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+title = "Software pipelining needs MVE"
+author = "Yann Herklotz"
+tags = []
+categories = []
+backlinks = ["3c2", "1c6a"]
+forwardlinks = ["2b2", "1c6c"]
+zettelid = "1c6b"
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+
+Modulo variable expansion (MVE) is needed because sometimes the
+lifetimes of variables exceed the II, meaning they get overwritten after
+II clock cycles even though they are needed afterwards. This is because
+the next iteration of the loop is already executing and overwriting the
+previous value of the register that was stored there. Hardware support
+for rotating registers ([\#2b2]) could prevent this from happening,
+otherwise, modulo variable expansion needs to be added to have the
+correct lifetimes for each variable. For example, if the II is 3 and the
+maximum lifetime of a variable is 12 cycles, then the loop needs to be
+unrolled 4 times, as after 12 cycles the initial register can finally be
+reused. Otherwise, a new register needs to be used for each value.
+
+  [\#2b2]: /zettel/2b2