Copy-in rather than copy-out in transpiler#11176
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This shifts the deepcopy of instructions in the transpiler to be at the input stage, rather than the output stage. This more closely matches our behaviour before the passmanager refactoring, but also has a performance benefit for circuits that require significant routing and is typically safer for transpiler passes. Output circuits are typically larger than input ones, so copy-in means less copying, and also makes the ownership model for tranpsiler passes clearer: a pass can assume the input operations are entirely owned by the circuit it receives, and that a pass must output a circuit that entirely owns its operations.
jakelishman
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ElePT
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Nov 2, 2023
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Given that this fixes an unintended behavioral change, I think that it makes a lot of sense to include it into 0.45.0. Thanks Jake!
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This shifts the deepcopy of instructions in the transpiler to be at the input stage, rather than the output stage. This more closely matches our behaviour before the passmanager refactoring, but also has a performance benefit for circuits that require significant routing and is typically safer for transpiler passes. Output circuits are typically larger than input ones, so copy-in means less copying, and also makes the ownership model for tranpsiler passes clearer: a pass can assume the input operations are entirely owned by the circuit it receives, and that a pass must output a circuit that entirely owns its operations. (cherry picked from commit eca3478)
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This shifts the deepcopy of instructions in the transpiler to be at the input stage, rather than the output stage. This more closely matches our behaviour before the passmanager refactoring, but also has a performance benefit for circuits that require significant routing and is typically safer for transpiler passes. Output circuits are typically larger than input ones, so copy-in means less copying, and also makes the ownership model for tranpsiler passes clearer: a pass can assume the input operations are entirely owned by the circuit it receives, and that a pass must output a circuit that entirely owns its operations. (cherry picked from commit eca3478) Co-authored-by: Jake Lishman <jake.lishman@ibm.com>
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Summary
This shifts the deepcopy of instructions in the transpiler to be at the input stage, rather than the output stage. This more closely matches our behaviour before the passmanager refactoring, but also has a performance benefit for circuits that require significant routing and is typically safer for transpiler passes. Output circuits are typically larger than input ones, so copy-in means less copying, and also makes the ownership model for tranpsiler passes clearer: a pass can assume the input operations are entirely owned by the circuit it receives, and that a pass must output a circuit that entirely owns its operations.
Details and comments
The behaviour before #10127 was copy-in, as this PR reinstates, and I think it just got mistakenly swapped over in that - I didn't notice during the review either.
Ideally this goes into 0.45.0, so the behavioural change from #10127 doesn't get released, but it's not a big deal if it goes into 0.45.1 instead, I think.