tree 0c5082406afaac1c068a204232bf8ae7f0140287
parent 0dc2b5517c57b0e75b91c045289c79f20703ea71
author Pavel Labath <pavel@labath.sk> 1783422935 +0200
committer Copybara-Service <copybara-worker@google.com> 1783423255 -0700

[libc] Add inet_ntop (#204143)

The function converts from IPv4/6 addresses to their string forms. The
complication comes from the v6 part due to address compression and
v4-mapping.

The traditional implementation of this is to sprintf the address into a
temporary buffer. We cannot do that here due to the ban on calling other
entry points. Using the lower-level constructs is possible, but a
straight forward application of IntegerToString does not result in
particularly impressive performance: 50%-100% slower than what's in this
patch (and slower than glibc). It also doesn't make the code much
smaller as the majority of it is dedicated to v6 compression.

The IPv6 implementation uses a temporary buffer, but it also has a
fast-path which skips the buffer if its size is guaranteed to be
sufficient. The IPv4 implementation also has a fast path, but the
fallback here is to compute the precise length of the string instead
(the buffer option was slower and IPv4 makes it easier to compute the
precise length).

The v6 compression code is unrolled to make it easier to optimize. While
it is possible to write a single printing loop that handles all cases,
its branchiness does not make it amenable for optimization.

The resulting function is faster than its glibc counterpart, though the
exact ratio varies depending on the input:
- random IPv4: 15% faster
- random IPv6: 6% faster
- "::": 50% faster

GitOrigin-RevId: 4db63ee847f2a9154a9869da51a8c575eb123923