First, what a great resource you've put together! You're presenting a lot of useful material clearly and concisely, together with the reasoning behind it. I wish I had this when I started doing performance work.
> For a perfect drop-in replacement of std::lower_bound, the best you can do without breaking anything is to make the search branchless and maybe add some prefetching [1]. Some compilers actually try to implement std::lower_bound this way, but these things can sometimes break from version to version because there are no semantics to reliably make compiler use predication instead of branching.
What's imperfect about a radix-4 (or higher) search instead of a binary search for replacing std::lower_bound? For a radix-k search, you'll have k-1 fetches in flight before you decide which subarray to recurse on, and your dep chain will be log_2(k) times shorter.
> For a perfect drop-in replacement of std::lower_bound, the best you can do without breaking anything is to make the search branchless and maybe add some prefetching [1]. Some compilers actually try to implement std::lower_bound this way, but these things can sometimes break from version to version because there are no semantics to reliably make compiler use predication instead of branching.
What's imperfect about a radix-4 (or higher) search instead of a binary search for replacing std::lower_bound? For a radix-k search, you'll have k-1 fetches in flight before you decide which subarray to recurse on, and your dep chain will be log_2(k) times shorter.