Description
Office is trying to adopt gsl::span more broadly, but we are seeing some unexpected performance regressions when compiling with clang:
A - range-for loops
B - std algorithms
On a mail thread @StephanTLavavej suggested:
I'd recommend reaching out to the libc++ devs on either the LLVM Discord server or their Discourse forum, about whether they have an "unwrapping" system similar to what MSVC's STL has. (Presumably you'd want to do the same for libstdc++.)
We use libc++'s portable subset of tests, and we're aligned on a few implementation things (backporting import std; to C++20, names of compiler builtins, etc.), but our internal mechanisms are wildly different - there is no commonality between our unwrapping mechanisms.
In C++20 we can detect arbitrary contiguous iterators, which lets us extract raw pointers, but in C++14/17 that's not physically possible so iterators really need to opt in to unwrapping.
Could one of the maintainers please follow up?