Vax performance V-008 SP User's Guide download pdf (Page 95)

Improving the Performance of Ported Code

4.4 Common-Based Referencing

On an OpenVMS Alpha system, references to data cells generally require two

memory references—one reference to load the data cell address from the linkage

section and another reference to the data cell itself. If several data cells are

located in proximity to one other, and the ADDRESSES optimization is used, the

compiler can load a register with a common base address and then reference the

individual data cells as offsets from that base address. This eliminates the load of

each individual data cell address and is known as common-based referencing.

The compiler performs this optimization automatically for local data psects when

the ADDRESSES optimization is turned on. The compiler generates symbols of

the form $PSECT_BASEn to use as the base of a local psect.

To use common-based referencing for external data psects, you must create a

preﬁx ﬁle which deﬁnes symbols as offsets from a common base. The preﬁx ﬁle

cannot be used when assembling the module for OpenVMS VAX because the VAX

MACRO assembler does not allow symbols to be deﬁned as offsets from external

symbols.

4.4.1 Creating a Preﬁx File for Common-Based Referencing

The following example illustrates the beneﬁts of creating a preﬁx ﬁle to use

common-based referencing. It shows:

• Code generated without the use of a preﬁx ﬁle

• How to create a preﬁx ﬁle

• Code generated with the use of a preﬁx ﬁle

Consider the following simple code section (CODE.MAR), which refers to data

cells in another module (DATA.MAR):

Module DATA.MAR:

.PSECT DATA NOEXE

BASE::

A:: .LONG 1

B:: .LONG 2

C:: .LONG 3

D:: .LONG 4

.END

Module CODE.MAR:

.PSECT CODE NOWRT

E:: .CALL_ENTRY

MOVL A,R1

MOVL B,R2

MOVL C,R3

MOVL D,R4

RET

.END

When compiling CODE.MAR without using common-based referencing, the

following code is generated:

In the linkage section:

.ADDRESS A

.ADDRESS B

.ADDRESS C

.ADDRESS D

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Vax performance V-008 SP User's Guide Page 95

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