Deprecate DAGNode.sort_key attribute #13744
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Prior to having the DAGCircuit in rust the sort_key attribute was added as a cache to speed up the lexicographical topological sort. Prior to having this attribute the topological sort method would compute the sort key as a string on the fly which ended up being a large bottleneck in transpilation (see: Qiskit#4040 for more details). However, since migrating the DAGCircuit implementation to Rust this sort_key attribute isn't needed anymore because we call the rustworkx-core function with a tuple of bit objects instead of a string. The sort_key atribute was left on in place for backwards compatibility (it shouldn't have been public, this was a mistake in Qiskit#4040) and when we create a python DAGNode object that will be returned to Python the creation of the sort key is unnecessary overhead (it takes roughly 1% of profile time to format the sort_key string during transpilation). Since nothing in DAGCircuit is actually using it this commit removes it to save the CPU cycles creating the string on each dag creation. This attribute is removed in Qiskit 2.0.0 in Qiskit#13736
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Changelog: Deprecation
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Summary
Prior to having the DAGCircuit in rust the sort_key attribute was added as a cache to speed up the lexicographical topological sort. Prior to having this attribute the topological sort method would compute the sort key as a string on the fly which ended up being a large bottleneck in transpilation (see: #4040 for more details). However, since migrating the DAGCircuit implementation to Rust this sort_key attribute isn't needed anymore because we call the rustworkx-core function with a tuple of bit objects instead of a string. The sort_key atribute was left on in place for backwards compatibility (it shouldn't have been public, this was a mistake in #4040) and when we create a python DAGNode object that will be returned to Python the creation of the sort key is unnecessary overhead (it takes roughly 1% of profile time to format the sort_key string during transpilation). Since nothing in DAGCircuit is actually using it this commit removes it to save the CPU cycles creating the string on each dag creation.
Details and comments
This attribute is removed in Qiskit 2.0.0 in #13736