[mypyc] librt base64: use existing SIMD CPU dispatch by customizing build flags

[mr-c]

Fixes the current SSE4.2 requirement added in 1b6ebb1 / #20244

This PR fully enables the existing x86-64 CPU detection and dispatch code for SSSE3, SSE4.1, SSE4.2, AVX, and AVX2 in the base64 module.

To use the existing CPU dispatch from the upstream base64 code, one needs to compile the sources in each of the CPU specific codec directories with a specific compiler flag; alas this is difficult to do with setuptools, but I found a solution inspired by https://stackoverflow.com/a/68508804

Note that I did not enable the AVX512 path with this PR, as many intel CPUs that support AVX512 can come with a performance hit if AVX512 is sporadically used; the performance of the AVX512 (encoding) path need to be evaluated in the context of how mypyc uses base64 in various realistic scenarios. (There is no AVX512 accelerated decoding path in the upstream base64 codebase, it falls back to the avx2 decoder).

If there are additional performance concerns, then I suggest benchmarking with the openmp feature of base64 turned on, for multi-core processing.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[jhance]

I think there should also be a documented flag for setting the hardware floor (mostly useful for avx2 in order to avoid CPU dispatch if you know your hardware supports it)

[mr-c]

I think there should also be a documented flag for setting the hardware floor (mostly useful for avx2 in order to avoid CPU dispatch if you know your hardware supports it)

Thank you for the review @jhance ; is adding a flag for setting the hardware floor a blocker for merging?

According to upstream, and confirmed by my review of the code, the codec choice (as a result of the CPU detection) is only done once and is saved for the lifetime of the program.

mypy/mypyc/lib-rt/base64/lib.c

Lines 12 to 15 in 094f66d

	// These static function pointers are initialized once when the library is
	// first used, and remain in use for the remaining lifetime of the program.
	// The idea being that CPU features don't change at runtime.
	static struct codec codec = { NULL, NULL };

Prior to this PR, the CPU detection was already being run: on X86_64 systems BASE64_WITH_SSE42 was always defined, therefore HAVE_SSE42 was always defined prior to confirming the CPU support for SSE4.2

mypy/mypyc/lib-rt/base64/codec_choose.c

Lines 254 to 264 in 094f66d

	#if HAVE_SSE42
	// Check for SSE42 support:
	if (max_level >= 1) {
	__cpuid(1, eax, ebx, ecx, edx);
	if (ecx & bit_SSE42) {
	codec->enc = base64_stream_encode_sse42;
	codec->dec = base64_stream_decode_sse42;
	return true;
	}
	}
	#endif

In my opinion there is no performance advantage for bypassing the one-time CPU detection on X86_64 systems.

[mr-c]

Benchmarking results (n=100, AMD EPYC 9454 48-Core Processor)

master                         6.098s (0.0%)  | stdev 0.059s 
librt_base64_simd_cpu_dispatch 6.076s (-0.4%) | stdev 0.070s