[C++] Constexpr implementation for dynamic dispatch

[AntoinePrv]

Describe the enhancement requested

Right now the DynamicDispatch makes no assumption on the DynamicFunction::implementations().
However in practice we can assume the implementations to be a compile-time sequence: each available must have been compiled so there is no possibility to add more at runtime (doing so would require a dlopen style plugin loading, which I don't believe is what we want to go towards).

With C++17 and CTAD, it is straightforward to use a static array without needing with compile-time array sizes.
For e.g. it is already done with:

arrow/cpp/src/arrow/util/bpacking.cc

Lines 35 to 47 in e8b7b4e

	return std::array{
	#if defined(ARROW_HAVE_SSE4_2)
	Implementation{DispatchLevel::NONE, &bpacking::unpack_sse4_2<Uint>},
	#else
	Implementation{DispatchLevel::NONE, &bpacking::unpack_scalar<Uint>},
	#endif
	#if defined(ARROW_HAVE_RUNTIME_AVX2)
	Implementation{DispatchLevel::AVX2, &bpacking::unpack_avx2<Uint>},
	#endif
	#if defined(ARROW_HAVE_RUNTIME_AVX512)
	Implementation{DispatchLevel::AVX512, &bpacking::unpack_avx512<Uint>},
	#endif
	};

There are actually few places to change such as:

arrow/cpp/src/parquet/level_comparison.cc

Lines 57 to 63 in e8b7b4e

	static std::vector<std::pair<DispatchLevel, FunctionType>> implementations() {
	return {{DispatchLevel::NONE, standard::FindMinMaxImpl}
	#if defined(ARROW_HAVE_RUNTIME_AVX2)
	,
	{DispatchLevel::AVX2, FindMinMaxAvx2}
	#endif
	};

This will be a small immediate yet negligible improvement (the std::vector is actually just parsed once).
The big win, is to use that information to skip dynamic dispatch altogether when there is only one implementation available, as done (verbosly) here:

arrow/cpp/src/arrow/util/bpacking.cc

Lines 55 to 60 in e8b7b4e

	#if defined(ARROW_HAVE_NEON)
	return bpacking::unpack_neon(in, out, opts);
	#else
	static DynamicDispatch<UnpackDynamicFunction<Uint> > dispatch;
	return dispatch.func(in, out, opts);
	#endif

But not done below:

arrow/cpp/src/parquet/level_comparison.cc

Lines 74 to 77 in e8b7b4e

	MinMax FindMinMax(const int16_t* levels, int64_t num_levels) {
	static DynamicDispatch<MinMaxDynamicFunction> dispatch;
	return dispatch.func(levels, num_levels);
	}

Component(s)

C++

[pitrou]

Issue resolved by pull request 49840
#49840