Preserve constant loop bounds through adjoint-lowering#2959
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Relocate the forward pass into the same location as the reverse pass, as having the two pieces of the pass in different locations is very confusing. A subdirectory is introduced for the pass for better organization.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #2959 +/- ##
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+ Coverage 96.95% 96.97% +0.02%
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Files 166 166
Lines 19143 19209 +66
Branches 1772 1788 +16
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+ Hits 18560 18628 +68
+ Misses 430 429 -1
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joeycarter
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Jun 19, 2026
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Looks good, just a few editorial things and questions—nothing blocking. 👍
Co-authored-by: Joey Carter <joey.snarrcarter@gmail.com>
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As laid out in #2936, the
--adjoint-loweringpass converts all classical values used by the reverse pass to dynamic ones via a tape caching mechanism (LIFOlist.push/pop). This hinders analyzability for example inqp.specs. This PR skips the tape mechanism for constant loop bounds.As a pre-requisite to implementing the change, the pass is restructured into a more coherent structure (all in the same directory) of pass driver (
AdjointLowering.cpp), forward pass / classical value caching (Forward.cpp), reverse pass / inverse quantum instruction stream & adjoint propagation (Reverse.cpp), and a helper data structure (QuantumCache.cpp/hpp). The actual functional change is very small (2f6ad9e).closes #2936
[sc-122139]
Future improvements: One could apply this fix to any constant classical value cached by the tape.
Additional note: To "fix" the example in the issue one should run the canonicalization pass before the adjoint lowering (standard in our pipeline) so that the detection of constant values is facilitated.