commit 9d637ca923bb45691809405615095ed8dc2a2431
author Joe Tsai <joetsai@digital-static.net> Wed Sep 18 18:08:52 2019 -0700
committer Joe Tsai <joetsai@digital-static.net> Thu Sep 19 17:45:34 2019 +0000
tree 91becff795df9cd4e5deb97e28efcd04865eeca1
parent 7fa1ee5937c4ba56c2d4e6ff0436da6f4bb9b631

internal/impl: optimize reflect methods

This change performs two optimizations:
* It uses a pre-constructed rangeInfos slice to iterate over
all the fields. This is more performant since iterating over a slice
is faster than iterating over a map. Furthermore, this slice
does not contain fields that are part of a oneof. If a oneof has
N fields, the time to check presence on the oneof is now O(1)
instead of O(N).
* It uses a dense field info slice that is optmized for the common
case where the field number is relatively low and close in value
to the index itself.

We also fix a minor bug in the construction of oneofInfo where
it wasn't treating a typed nil pointer to a wrapper struct as if
it were unset. This ensures WhichOneof and Has always agree.

name             old time/op    new time/op    delta
Reflect/Has-4      7.81µs ± 3%    6.74µs ± 3%  -13.61%  (p=0.000 n=9+9)
Reflect/Get-4      12.7µs ± 1%    11.3µs ± 4%  -10.85%  (p=0.000 n=8+10)
Reflect/Set-4      19.5µs ± 5%    17.8µs ± 2%   -8.99%  (p=0.000 n=10+10)
Reflect/Clear-4    12.0µs ± 4%    10.2µs ± 3%  -14.86%  (p=0.000 n=9+10)
Reflect/Range-4    6.58µs ± 1%    4.17µs ± 2%  -36.65%  (p=0.000 n=8+9)

Change-Id: I2c48b4d3fb6103ab238924950529ded0d37f8c8a
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/196358
Reviewed-by: Damien Neil <dneil@google.com>