MNEMEE - Electronic Systems - Technische Universiteit Eindhoven

Memory hierarchy re-use
In addition to the copy candidates identified at branches and loops, more copy candidates can be
generated by two techniques:
• considering both loop-carried and non-loop-carried versions of a copy candidate at loop
boundaries,
considering different combinations of copy-candidates from different branches as a new copycandidate
(“XOR” algorithm).
These techniques are explained by examples from the MPEG-4 encoder application.
In a re-use graph, the direction of the arrows between copy candidates indicates the direction in which
data transfers between them have to occur. For example, consider Figure 25. The bi-directional arrow
between @1 and @4 indicates that @4 has to be initialized with data from @1 first, and after it has
been accessed, the contents of @4 has to be written back to @1, i.e., it has both loads and stores. The
arrow between @1 and @5, on the other hand, is uni-directional, meaning that @5 only has to load
data from @1, but nothing has to be written back (because @5 is only read, not written, by accesses in
the application).
With the simple reuse algorithm, a copy candidate at a higher level always covers all the copy
candidates at the levels below it. So, in the example, only @12C would be extracted at the “loop v”
level. In reality, it is not always true that such a copy candidate covering all of its children represents
the best solution. Sometimes, it can be better to leave out certain children. In this case, the tool has
analyzed that there is a possibly better solution at that level that doesn’t cover the write accesses,
namely @14C. This is what the more complex algorithm does: it does not only consider copy
candidates that are covering all of their children, but also partial combinations.
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