Which groups are written is decided first (KeptIndices, in group order), then
each written group's MergedReflection is formed on its own, on all threads,
into a result sized once - where the serial loop pushed back up to millions of
records, growing a vector that each merge allocated afresh. The same records in
the same order.
p.mtz md5 unchanged on myob, cytc, thau, kdp, 8a1a and 8qaw (GPU build).
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SVmAWnzCmRKAXVUCdc4iNi
leonarski_fandClaude Opus 5.5
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