Reduce the size of precomputed signing table for lowmemory

[RCasatta]

Reduce the size of precomputed signing table (ECMULT_GEN_PREC_BITS) for lowmemory feature

in rust-bitcoin embedded test secp preallocate_size become 33472 from 66240

[elichai]

Can I ask what's the motivation behind this? a specific setup where this was too much memory?
I'm for reducing ECMULT_GEN_PREC_BITS because it saves 2048 bytes, which can be important for lowmemory
But reducing ECMULT_WINDOW_SIZE from 4 to 2 only save 24 bytes in the context and costs ~16% decrease in performance on my laptop

(i9-9980HK, turbo off, window size 4, gen prec bits 2)

ecdsa_verify: min 80.9us / avg 81.2us / max 81.4us
ecdsa_verify: min 81.1us / avg 81.9us / max 83.4us
ecdsa_verify: min 81.3us / avg 82.0us / max 83.6us

(window size 2, gen prec bits 2)

ecdsa_verify: min 95.7us / avg 96.0us / max 97.0us
ecdsa_verify: min 95.6us / avg 97.1us / max 100us
ecdsa_verify: min 95.6us / avg 96.1us / max 98.1us

[RCasatta]

But reducing ECMULT_WINDOW_SIZE from 4 to 2 only save 24 bytes in the context and costs ~16% decrease in performance on my laptop

sorry, I did a test changing ECMULT_WINDOW_SIZE but it was not intended to be changed in the commit (as you can see from the commit message there is no reference to it)

I'm for reducing ECMULT_GEN_PREC_BITS because it saves 2048 bytes,

Why do you say 2048 bytes, is this line wrong? https://github.com/rust-bitcoin/rust-bitcoin/blob/df4d70a37e96e6f82c5231063635e877cb6b4f77/embedded/src/main.rs#L37

Can I ask what's the motivation behind this? a specific setup where this was too much memory?

In general lower required memory enlarges the possible usable MCUs, and personally, I would like to run it on the stm32f4xx Blackpill (64k ram)

[elichai]

Why do you say 2048 bytes, is this line wrong? https://github.com/rust-bitcoin/rust-bitcoin/blob/master/embedded/src/main.rs#L37

I completely forgot that our precompute size is in blocks of 16 bytes and not in bytes(funny considering I changed that lol)
so you're right, changing the window size from 4 to 2 decreases by 384 bytes, and the precision bits from 4 to 2 decreases by 32,768 bytes.

[elichai]

tACK

[apoelstra]

ACK 505b04d

Not sure why CI is not running. Passes locally for me.

[elichai]

So apparently when you try to use this library inside Fortanix SGX enclave you get a SIGBUG, but with this PR + lowmemory you don't.