Reduce BearSSL code size by not -O2 FSMs by earlephilhower · Pull Request #7807 · esp8266/Arduino (original) (raw)

I wasn't able to measure a consistent performance difference (there is always some wiggle, this change is below that threshold) using the validation example's time-tests.

Encryption/BW is unimpacted as well, as far as I can tell (again, always some wiggle). Only the FSM (think of it as the control plane) had -Os used. Everything else is still at -O2 (AES, Elliptic Curve, SHA, etc.)

The FSM basically was inlining a copy-small-no-of-bytes-from-a-to-b multiple times. Saves a few register stores and call/ret, but it's only done 1-2 times per an entire connection.

The FSM itself doesn't have much option for logic optimization, it's a machine-generated, Forth-like interpreter with a byte-byte processor so each case is pretty minimal, and the case transitions are driven off of a big table, so GCC really can't look into things.

So, AFAICT, it's ~0 perf impact and 1-2K (depends on your calls) size savings.