https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109400
Bug ID: 109400
Summary: Missing 'advance' optimizations for
std::istreambuf_iterator
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: enhancement
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
We have a custom overload of std::advance for std::istreambuf_iterator which
operates directly on the streambuf's get area, instead of incrementing
character by character.
But it doesn't get used when calling std::next, because that calls std::advance
without ADL, and the custom overload hasn't been declared yet. We either need
to declare istreambuf_iterator and its std::advance, or add a custom overload
of std::next.
Additionally, std::ranges::advance is not specialized for
std::istreambuf_iterator so just increments character by character. The
ranges::advance(i, n) form should use std::advance. The ranges::advance(i,
sentinel) form will already be optimal when the sentinel is the same type as
the iterator, but will be slow when using default_sentinel_t. The
ranges::advance(i, n, sentinel) form could be done efficiently, and has the
advantage that it's not undefined if EOF is reached before advancing n times.
