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[SECP256K1] Optimization: track f,g limb count and pass to new variable-time update_fg_var
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Authored by Fabien on Tue, Apr 13, 18:33.

Details

Summary
The magnitude of the f and g variables generally goes down as the
algorithm progresses. Make use of this by keeping tracking how many
limbs are used, and when the number becomes small enough, make use of
this to reduce the complexity of arithmetic on them.

Partial backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/ebc1af700f9ec6e96586152b7090a2a6494308c3#diff-91cfb587705679268ee32d45895a62884faf262add85ba385cf55f74fcd51471R32-R575

Depends on D9410.

Test Plan
ninja check-secp256k1

Diff Detail

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