User`s guide

How the MACRO Compiler Functions on Different Platforms

2.11 Preserving VAX Atomicity and Granularity

To preserve the granularity of a MOVL R1,(R2) instruction, the compiler always

writes whole longwords with a STL instruction, even if the address to which

it is writing is assumed to be unaligned. If the address is unaligned, the STL

instruction will cause an unaligned memory reference fault. The PALcode

unaligned fault handler will then do the loads, masks, and stores necessary to

write the unaligned longword. However, since PALcode is noninterruptible, this

ensures that the surrounding memory locations are not corrupted.

When porting an application to an OpenVMS Alpha system, you should determine

whether the application performs byte, word, or unaligned longword writes to

memory that is shared either with processes executing on the local processor,

or with processes executing on another processor in the system, or with an

AST routine or condition handler. See Migrating to an OpenVMS AXP System:

Recompiling and Relinking Applications for a more complete discussion of the

programming issues involved in granularity operations in an OpenVMS Alpha

system.

Note

INSV instructions do not generate code that correctly preserves

granularity when granularity is turned on.

2.11.3 Precedence of Atomicity Over Granularity

If you enable the preservation of both granularity and atomicity, and the compiler

encounters VAX code that requires that both be preserved, atomicity takes

precedence over granularity.

For example, the instruction INCW 1(R0), when compiled with

.PRESERVE=GRANULARITY, retries the write of the new word value, if it

is interrupted. However, when compiled with .PRESERVE=ATOMICITY, it will

also refetch the initial value and increment it, if interrupted. If both options are

speciﬁed, it will do the latter.

In addition, while the compiler can successfully generate code for unaligned words

and longwords that preserves granularity, it cannot generate code for unaligned

words or longwords that preserves atomicity. If both options are speciﬁed, all

memory references must be to aligned addresses.

2.11.4 When Atomicity Cannot Be Guaranteed

Because compiler atomicity guarantees only affect memory modiﬁcation operands

in VAX instructions, you should take special care in examining VAX MACRO

sources for coding problems that /PRESERVE=ATOMICITY cannot resolve on

OpenVMS Alpha or OpenVMS I64 systems.

For example, consider the following VAX instruction:

ADDL2 (R1),4(R1)

For this instruction, the compiler generates an Alpha code sequence such as

the following, when /PRESERVE=ATOMICITY (or .PRESERVE ATOMICITY) is

speciﬁed:

2–26 How the MACRO Compiler Functions on Different Platforms