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?

[real-or-random]

Suggested change

	int8_t wnaf_na_1[129];
	int_least8_t wnaf_na_1[129];

This is technically better to retain support for platforms that don't offer int8_t. I don't think there are any relevant platforms of that kind, but why not... 🤷

edit: Now that I think about it again, a platform that does not offer int8_t will most likely one for which CHAR_BITS != 8. And then it's neither relevant nor it's likely to offer int32_t or int64_t which we need anyway. So I guess there's no difference between int8_t and int_least8_t.

[hebasto]

This is technically better to retain support for platforms that don't offer int8_t.

int8_t is already used in src/assumptions.h.

[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.