[herd] Check the liveness of some read-from relations #997
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This commit also performs a change of vocabulary: "too far" is changed into the more appropriate "cutoff", as such events signals the position where program order is pruned.
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The warning is now emitted as soon as there is an execution candidate with a cuttof node, would it be validated of not by the model.
The condition will reject executions for which there exists some `fr` source in a pruned `po` that is not followed by a `rf` target: That is, given +-fr->W | | R ----po---> CutOff there does not exists a rf s.t. +-fr->W-rf-+ | | | \ / R ----po---R-----> CutOff Notice that this "liveness" condition applies to writes that are followed by a DSB effect (_i.e._ stores followed by a DSB fence instruction).
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Intuitively, this "liveness" property is that a read cannot indefinitely read from a non-final write. Our formulation involves 'pruned' executions and requires that, for all locations x the last read of x in a pruned po is from the final write to x.
Our implementation builds on the "CutOff" (ex "TooFar") feature. Namely, for the sake of avoiding infinite or excessive running times, some executions are pruned, resulting in execution prefixes. Those are normally discarded, unless
-variant CutOffis specified. In those executions, prunedporelation end with a specificCutOffeffect.The liveness condition applies to those execution prefixes. The condition will reject executions for which there exists
some
frsource in a pruned po that is not followed by arftarget:That is, given
there does not exists a rf s.t.
Notice that this "liveness" condition applies to writes that are followed by a DSB effect (i.e. stores followed by a DSB fence instruction).