commit 6e516e800c311ffe8f7db6dd1fd732601968d186
author Sami Kyostila <skyostil@chromium.org> Sun Oct 08 23:20:34 2023 +0000
committer Sami Kyostila <skyostil@chromium.org> Sun Oct 08 23:20:34 2023 +0000
tree b9f9f01c4633822d26aa6fb92c41d7e8188edcca
parent a0e3185a0d0c647442a9beb14f267074ac34563d

protozero: Pack length fields for short nested messages

Previously protozero uses 4 bytes for all nested message length fields
despite the actual length of the message. This can be wasteful,
especially for small nested messages.

This patch adds logic to pack the length field for short messages
(<= 127 bytes) into a single byte. This is done by shifting back the
already-written message data into the preceeding length field. The
assumption is that this data will still be in the L1 cache, so the
cost of shifting it will be negligible.

Latency benchmarking results (cloudtop):

Before: go/paste/5009322705682432
After: go/paste/5758827482841088

(tl;dr: any changes seem to be within noise)

For a size benchmark, I loaded https://theverge.com on a fresh
Chrome profile while tracing all categories (just Chrome, no traced)
for 10 seconds. The uncompressed trace sizes were:

Before: 64302974 bytes
After: 56297094 bytes (-12.5%)

Bug: b/135511481
Change-Id: Ia2d4c81160af2bcdf7d2df342124408719d8f9fe