Vax performance V-008 SP User's Guide download pdf (Page 16)

Preparing to Port Macro-32 Code

1.1 Features of the MACRO Compiler

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:

• Qualiﬁers that allow you to control the kinds of messages the compiler

generates or to enforce VAX behavior in the generated code. For example, the

/FLAG qualiﬁer enables you to specify the types of informational messages

the compiler reports. Many of these messages identify porting problems,

including VAX architectural dependencies. The options to the /FLAG qualiﬁer

include reporting unaligned stack and memory references and reporting

unsupported directives. (For more information about the /FLAG qualiﬁer, see

Appendix A, MACRO Compiler Qualiﬁers.)

• Directives that indicate routine entry points and describe them to the

compiler or enforce VAX behavior for sections of code. For example,

.CALL_ENTRY declares the entry point of a called routine to the compiler.

Section 2.3, Section 2.5, and Chapter 3 discuss situations when the compiler

requires special directives. (See Appendix B, Specialized Directives.)

• For OpenVMS Alpha platforms, built-ins that allow you to access the Alpha

instructions that perform 64-bit operations and Alpha PALcode instructions.

(PALcode is shorthand for privileged architecture library code.) For example,

EVAX_ADDQ, with the appropriate operands, performs the quadword add

instruction. (See Appendix C, MACRO Compiler Built-ins.)

• For OpenVMS I64 platforms, built-ins that allow you to access certain I64

instructions. In addition, to aid in porting code from Alpha to I64, many

of the EVAX_builtins have been reimplemented for I64. (See Appendix C,

MACRO Compiler Built-ins.)

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 or OpenVMS

I64.

1.2 Differences Between the Compiler and the Assembler

It is important to remember that the MACRO 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 might move, replicate, or remove code

and interleave instructions. Furthermore, the faulting behavior of the ported

code might 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 OpenVMS Alpha or OpenVMS I64

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:

1–2 Preparing to Port Macro-32 Code

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Vax performance V-008 SP User's Guide Page 16

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