CPU features
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There are several ways to check for CPU features in your code, each with a different set of trade-offs.
ABI: Use the preprocessor's pre-defined macros
It's usually most convenient to determine the ABI at build time using #ifdef
in conjunction with:
__arm__for 32-bit ARM__aarch64__for 64-bit ARM__i386__for 32-bit X86__x86_64__for 64-bit X86
Note that 32-bit X86 is called __i386__, not __x86__ as you might expect!
CPU core counts: Use libc's sysconf(3)
sysconf(3) lets you
query both _SC_NPROCESSORS_CONF (the number of CPU cores in the system)
and _SC_NPROCESSORS_ONLN (the number of CPU cores currently online).
Features: Use libc's getauxval(3)
In API level 18 and newer,
getauxval(3)
is available in Android's C library. The AT_HWCAP and AT_HWCAP2 arguments
return bitmasks listing CPU-specific features. See the various hwcap.h
headers in the NDK for the constants to compare against, such as HWCAP_SHA512
for arm64's SHA512 instructions, or HWCAP_IDIVT for arm's Thumb integer
division instructions.
The Google cpu_features library
One problem with AT_HWCAP is that sometimes devices are mistaken. Some old
devices, for example, claim to have integer division instructions but do not.
Google's cpu_features library works
around such issues by applying its own knowledge of specific SoCs (by parsing
/proc/cpuinfo to work out the specific SoC in question).
This library is maintained for use by Google's first-party app teams, and has workarounds for every problematic device they've encountered in the wild.
The NDK cpufeatures library (deprecated)
The NDK still provides a deprecated library named cpufeatures for source
compatibility with apps that already use it. Unlike the newer and more complete
cpu_features library, this historical
library does not have workarounds for as many specific SoCs.