Document revision date: 30 March 2001 | |
Order Number: AA--PV64D--TE
This manual describes how to use the MACRO-32 Compiler for OpenVMS Alpha to port VAX MACRO code to an OpenVMS Alpha system. It also describes how to use the compiler's 64-bit addressing support.
Revision/Update Information: This manual supersedes Migrating to an OpenVMS AXP System: Porting VAX Macro Code, OpenVMS AXP Version 1.5.
Software Version:
OpenVMS Alpha Version 7.3
Compaq Computer Corporation Houston, Texas
© 2001 Compaq Computer Corporation
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ZK5601
The Compaq OpenVMS documentation set is available on CD-ROM.
Contents | Index |
This manual is for software engineers responsible for porting application code written in VAX MACRO. Therefore, it demands that the reader understand the OpenVMS VAX operating system and possess strong programming skills.
This manual is divided into two parts.
Part 1: Concepts and Methodology comprises the following four chapters:
Part 2: Reference comprises the following appendixes:
This manual refers readers to the following manuals for additional information on certain topics:
For additional information about OpenVMS products and services, access the following World Wide Web address:
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If you need help deciding which documentation best meets your needs, call 800-282-6672.
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This chapter describes a process that software engineers can use when planning to port VAX MACRO code to an OpenVMS Alpha system. The chapter contains the following:
The MACRO-32 Compiler for OpenVMS Alpha is provided for porting VAX MACRO code to OpenVMS Alpha. For any new development, Compaq recommends the use of mid- and high-level languages. |
The MACRO-32 Compiler for OpenVMS Alpha compiles VAX MACRO source code into machine code that runs on Alpha systems. While some code can be compiled without any changes, most code modules will require the addition of entry point directives. Many code modules will require other changes as well.
The compiler may detect only a few problems with a module at its initial compilation and then, after you have corrected them, the compiler may discover additional problems. In such cases, the resolution of one problem can allow the compiler to further examine the code and discover other problems the initial one concealed.
The compiler includes many features that make this process easier, such as:
The compiler also provides 64-bit addressing support, which is
documented in Chapter 5 and in Appendix E. Support for 64-bit
addressing was introduced in OpenVMS Alpha Version 7.0. This support is
provided for those rare instances when it is preferable to use VAX
MACRO to access 64-bit address space instead of using a high-level
language that is supported on OpenVMS Alpha.
1.2 Differences Between the Compiler and the Assembler
It is important to remember that the MACRO-32 compiler is a compiler,
not an assembler. It does not create output code that exactly
matches the input code. In its optimization process, the compiler may
move, replicate, or remove code and interleave instructions.
Furthermore, the faulting behavior of the ported code may not match
that of VAX code. These differences are described in the following
sections.
1.2.1 Moving Code
Mispredicted branches are expensive on an Alpha system. The compiler attempts to determine the most likely code path through the module and then generates code that consolidates that code path. Code paths deemed unlikely are moved out of line to the end of the module. Consider the following example:
$ASSIGN_S DEVNAM=DEVICE,CHAN=CHANNEL BLBS R0,10$ JSB PROCESS ERROR HALT 10$: |
In this example, the compiler will treat the HALT as an unlikely code path and detect that the two code streams do not rejoin at 10$. Because of these conditions, it will determine that the branch is likely to be taken. It will then move the intervening instructions out of line to the end of the module, change the BLBS instruction to a BLBC that branches to the moved code, and continue with in-line code generation at the label 10$, as follows:
$ASSIGN_S DEVNAM=DEVICE,CHAN=CHANNEL BLBC L1$ 10$: . . . (routine exit) L1$: JSB PROCESS ERROR HALT |
You can change the compiler's determination of the likelihood of
conditional branches with the compiler directives .BRANCH_LIKELY and
.BRANCH_UNLIKELY (see Section 4.2).
1.2.2 Replicating Code
The compiler may replicate small sections of code multiple times to eliminate excessive branching. For example, when compiling branches to the following VAX code, the compiler will replicate the MOVL at each branch to ERROR1 and then branch directly to COMMON_ERROR.
ERROR1: MOVL #ERROR1,R0 BRW COMMON_ERROR |
The compiler's optimizations may determine that some instructions do not contribute to the code flow. In such instances, the instructions may be removed. An example of this is a CMP or TST instruction with no subsequent conditional branch, such as the following:
CMPB (R2),511(R2) JSB EXE$SENDMSG |
Removal of this CMPB instruction could cause a problem if its purpose was to touch two memory locations to ensure that the memory pages were faulted in before calling the routine. This would likely have to be changed in porting to OpenVMS Alpha anyway because of the different page sizes of VAX and Alpha systems. In addition to changing the page size, you should replace the instruction with MOVx instructions, such as the following:
MOVB (R2),R1 MOVB 8191(R2),R0 JSB EXE$SENDMSG |
Note that the two MOVB instructions operated on two different registers. The compiler does not currently remove instructions that load values into a register which is never subsequently read before being overwritten. However, this optimization may be done in the future.
In general, code which requires that a memory read reference actually touch memory should be examined carefully, as current or future optimizations may move or remove the references. |
Instruction scheduling, which is performed by default (see
Section 4.3), will interleave the Alpha instructions generated from
one VAX instruction with the Alpha instructions generated by
surrounding VAX instructions.
1.2.5 Reserved Operand Faults
On VAX systems, some VAX MACRO instructions may generate a reserved operand fault if certain operands are out of a required range. For example, on a bit manipulation instruction such as INSV, if the size operand is greater than 32, a VAX system will generate a run-time reserved operand fault.
On Alpha systems, if the operand that is out of range is a compile-time
constant, the compiler will flag this condition with an error message.
However, if this operand is variable at run time, the compiler makes no
attempt to generate run-time range checks on it. If the operand is out
of range, the resulting operation may cause incorrect results yet not
create a fault.
1.3 Step-by-Step Porting Process
The following steps have proven to be an efficient means for porting VAX MACRO code to OpenVMS Alpha:
Once you have some experience in porting VAX MACRO modules, it will be easier to recognize certain problems while inspecting the source and to fix them before your initial invocation of the compiler.
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