What is the minimum version of OS X for use with AVX/AVX2?

Question

I have an image drawing routine which is compiled multiple times for SSE, SSE2, SSE3, SSE4.1, SSE4.2, AVX and AVX2. My program dynamically dispatches one of these binary variations by checking CPUID flags.

On Windows, I check the version of Windows and disable AVX/AVX2 dispatch if the OS doesn't support them. (For example, only Windows 7 SP1 or later supports AVX/AVX2.)

I want to do the same thing on Mac OS X, but I'm not sure what version of OS X supports AVX/AVX2.

Note that what I want to know is the minimum version of OS X for use with AVX/AVX2. Not machine models which are capable of AVX/AVX2.

You may find this command in Terminal/shell to be useful sysctl -a | grep -i avx to see CPU features. — Mark Setchell

Z boson Z boson · Accepted Answer · 2016-07-13T07:38:22

For detecting instruction set features there are two source files I reference:

Mysticial's cpu_x86.cpp
Agner Fog's instrset_detect.cpp

Both of these files will tell you how to detect SSE through AVX2 as well as XOP, FMA3, FMA4, if your OS supports AVX and other features.

I am used to Agner's code (one source file for MSVC, GCC, Clang, ICC) so let's look at that first.

Here are the relevant code fragments from instrset_detect.cpp for detecting AVX:

iset = 0;                                              // default value
int abcd[4] = {0,0,0,0};                               // cpuid results
cpuid(abcd, 0);                                        // call cpuid function 0
//....
iset = 6;                                              // 6: SSE4.2 supported
if ((abcd[2] & (1 << 27)) == 0) return iset;           // no OSXSAVE
if ((xgetbv(0) & 6) != 6)       return iset;           // AVX not enabled in O.S.
if ((abcd[2] & (1 << 28)) == 0) return iset;           // no AVX
iset = 7;                                              // 7: AVX supported

with xgetbv defined as

// Define interface to xgetbv instruction
static inline int64_t xgetbv (int ctr) {    
#if (defined (_MSC_FULL_VER) && _MSC_FULL_VER >= 160040000) || (defined (__INTEL_COMPILER) && __INTEL_COMPILER >= 1200) // Microsoft or Intel compiler supporting _xgetbv intrinsic

    return _xgetbv(ctr);                                   // intrinsic function for XGETBV

#elif defined(__GNUC__)                                    // use inline assembly, Gnu/AT&T syntax

   uint32_t a, d;
   __asm("xgetbv" : "=a"(a),"=d"(d) : "c"(ctr) : );
   return a | (uint64_t(d) << 32);

#else  // #elif defined (_WIN32)                           // other compiler. try inline assembly with masm/intel/MS syntax

  //see the source file
}

I did not include the cpuid function (see the source code) and I removed the non GCC inline assembly from xgetbv to make the answer shorter.

Here is the detect_OS_AVX() from Mysticial's cpu_x86.cpp for detecting AVX:

bool cpu_x86::detect_OS_AVX(){
    //  Copied from: http://stackoverflow.com/a/22521619/922184

    bool avxSupported = false;

    int cpuInfo[4];
    cpuid(cpuInfo, 1);

    bool osUsesXSAVE_XRSTORE = (cpuInfo[2] & (1 << 27)) != 0;
    bool cpuAVXSuport = (cpuInfo[2] & (1 << 28)) != 0;

    if (osUsesXSAVE_XRSTORE && cpuAVXSuport)
    {
        uint64_t xcrFeatureMask = xgetbv(_XCR_XFEATURE_ENABLED_MASK);
        avxSupported = (xcrFeatureMask & 0x6) == 0x6;
    }

    return avxSupported;
}

Mystical apparently came up with this solution from this answer.

Notice that both source files do basically the same thing: check the OSXSAVE bit 27, check the AVX bit 28 from CPUID, check a result from xgetbv.

What is the minimum version of OS X for use with AVX/AVX2?

2 Answers