Multi-stage Reverse Mode with Intermediate Shadow Reuse

[pnik073]

Hi Enzyme Team,

I am building a simulation engine that works in two distinct calculation stages:

Calculation flow

Stage 1 (Common calculation):
Generates a 3D "Risk Factor Cube" (Factors $\times$ Times $\times$ Paths).

This cube will be re-used from list of individual (entity) calculations afterwards

Stage 2 (Entity - unique):
Each individual calculation entity reads from the 3D Cube to generate a 2D "Projection Matrix" of results (Times $\times$ Paths). This matrix is then is aggregated into a single scalar result of we which I need to differentiate.

Hence, I need to evaluate the derivatives of the "list" of the dependent scalars with respect to the independent variables.

Contraint

Because I have thousands of individual entities to calculate, I cannot store simultaneously all the 2D "Projection Matrices" at once. So I need to stage the results of each individual "entity" one at the time, to keep RAM usage constant.

My Proposed Loop:

Step A (Entity AD: from Stage 2 -> 1):
Run __enzyme_autodiff on the Entity Simulation result. I capture the gradient of the scalar with respect to the 3D Cube (from Calculation Stage 1) in a temporary shadow buffer (d_Cube_Staging).

Step B (Bridge AD: from Stage 1 -> 0):
Run __enzyme_autodiff on the Common Simulation. I seed it with the d_Cube_Staging from Step A to propagate the sensitivity back to my initial model parameters.

Step C (Cleanup the staging 2D matrix):
memset the staging shadow buffer (of staged gradients) to zero and repeat for the next entity.

Questions:

1. Is it supported to "seed" the second Enzyme call using the shadow output of the first call?
2. Is manually zeroing and reusing the 3D shadow buffer (d_Cube_Staging) inside a loop a safe and recommended pattern?
3. Since the 3D Cube is the same for every entity, is there a way to avoid re-running the adjoint of the Common Simulation every time, or is this sequential "Bridge" approach the standard for keeping memory usage flat?

[pnik073]

Closing in favour of a more detailed report with confirmed measurements and related issue references: #2787.