Qiskit / qiskit / 10065639500

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github

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Commit Message

Rebalance `CircuitInstruction` and `PackedInstruction` (#12730)

* Rebalance `CircuitInstruction` and `PackedInstruction`

This is a large overhaul of how circuit instructions are both stored in
Rust (`PackedInstruction`) and how they are presented to Python
(`CircuitInstruction`).  In summary:

* The old `OperationType` enum is now collapsed into a manually managed
  `PackedOperation`.  This is logically equivalent, but stores a
  `PyGate`/`PyInstruction`/`PyOperation` indirectly through a boxed
  pointer, and stores a `StandardGate` inline.  As we expect the vast
  majority of gates to be standard, this hugely reduces the memory
  usage.  The enumeration is manually compressed to a single pointer,
  hiding the discriminant in the low, alignment-required bytes of the
  pointer.

* `PackedOperation::view()` unpacks the operation into a proper
  reference-like enumeration `OperationRef<'a>`, which implements
  `Operation` (though there is also a `try_standard_gate` method to get
  the gate without unpacking the whole enumeration).

* Both `PackedInstruction` and `CircuitInstruction` use this
  `PackedOperation` as the operation storage.

* `PackedInstruction` is now completely the Rust-space format for data,
  and `CircuitInstruction` is purely for communication with Python.

On my machine, this commit brings the utility-scale benchmarks to within
10% of the runtime of 1.1.0 (and some to parity), despite all the
additional overhead.

Changes to accepting and building Python objects
------------------------------------------------

* A `PackedInstruction` is created by copy constructor from a
  `CircuitInstruction` by `CircuitData::pack`.  There is no `pack_owned`
  (really, there never was - the previous method didn't take ownership)
  because there's never owned `CircuitInstruction`s coming in; they're
  Python-space interop, so we never own them (unless we clone them)
  other than when we're unpacking them.

* `PackedInstruction` is currently just cr... (continued)

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