Fix wnaf_table overallocation#2437
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For window size `w`, wNAF digits are odd with magnitude at most `2^(w-1) - 1`. The table is indexed by `|digit| / 2`, so the maximum index is `(2^(w-1) - 1) / 2 = 2^(w-2) - 1`, requiring `2^(w-2)` entries. The previous `2^(w-1)` allocation computed twice as many odd multiples as needed, wasting point additions during table setup. Also corrects the module-level doc comment to reflect the `2^(w-2)` table size.
tarcieri
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Jun 6, 2026
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For window size
w, wNAF digits have max magnitude2^(w-1) - 1. The table is indexed by|digit| / 2, so the maximum index is(2^(w-1) - 1) / 2 = 2^(w-2) - 1, requiring2^(w-2)entries.The previous
2^(w-1)allocation computed twice as many odd multiples as needed, wasting point additions during table setup.Ran the
k256Schnorr signing and verifying benches and gave around 20% improvements for hot paths, however@tarcieri please note my benchmarks were ran against this branch that does GLV decomposition + shared doublings, where the larger table + extra allocations actually hurt.
While the fix is provably correct, the current vendored wNAF path only allocates a single table, so the effects a not really measurable.
2^(w-2)(fix)2^(w-1)(no fix)lincomb)