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Keep the augmented call when it returns an empty tape directly - #3376
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When a call is differentiated with an augmented forward pass that returns
only its tape (tapeIdx == -1), the tape value is the augmented call itself.
If the tape type is empty ({}), the isEmptyTy() branch erased the tape to
replace it with undef, which deleted the augmented call and the primal side
effects of the callee.
Only erase the tape when it is an extractvalue of the augmented call.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G9QZewtK4oCErzZphHWd8u
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Requested by Valentin · project thread
Before: when a callee is differentiated in split mode and its augmented forward pass returns only an empty tape (
{}), the augmented call is deleted from the caller's augmented forward pass. The callee's primal side effects (for example a store to an output argument) are silently lost.After: the augmented call is kept, and the empty tape is still replaced by
undef.How: in
AdjointGenerator.h, withtapeIdx == -1the tape value is the augmented call itself, so theisEmptyTy()branch now erases the tape only when it is anextractvalueof the call.Test:
ReverseMode/custom-split-empty-tape.ll.@squarehas a customenzyme_augmentreturning{}that stores to%out;@topreads%outafter calling@caller, so@calleris differentiated in split mode. On main,@augmented_callerhas no call to@augment_square; with the fix it does. The fulltest/Enzymelit suite against LLVM 18 has the same set of failures with and without the change (the pre-existing typed-pointer failures in this setup).Found while writing the test for #3373.
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https://claude.ai/code/session_01G9QZewtK4oCErzZphHWd8u
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