Cpu feature detection issues M1 mac

[gbaraldi]

Julia currently doesn't detect any cpu features when calling Base.BinaryPlatforms.CPUID.cpu_isa on an M1 mac. Julia isn't the only one if issues with runtime feature detection in the ARM-Apple space.
Go had this issue and has an implementation golang/go#42747.
I haven't checked, but reading the code we try to check the features via getauxval(), which isn't available on the darwin libc, and the fallback is a hardcoded name, which isn't available.

julia/src/processor_arm.cpp

Lines 1184 to 1308 in a08a3ff

	static NOINLINE std::pair<uint32_t,FeatureList<feature_sz>> _get_host_cpu()
	{
	FeatureList<feature_sz> features = {};
	// Here we assume that only the lower 32bit are used on aarch64
	// Change the cast here when that's not the case anymore (and when there's features in the
	// high bits that we want to detect).
	features[0] = (uint32_t)jl_getauxval(AT_HWCAP);
	features[1] = (uint32_t)jl_getauxval(AT_HWCAP2);
	#ifdef _CPU_AARCH64_
	if (test_nbit(features, 31)) // HWCAP_PACG
	set_bit(features, Feature::pauth, true);
	#endif
	auto cpuinfo = get_cpuinfo();
	auto arch = get_elf_arch();
	#ifdef _CPU_ARM_
	if (arch.version >= 7) {
	if (arch.klass == 'M') {
	set_bit(features, Feature::mclass, true);
	}
	else if (arch.klass == 'R') {
	set_bit(features, Feature::rclass, true);
	}
	else if (arch.klass == 'A') {
	set_bit(features, Feature::aclass, true);
	}
	}
	switch (arch.version) {
	case 8:
	set_bit(features, Feature::v8, true);
	JL_FALLTHROUGH;
	case 7:
	set_bit(features, Feature::v7, true);
	break;
	default:
	break;
	}
	#endif
	std::set<uint32_t> cpus;
	std::vector<std::pair<uint32_t,CPUID>> list;
	// Ideally the feature detection above should be enough.
	// However depending on the kernel version not all features are available
	// and it's also impossible to detect the ISA version which contains
	// some features not yet exposed by the kernel.
	// We therefore try to get a more complete feature list from the CPU name.
	// Since it is possible to pair cores that have different feature set
	// (Observed for exynos 9810 with exynos-m3 + cortex-a55) we'll compute
	// an intersection of the known features from each core.
	// If there's a core that we don't recognize, treat it as generic.
	bool extra_initialized = false;
	FeatureList<feature_sz> extra_features = {};
	for (auto info: cpuinfo) {
	auto name = (uint32_t)get_cpu_name(info);
	if (name == 0) {
	// no need to clear the feature set if it wasn't initialized
	if (extra_initialized)
	extra_features = FeatureList<feature_sz>{};
	extra_initialized = true;
	continue;
	}
	if (!check_cpu_arch_ver(name, arch))
	continue;
	if (cpus.insert(name).second) {
	if (extra_initialized) {
	extra_features = extra_features & find_cpu(name)->features;
	}
	else {
	extra_initialized = true;
	extra_features = find_cpu(name)->features;
	}
	list.emplace_back(name, info);
	}
	}
	features = features | extra_features;
	// Not all elements/pairs are valid
	static constexpr CPU v8order[] = {
	CPU::arm_cortex_a35,
	CPU::arm_cortex_a53,
	CPU::arm_cortex_a55,
	CPU::arm_cortex_a57,
	CPU::arm_cortex_a72,
	CPU::arm_cortex_a73,
	CPU::arm_cortex_a75,
	CPU::arm_cortex_a76,
	CPU::arm_neoverse_n1,
	CPU::arm_neoverse_n2,
	CPU::arm_neoverse_v1,
	CPU::nvidia_denver2,
	CPU::nvidia_carmel,
	CPU::samsung_exynos_m1,
	CPU::samsung_exynos_m2,
	CPU::samsung_exynos_m3,
	CPU::samsung_exynos_m4,
	CPU::samsung_exynos_m5,
	};
	shrink_big_little(list, v8order, sizeof(v8order) / sizeof(CPU));
	#ifdef _CPU_ARM_
	// Not all elements/pairs are valid
	static constexpr CPU v7order[] = {
	CPU::arm_cortex_a5,
	CPU::arm_cortex_a7,
	CPU::arm_cortex_a8,
	CPU::arm_cortex_a9,
	CPU::arm_cortex_a12,
	CPU::arm_cortex_a15,
	CPU::arm_cortex_a17
	};
	shrink_big_little(list, v7order, sizeof(v7order) / sizeof(CPU));
	#endif
	uint32_t cpu = 0;
	if (list.empty()) {
	cpu = (uint32_t)generic_for_arch(arch);
	}
	else {
	// This also covers `list.size() > 1` case which means there's a unknown combination
	// consists of CPU's we know. Unclear what else we could try so just randomly return
	// one...
	cpu = list[0].first;
	}
	// Ignore feature bits that we are not interested in.
	mask_features(feature_masks, &features[0]);
	return std::make_pair(cpu, features);
	}

LLVM does use the features since it has a separate flag -mcpu=apple-a12 so it isn't something very urgent.

Using something like https://developer.apple.com/documentation/kernel/1387446-sysctlbyname seems like an option in other not to hardcode it, for future apple silicon CPUs.

[yuyichao]

Dup of #40876

[gbaraldi]

I'm not that familiar, is this the same codepath for feature detection that LLVM uses for codegen?