This series adds a build-time knob to allow selecting an alternative
SHA-1 implementation for non-cryptographic hashing within Git, starting
with the `hashwrite()` family of functions.
This series is the result of starting to roll out verbatim multi-pack
reuse within GitHub's infrastructure. I noticed that on larger
repositories, it is harder thus far to measure a CPU speed-up on clones
where multi-pack reuse is enabled.
After some profiling, I noticed that we spend a significant amount of
time in hashwrite(), which is not all that surprising. But much of that
time is wasted in GitHub's infrastructure, since we are using the same
collision-detecting SHA-1 implementation to produce a trailing checksum
for the pack which does not need to be cryptographically secure.
This series teaches a new set of build-time knobs: OPENSSL_SHA1_FAST,
BLK_SHA1_FAST, and APPLE_COMMON_CRYPTO_SHA1_FAST, which can be used to
select an alterantive SHA-1 implementation for non-cryptographic uses
within Git.
The series is laid out as follows:
- The first two patches are preparatory, allowing us to include
multiple SHA-1 wrapper headers and adding scaffolding functions for
the _fast() variants, respectively.
- The third patch introduces the build-time knobs for selecting which
SHA-1 implementation is used for non-cryptographic purposes.
- The fourth and final patch updates the hashwrite() implementation to
use the _fast() variants.
