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flashsoftmax
confirmedC++ · CPUSystems & data structuresJuly 6, 2026
the question
What does the online (flash-attention) softmax reduction actually cost?
what came out
It's numerically identical to two-pass at every context length -- the streaming rescale accumulates nothing -- but ~2.8x slower standalone. Its win is fusion (avoiding a second pass over memory), not raw speed. Pre-registered, long-double oracle.
method & receipts
- Result: confirmed
- Pre-registered — the prediction was written down and committed before the run.
- Reproducible — one script re-runs the whole thing from scratch.
- Independently reviewed — an adversarial review pass, kept in the repo.
- Machine-readable claims — every reported number, in a checkable file.
- Tested — a correctness/benchmark suite ships alongside the code.
→ read the code and re-run it
github.com/v-code01/flashsoftmax