Use singleton SwapGate in Sabre reconstruction

[jakelishman]

Summary

Since SwapGate is now a singleton instance in most cases, we can directly reuse the same instance during Sabre reconstruction rather than wasting cycles re-retrieving the singleton instance.

Details and comments

See #10314 for the root of this.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[alexanderivrii]

With #10314 merged, both swap_singleton and SwapGate() return the same shared singleton instance, right? How much of a performance improvement does this give?

[mtreinish]

In my local testing the overhead of calling SwapGate() is ~5us per call:

In [4]: %timeit qiskit.circuit.library.SwapGate()
5.06 µs ± 72.1 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

with #10314. This is actually a significant regression because before #10314 it took ~666ns:

In [3]: %timeit qiskit.circuit.library.SwapGate()
666 ns ± 2.18 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)

so doing this can save a noticeable amount of time depending on how many swaps we need to create.

[mtreinish]

This LGTM, it's a straightforward optimization. We should look at the runtime overhead of SingletonGate.__new__ in parallel to try and minimize the overhead there.

[jakelishman]

Fwiw, there's a bunch of "hidden" overhead of SingletonGate.__new__ in that:

• making the __new__ an extra Python-space call over object.__new__ adds cost of itself
• even if we return the singleton instance from __new__, the resulting __init__ still gets called if the type of return value is in the inheritance chain of the original object.

One potential way around this is to have the singleton class be a metaclass that overrides type.__call__ with a new method that returns the singleton instance without calling the __new__ machinery at all. That will avoid __init__ from being called at all, so we'll gain really a bunch of the time there. The downside is that there's a metaclass conflict: Operation's metaclass is ABCMeta, so we can't (naively) make SingletonGate a metaclass. However, we could make SingletonGateMeta derive from ABCMeta, which would resolve the conflict, even if we'd need to take care to maintain that relationship were Operation ever to change.

[jakelishman]

Now rebased over #10782.

[coveralls]

Pull Request Test Coverage Report for Build 6250595529

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 5 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.03%) to 87.279%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	5	91.92%

Totals
Change from base Build 6250386385:	0.03%
Covered Lines:	74337
Relevant Lines:	85172

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💛 - Coveralls