Power ISA™ Version 2.03 - Power.org

Version 2.03
Programming Note
This Programming Note describes several aspects of
using dcbt instructions in which TH 0 =1.
A non-transient data stream having unlimited
length can be completely specified, including providing
the hint that the program will probably soon
load from it, using one dcbt instruction. The corresponding
specification for a data stream having
other attributes requires three dcbt instructions.
However, one dcbt instruction with TH=0b1010
and GO=1 can apply to a set of the data streams
described in the preceding sentence, so the corresponding
specification for n such data streams
requires 2×n+1 dcbt instructions. (There is no
need to execute a dcbt instruction with
TH=0b1010 and S=0b10 for a given stream ID
before using the stream ID for a new data stream;
the implicit portion of the hint provided by dcbt
instructions that describe data streams suffices.)
If it is desired that the hint provided by a given
dcbt instruction be provided in program order with
respect to the hint provided by another dcbt
instruction, the two dcbt instructions must be separated
by an eieio (or sync) instruction. For
example, if a dcbt instruction with TH=0b1010 and
GO=1 is intended to indicate that the program will
probably soon load from nascent data streams
described (completely) by preceding dcbt instructions,
and is intended not to indicate that the program
will probably soon load from nascent data
streams described (completely) by following dcbt
instructions, an eieio (or sync) instruction
must separate the dcbt instruction with GO=1 from
the preceding dcbt instructions, and another
eieio (or sync) instruction must separate that
dcbt instruction from the following dcbt instructions.
In practice, the second eieio (or sync)
described above can sometimes be omitted. For
example, if the program consists of an outer loop
that contains the dcbt instructions and an inner
loop that contains the Load instructions that load
from the data streams, the characteristics of the
inner loop and of the implementation’s branch prediction
mechanisms may make it highly unlikely
that hints corresponding to a given iteration of the
outer loop will be provided out of program order
with respect to hints corresponding to the previous
iteration of the outer loop. (Also, any providing of
hints out of program order affects only performance,
not program correctness.)
To mitigate the effects of interrupts on data
streams, it may be desirable to specify a given
“logical” data stream as a sequence of shorter,
component data streams. Similar considerations
apply to conditions and events that, depending on
the implementation, may cause an existing data
stream to cease to exist.
If it is desired to specify data streams without
regard to the number of stream IDs provided by
the implementation, stream IDs should be
assigned to data streams in order of decreasing
stream importance (stream ID 0 to the most important
stream, stream ID 1 to the next most important
stream, etc.). This order ensures that the hints for
the most important data streams will be provided.
Chapter 3. Storage Control Instructions
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