction specified by the command. (For example, the compiler might specify BIS R31,R31,R31, an instruction that does nothing and uses very few CPU cycles.) The replacement of the first LDQ instruction anticipates the replacement of the JSR instruction and eliminates a memory reference, that is, the reference to the linkage pair that contains the address of the procedure entry for B. If D2 is too large to be expressed as a signed 21-bit integer, the linker does not do the replacement. The linker cannot delete the storage that contained the LDQ instruction (instead of replacing it with a NOP instruction) because the offset d that was calculated by the compiler would no longer be valid.

Figure B-11 Optimization of a Standard Call

The ETIR$C_STC_LDA command passes the addresses used to calculate the offset D1. If D1 can be expressed as a signed 16-bit integer, the linker replaces the second LDQ instruction with the LDA instruction supplied by the command and fills in the 16-bit offset field with D1. This replacement eliminates a memory reference to the second half of the linkage pair, which contains the address of the procedure descriptor for B. If D1 is too large to be expressed as a signed 16-bit integer, the linker does not do the replacement. This replacement can happen independently of the replacement of the other LDQ and JSR instructions.

Finally, the ETIR$C_STC_BSR command passes the same addresses as ETIR$C_STC_NOP (so that they will fail or succeed together), and the same test is done again. If the test succeeds, the linker replaces the JSR instruction with the BSR instruction supplied by the command and fills in the 21-bit offset field with D2.

Table B-15 lists the commands for optimizing the instruction stream. The default instruction stream must be placed in memory (typically with an ETIR$C_STO_IMM command) before the instruction-related commands are issued. The linker cannot detect attempts to optimize instructions that have not previously been placed in memory. Each command in the table has the following arguments (excluding the ETIR$C_STC_NBH_PS and ETIR$C_STC_NBH_GBL commands, which are not supported in structure level 2). The argument abbreviations are used by the commands in Table B-15 to describe the operation of the command.

The Linkage Index argument must be nonzero and point to the linkage declared with an address-related conditional store command. The PS1 and OFF1 arguments are used to calculate the location of the instruction that needs to be replaced. For the commands that deal with NOP and BSR instructions, the PS2 and OFF2 arguments point to the base of the calculation, typically, JSR + 4 . The commands for replacing the LDA instruction use PS2 and OFF2 to point to the base of the linkage section. The PS3 and OFF3 arguments point to the procedure entry point for the NOP and BSR commands and to the procedure descriptor for the LDA commands.

Instruction replacement will fail when any of the following conditions occur:

• The linker qualifier /NOREPLACE is specified.
• The displacement cannot be expressed in the number of bits specified for each command listed in Table B-15.
• The linker qualifier /SECTION_BINDING=CODE is specified and the call and the destination are in different image sections.
• A command with the _GBL suffix is used and the global name is defined by a shareable image.