ecmult_multi: reduce strauss memory usage by 30%

[jonasnick]

This is a draft because I'm not sure about the cleanest way to implement it.

[real-or-random]

This is a draft because I'm not sure about the cleanest way to implement it.

The current approach looks clean. What other approaches do you have in mind?

[hebasto]

Concept ACK.

[hebasto]

On my x86_64 system, this PR reduces the memory allocated on the scratch space from 2224 bytes to 1452 bytes per point.

Please ping me once it’s undrafted.

[jonasnick]

The current approach requires a temporary array int wnaf_tmp[256]; to provide to secp256k1_ecmult_wnaf which looks unclean. The alternatives are

1. copy almost all of secp256k1_ecmult_wnaf into secp256k1_ecmult_wnaf_small, or
2. remove secp256k1_ecmult_wnaf_small and write an secp256k1_ecmult macro.

Both options seem to be worse.

[hebasto]

The current approach requires a temporary array int wnaf_tmp[256]; to provide to secp256k1_ecmult_wnaf which looks unclean. The alternatives are

1. copy almost all of secp256k1_ecmult_wnaf into secp256k1_ecmult_wnaf_small, or

2. remove secp256k1_ecmult_wnaf_small and write an secp256k1_ecmult macro.

Both options seem to be worse.

I might suggest a third option: hebasto@5c0d6ee.