Address and data dependencies

Contents

• ARM
• Power

This note illustrates that address and data dependencies may behave differently. It is reminded that an address dependency occurs when the effective address of a store depends upon the value read by a previous load instruction; while a data dependency occurs when the the value stored by a store instruction depends upon the value. The difference is observable on ARM hardware only, as the tests derive from the LB and S+ffence+po idioms, which are not observed on Power.

ARM

The above table lists some tests that differ only in some dependencies from one load to one store: the dependency being either an address dependency or a data dependency. Names should help to identify the matching tests. For instance LB+addrs+WW matches LB+datas+WW.

Then, one sees that tests with a data dependency are allowed by the model and observed on hardware; while tests with an address dependency are forbidden by the model and not observed on hardware.

Power

This document was translated from L^AT_EX by H^EV^EA.