Draft: Improve performance of QuantumCircuit.measure_all and QuantumCircuit.barrier by using _append fast path

[eendebakpt]

Summary

Improve performance of QuantumCircuit.measure_all and QuantumCircuit.barrier by using _append fast path

Details and comments

The register argument are obtained from the QuantumCircuit directly, so do not need to be validated.

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[jakelishman]

When dropping to the lower-level _append primitive, there a quite a lot of additional considerations that must be validated by the caller:

• any control-flow builder scopes must be resolved
• the qubit counts and clbit counts should be known to match the instruction (Measure is a 1q/1c operation, for example)
• _append should take a single CircuitInstruction (which comes with its own validation requirements), not three arguments

It's worth looking at end of append's implementation to be sure that all the invariants it enforces are being respected here as well.

Out of curiosity, in what situations is this function even visible in performance benchmarking?

[eendebakpt]

When dropping to the lower-level _append primitive, there a quite a lot of additional considerations that must be validated by the caller:

• any control-flow builder scopes must be resolved
• the qubit counts and clbit counts should be known to match the instruction (Measure is a 1q/1c operation, for example)
• _append should take a single CircuitInstruction (which comes with its own validation requirements), not three arguments

It's worth looking at end of append's implementation to be sure that all the invariants it enforces are being respected here as well.

Out of curiosity, in what situations is this function even visible in performance benchmarking?

Thanks for the info. The barrier and (to lesser extend measure) show up in randomized benchmarking. I did some more tests, but the performance gain is not so high as the profiling suggests. I will close the PR.