Use std::less<> for Labels to enable heterogeneous lookup on C++14+

[aeroyorch]

No description provided.

[gjasny]

Hello,

do you have a benchmark that shows improvements with this change? I'm a little bit hesitant to make the public ABI dependent on __cpp_lib_generic_associative_lookup. If the change is worth it, I'd raise the required C++ standard to 14 (or newer).

But most of the comparisons happen with Labels and thus with std::string.

Thanks,
Gregor

[aeroyorch]

Hello,

do you have a benchmark that shows improvements with this change? I'm a little bit hesitant to make the public ABI dependent on __cpp_lib_generic_associative_lookup. If the change is worth it, I'd raise the required C++ standard to 14 (or newer).

But most of the comparisons happen with Labels and thus with std::string.

Thanks,
Gregor

Ok, I'll try to put together a benchmark next week. I don't expect a big improvement inside the library itself, but it could help users who use the Labels alias directly, especially when doing lookups with std::string_view or string literals.

[aeroyorch]

Added a benchmark (core/benchmarks/labels_bench.cc), plain std::map vs std::less<>, looked up with a std::string_view. g++ 13, release, medians:

entries / key len	plain (temp string)	std::less<>
4 / 8	24.5 ns	17.6 ns
16 / 8	30.3 ns	27.0 ns
4 / 32	38.5 ns	17.1 ns
16 / 32	44.1 ns	24.0 ns

So roughly 1.1 to 1.4x faster for short keys, and about 2x for keys past the SSO size, where the temporary std::string also heap-allocates (1 alloc/lookup vs 0). With a std::string key the two are identical within noise, so there is no regression on the path the library itself uses.

No win inside the library then, but a real one for users looking up Labels with a string_view or literal.