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_ProblemBlockBase.evaluate_hessian_component accumulated the pure d2F/dm2 term and each mixed d2F/(dm dc2) term into the same output vector with repeated assemble_compiled_form calls. Each call ends in scatter_reverse(add) + scatter_forward, which leaves the ghost entries holding copies of their owners' values, so the next call's reverse scatter added those copies back into the owners again: every shared dof of the first term was counted twice. Serial runs have no ghosts and were unaffected; in parallel any solve with a non-zero mixed term (e.g. a control multiplying the previous state in a time-stepping right-hand side) got a wrong Hessian action, visible as a Hessian Taylor rate of 2 instead of 3. Add assemble_compiled_forms, which sums every form's local contribution and exchanges ghosts once, and use it there. The new regression test fails under CI's `mpirun -n 2` run without the fix. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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_ProblemBlockBase.evaluate_hessian_component accumulated the pure d2F/dm2 term and each mixed d2F/(dm dc2) term into the same output vector with repeated assemble_compiled_form calls. Each call ends in scatter_reverse(add) + scatter_forward, which leaves the ghost entries holding copies of their owners' values, so the next call's reverse scatter added those copies back into the owners again: every shared dof of the first term was counted twice.
Serial runs have no ghosts and were unaffected; in parallel any solve with a non-zero mixed term (e.g. a control multiplying the previous state in a time-stepping right-hand side) got a wrong Hessian action, visible as a Hessian Taylor rate of 2 instead of 3.
Add assemble_compiled_forms, which sums every form's local contribution and exchanges ghosts once, and use it there. The new regression test fails under CI's
mpirun -n 2run without the fix.