Commits on Nov 14, 2024

1. Add new multithreaded TwoQubitPeepholeOptimization pass …

   This commit adds a new transpiler pass for physical optimization,
   TwoQubitPeepholeOptimization. This replaces the use of Collect2qBlocks,
   ConsolidateBlocks, and UnitarySynthesis in the optimization stage for
   a default pass manager setup. The pass logically works the same way
   where it analyzes the dag to get a list of 2q runs, calculates the matrix
   of each run, and then synthesizes the matrix and substitutes it inplace.
   The distinction this pass makes though is it does this all in a single
   pass and also parallelizes the matrix calculation and synthesis steps
   because there is no data dependency there.
   
   This new pass is not meant to fully replace the Collect2qBlocks,
   ConsolidateBlocks, or UnitarySynthesis passes as those also run in
   contexts where we don't have a physical circuit. This is meant instead
   to replace their usage in the optimization stage only. Accordingly this
   new pass also changes the logic on how we select the synthesis to use
   and when to make a substituion. Previously this logic was primarily done
   via the ConsolidateBlocks pass by only consolidating to a UnitaryGate if
   the number of basis gates needed based on the weyl chamber coordinates
   was less than the number of 2q gates in the block (see Qiskit#11659 for
   discussion on this). Since this new pass skips the explicit
   consolidation stage we go ahead and try all the available synthesizers
   
   Right now this commit has a number of limitations, the largest are:
   
   - Only supports the target
   - It doesn't support any synthesizers besides the TwoQubitBasisDecomposer,
     because it's the only one in rust currently.
   
   For plugin handling I left the logic as running the three pass series,
   but I'm not sure this is the behavior we want. We could say keep the
   synthesis plugins for `UnitarySynthesis` only and then rely on our
   built-in methods for physical optimiztion only. But this also seems less
   than ideal because the plugin mechanism is how we support synthesizing
   to custom basis gates, and also more advanced approximate synthesis
   methods. Both of those are things we need to do as part of the synthesis
   here.
   
   Additionally, this is currently missing tests and documentation and while
   running it manually "works" as in it returns a circuit that looks valid,
   I've not done any validation yet. This also likely will need several
   rounds of performance optimization and tuning. t this point this is
   just a rough proof of concept and will need a lof refinement along with
   larger changes to Qiskit's rust code before this is ready to merge.
   
   Fixes Qiskit#12007
   Fixes Qiskit#11659

   

   mtreinish committed Nov 14, 2024
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