Updated: 11 December 1998

Porting VAX MACRO Code to OpenVMS Alpha

This directive allows the user to override the compiler's alignment assumptions, and also allows implicit reads/writes of registers to be declared.

Format

.SET_REGISTERS argument-list

Parameters

argument-list

One or more of the arguments listed in the following table. For each argument, you can specify one or more registers.

Option	Description
aligned=<>	Declares one or more registers to be aligned on longword boundaries.
unaligned=<>	Declares one or more registers to be unaligned. Because this is an explicit declaration, this unaligned condition will not produce a fault at run time.
read=<>	Declares one or more registers, which otherwise the compiler could not detect as input registers, to be read.
written=<>	Declares one or more registers, which otherwise the compiler could not detect as output registers, to be written to.

Description

The aligned and unaligned qualifiers to this directive allow the user to override the compiler's alignment assumptions. Using the directive for this purpose in certain cases can produce more efficient code (see Section 4.1).

The read and written qualifiers to this directive allow implicit reads and writes of registers to be declared. They are generally used to declare the register usage of called routines and are useful for documenting your program.

With one exception, the .SET_REGISTERS directive remains in effect (ensuring proper alignment processing) until the routine ends, unless you change the value in the register. The exception can occur under certain conditions when a flow path joins the code following a .SET_REGISTERS directive.

The following example illustrates such an exception. R2 is declared aligned, and at a subsequent label, 10$, which is before the next write access to the register, a flow path joins the code. R2 will be treated as unaligned following the label, because it is unaligned from the other path.

INCL R2 ; R2 is now unaligned . . . BLBC R0, 10$ . . . MOVL R5, R2 .SET_REGISTERS ALIGNED=R2 MOVL R0, 4(R2) 10$: MOVL 4(R2), R3 ; R2 considered unaligned ; due to BLBC branch

The .SET_REGISTERS directive and its read and written qualifiers are required on every routine call that passes or returns data in any register from R2 through R12, if you specify the command line qualifier and option /OPTIMIZE=VAXREGS. That is because the compiler allows the use of unused VAX registers as temporary registers when you specify /OPTIMIZE=VAXREGS.

Examples

DIVLR0,R1 
.SET_REGISTERS ALIGNED=R1 
MOVL     8(R1), R2          ; Compiler will use aligned load. 
 
      

In this example, the compiler would normally consider R1 unaligned after the division. Any memory references using R1 as a base register (until it is changed again) would use unaligned load/stores. If it is known that the actual value will always be aligned, performance could be improved by adding a .SET_REGISTERS directive, as shown.

MOV1     4(R0), R1          ; Stored memory addresses assumed 
.SET_REGISTERS UNALIGNED=R1 ; aligned so explicitly set it unaligned 
MOVL     4(R1), R2          ; to avoid run-time fault. 
      

In this example, R1 would be considered longword aligned after the MOVL. If it is actually unaligned, an alignment fault would occur on memory reference that follows at run time. To prevent this, the .SET_REGISTERS directive can be used, as shown.

.SET_REGISTERS READ=<R3,R4>, WRITTEN=R5 
JSB     DO_SOMETHING_USEFUL  
      

In this example, the read/written attributes are used to explicitly declare register uses which the compiler cannot detect. R3 and R4 are input registers to the JSB target routine, and R5 is an output register. This is particularly useful if the routine containing this JSB does not use these registers itself, or if the SET_REGISTERS directive and JSB are embedded in a macro. When compiled with /FLAG=HINTS, routines which use the macro would then have R3 and R4 listed as possible input registers, even if they are not used in that routine.

This directive associates an alignment attribute with a symbol definition for a register offset. You can use this directive when you know the alignment of the base register. This attribute guarantees to the compiler that the base register has the same alignment, which enables the compiler to generate optimal code.

Format

.SYMBOL_ALIGNMENT argument-list

Parameters

argument-list

One of the arguments listed in the following table.

Option	Description
long	Declares longword alignment for any symbol that you declare after this directive.
quad	Declares quadword alignment for any symbol that you declare after this directive.
none	Turns off the alignment specified by the preceding .SYMBOL_ALIGNMENT directive.

Description

The .SYMBOL_ALIGNMENT directive is used to associate an alignment attribute with the fields in a structure when you know the base alignment. It is used in pairs. The first .SYMBOL_ALIGNMENT directive associates either longword (long) or quadword (quad) alignment with the symbol or symbols that follow. The second directive, .SYMBOL_ALIGNMENT none, turns it off.

Any time a reference is made with a symbol with an alignment attribute, the base register of that reference, in effect, inherits the symbol's alignment. The compiler also resets the base register's alignment to longword for subsequent alignment tracking. This alignment guarantee enables the compiler to produce more efficient code sequences.

Example

OFFSET1 = 4 
.SYMBOL_ALIGNMENT LONG 
OFFSET2 = 8 
OFFSET3 = 12 
.SYMBOL_ALIGNMENT QUAD  
OFFSET4 = 16 
.SYMBOL_ALIGNMENT NONE 
OFFSET5 = 20 
    . 
    . 
    . 
CLR1 OFFSET2(R8) 
    . 
    . 
    . 
MOVL R2, OFFSET4(R6) 
 
      

For OFFSET1 and OFFSET5, the compiler will use only its tracking information for deciding if Rn in OFFSET1(Rn) is aligned or not. For the other references, the base register will be treated as longword (OFFSET2 and OFFSET3) or quadword (OFFSET4) aligned.

After each use of OFFSET2 or OFFSET4, the base register in the reference is reset to longword alignment. In this example, the alignment of R8 and R6 will be reset to longword, although the reference to OFFSET4 will use the stronger quadword alignment.

This appendix describes the two sets of built-ins provided with the MACRO-32 Compiler for OpenVMS Alpha. They are:

• Alpha instruction built-ins which are used to access Alpha instructions for which there are no VAX equivalents.
• Alpha PALcode built-ins which are used to emulate the VAX instructions for which there are no Alpha equivalents and to perform other functions such as quadword queue manipulations.

Both sets of built-ins are presented in tables. The second column of each table specifies the operands the built-in expects, where:

• WL = write longword
• ML = modify longword
• AL = address of longword
• WQ = write quadword
• RQ = read quadword
• MQ = modify quadword
• AQ = address of quadword
• AB = address of byte
• AW = address of word
• WB = write byte
• WW = write word

Ported VAX MACRO code sometimes requires access to Alpha native instructions to deal directly with a 64-bit quantity or to include an Alpha instruction that has no VAX equivalent. The compiler provides built-ins to allow you access to these instructions.

You use these built-ins in the same way that you use native VAX instructions, using any VAX operand mode. For example, EVAX_ADDQ 8(R0),(SP)+,R1 is legal. The only exception is that the first operand of any Alpha load/store built-in (EVAX_LD*, EVAX_ST*) must be a register.

It is recommended that you place any built-in within an ".IF DF,EVAX" conditional code block unless the module is Alpha specific. They can appear in Alpha specific portions of the macro definitions described in Appendix D.

The following byte and word built-ins are included in the MACRO-32 compiler, starting with OpenVMS Alpha Version 7.1:

• EVAX_LDBU
• EVAX_LDWU
• EVAX_STB
• EVAX_STW
• EVAX_SEXTB
• EVAX_SEXTW

The best environment in which to run code that contains the byte and word built-ins is on an Alpha computer that implements these instructions in hardware. If you run such code on an OpenVMS Alpha system that implements them by software emulation, the following limitations exist:

• Significant performance loss
  The overhead of handling the exception to trigger the software emulation causes a significant performance loss. If software emulation is in effect, you will see the following message:

  %SYSTEM-I-EMULATED, an instruction not implemented on this processor was emulated

• Some capabilities not present in the software emulation
  The software emulation is not capable of providing all the capabilities that would be present on a system that implemented the the instructions in hardware. Code that executes in inner access modes and at elevated IPL is allowed to use these instructions. For example, activation of the software emulator above IPL 2 will not cause a bugcheck. However, certain applications where these instructions might be useful, such as direct writes to hardware control registers, will be impossible, because such applications require the presence of address lines whose function cannot be emulated.

Furthermore, if the code with these built-ins executes on a system without either the byte and word software emulator or a processor that implements the byte and word instructions in hardware, it will incur a fatal exception, such as the following:

%SYSTEM-F-OPCDEC, opcode reserved to Digital fault at PC=00000000000020068,PS=0000001B

Table C-1 summarizes the Alpha built-ins supported by the compiler.

Table C-1 Alpha Instruction Built-Ins

Built-in	Operands	Description
EVAX_SEXTB	<RQ,WB>	Sign extend byte
EVAX_SEXTW	<RQ,WW>	Sign extend word
EVAX_SEXTL	<RQ,WL>	Sign extend longword
EVAX_LDBU	<WQ,AB>	Load zero-extended byte from memory
EVAX_LDWU	<WQ,AQ>	Load zero-extended word from memory
EVAX_LDLL	<WL,AL>	Load longword locked
EVAX_LDAQ	<WQ,AQ>	Load address of quadword
EVAX_LDQ	<WQ,AQ>	Load quadword
EVAX_LDQL	<WQ,AQ>	Load quadword locked
EVAX_LDQU	<WQ,AQ>	Load unaligned quadword
EVAX_STB	<RQ,AB>	Store byte from register to memory
EVAX_STW	<RQ,AW>	Store word from register to memory
EVAX_STLC	<ML,AL>	Store longword conditional
EVAX_STQ	<RQ,AQ>	Store quadword
EVAX_STQC	<MQ,AQ>	Store quadword conditional
EVAX_STQU	<RQ,AQ>	Store unaligned quadword
EVAX_ADDQ	<RQ,RQ,WQ>	Quadword add
EVAX_SUBQ	<RQ,RQ,WQ>	Quadword subtract
EVAX_MULQ	<RQ,RQ,WQ>	Quadword multiply
EVAX_UMULH	<RQ,RQ,WQ>	Unsigned quadword multiply high
EVAX_AND	<RQ,RQ,WQ>	Logical product
EVAX_OR	<RQ,RQ,WQ>	Logical sum
EVAX_XOR	<RQ,RQ,WQ>	Logical difference
EVAX_BIC	<RQ,RQ,WQ>	Bit clear
EVAX_ORNOT	<RQ,RQ,WQ>	Logical sum with complement
EVAX_EQV	<RQ,RQ,WQ>	Logical equivalence
EVAX_SLL	<RQ,RQ,WQ>	Shift left logical
EVAX_SRL	<RQ,RQ,WQ>	Shift right logical
EVAX_SRA	<RQ,RQ,WQ>	Shift right arithmetic
EVAX_EXTBL	<RQ,RQ,WQ>	Extract byte low
EVAX_EXTWL	<RQ,RQ,WQ>	Extract word low
EVAX_EXTLL	<RQ,RQ,WQ>	Extract longword low
EVAX_EXTQL	<RQ,RQ,WQ>	Extract quadword low
EVAX_EXTBH	<RQ,RQ,WQ>	Extract byte high
EVAX_EXTWH	<RQ,RQ,WQ>	Extract word high
EVAX_EXTLH	<RQ,RQ,WQ>	Extract longword high
EVAX_EXTQH	<RQ,RQ,WQ>	Extract quadword high
EVAX_INSBL	<RQ,RQ,WQ>	Insert byte low
EVAX_INSWL	<RQ,RQ,WQ>	Insert word low
EVAX_INSLL	<RQ,RQ,WQ>	Insert longword low
EVAX_INSQL	<RQ,RQ,WQ>	Insert quadword low
EVAX_INSBH	<RQ,RQ,WQ>	Insert byte high
EVAX_INSWH	<RQ,RQ,WQ>	Insert word high
EVAX_INSLH	<RQ,RQ,WQ>	Insert longword high
EVAX_INSQH	<RQ,RQ,WQ>	Insert quadword high
EVAX_TRAPB	<>	Trap barrier
EVAX_MB	<>	Memory barrier
EVAX_RPCC	<WQ>	Read process cycle counter
EVAX_CMPEQ	<RQ,RQ,WQ>	Integer signed compare, equal
EVAX_CMPLT	<RQ,RQ,WQ>	Integer signed compare, less than
EVAX_CMPLE	<RQ,RQ,WQ>	Integer signed compare, less equal
EVAX_CMPULT	<RQ,RQ,WQ>	Integer unsigned compare, less than
EVAX_CMPULE	<RQ,RQ,WQ>	Integer unsigned compare, less equal
EVAX_BEQ	<RQ,AQ>	Branch equal
EVAX_BLT	<RQ,AQ>	Branch less than
EVAX_BNE	<RQ,AQ>	Branch not equal
EVAX_CMOVEQ	<RQ,RQ,WQ>	Conditional move/equal
EVAX_CMOVNE	<RQ,RQ,WQ>	Conditional move/not equal
EVAX_CMOVLT	<RQ,RQ,WQ>	Conditional move/less than
EVAX_CMOVLE	<RQ,RQ,WQ>	Conditional move/less or equal
EVAX_CMOVGT	<RQ,RQ,WQ>	Conditional move/greater than
EVAX_CMOVGE	<RQ,RQ,WQ>	Conditional move/greater or equal
EVAX_CMOVLBC	<RQ,RQ,WQ>	Conditional move/low bit clear
EVAX_CMOVLBS	<RQ,RQ,WQ>	Conditional move/low bit set
EVAX_MF_FPCR	<WQ>	Move from floating-point control register
EVAX_MT_FPCR	<WQ,RQ>	Move to floating-point control register
EVAX_ZAP	<RQ,RQ,WQ>	Zero bytes
EVAX_ZAPNOT	<RQ,RQ,WQ>	Zero bytes with NOT mask

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