arm 的neon指令等同于arm64的asimd指令

在ceph 的src/test/test_arch.cc 中我们可以看到下面的code ，从kernel中的/proc/cpuinfo 中的feature
来判断arm是否支持neon指令，需要注意这个指令在armv8 之后被称为asimd指令
TEST(Arch, all)
{
  ceph_arch_probe();
  EXPECT_TRUE(ceph_arch_probed);

#if (__arm__ || __aarch64__ || __x86_64__) && __linux__
  char flags[FLAGS_SIZE];
  FILE *f = popen("grep '^\\(flags\\|Features\\)[        ]*:' "
                  "/proc/cpuinfo | head -1", "r");
  if(f == NULL || fgets(flags, FLAGS_SIZE - 1, f) == NULL) {
    // silently do nothing if /proc/cpuinfo does exist, is not
    // readable or does not contain the expected information
    if (f)
      pclose(f);
    return;
  }
  pclose(f);
  flags[strlen(flags) - 1] = ' ';

  int expected;

#if (__arm__ || __aarch64__)
#从这里知道neon和asimd 是一回事
  expected = (strstr(flags, " neon ") || strstr(flags, " asimd ")) ? 1 : 0;
  EXPECT_EQ(expected, ceph_arch_neon);

#endif
除了通过/proc/cpuinfo 外更通用的方式是通过/proc/self/auxv 来获取硬件feature

static unsigned long get_auxval(unsigned long type)
{
	unsigned long result = 0;
	#打开这个文件
	FILE *f = fopen("/proc/self/auxv", "r");
	if (f) {
		ElfW(auxv_t) entry;
		#读取这个文件的AT_HWCAP
		while (fread(&entry, sizeof(entry), 1, f) == 1) {
			if (entry.a_type == type) {
				result = entry.a_un.a_val;
				break;
			}
		}
		fclose(f);
	}
	return result;
}

static unsigned long get_hwcap(void)
{
	return get_auxval(AT_HWCAP);
}

#endif // __linux__

int ceph_arch_arm_probe(void)
{
#从下面这段code 也可以知道arm下的neon 指令等同于arm64下的asimd指令
#if __arm__ && __linux__
	ceph_arch_neon = (get_hwcap() & HWCAP_NEON) == HWCAP_NEON;
#elif __aarch64__ && __linux__
	ceph_arch_neon = (get_hwcap() & HWCAP_ASIMD) == HWCAP_ASIMD;
# if defined(HAVE_ARMV8_CRC) && defined(HWCAP_CRC32)
	ceph_arch_aarch64_crc32 = (get_hwcap() & HWCAP_CRC32) == HWCAP_CRC32;
# endif
#else
	if (0)
		get_hwcap();  // make compiler shut up
#endif
	return 0;
}