Are older SIMD-versions available when using newer

问题:

When I can use SSE3 or AVX, are then older SSE versions as SSE2 or MMX available -
or do I still need to check for them separately?

回答1:

In general, these have been additive but keep in mind that there are differences between Intel and AMD support for these over the years.

If you have AVX, then you can assume SSE, SSE2, SSE3, SSSE3, SSE4.1, and SSE 4.2 as well. Remember that to use AVX you also need to validate the OSXSAVE CPUID bit is set to ensure the OS you are using actually supports saving the AVX registers as well.

You should still explicitly check for all the CPUID support you use in your code for robustness (say checking for AVX, OSXSAVE, SSE4, SSE3, SSSE3 at the same time to guard your AVX codepaths).

#include <intrin.h>

inline bool IsAVXSupported()
{
#if defined(_M_IX86 ) || defined(_M_X64)
   int CPUInfo[4] = {-1};
   __cpuid( CPUInfo, 0 );

   if ( CPUInfo[0] < 1  )
       return false;

    __cpuid(CPUInfo, 1 );

    int ecx = 0x10000000 // AVX
              | 0x8000000 // OSXSAVE
              | 0x100000 // SSE 4.2
              | 0x80000 // SSE 4.1
              | 0x200 // SSSE3
              | 0x1; // SSE3

    if ( ( CPUInfo[2] & ecx ) != ecx )
        return false;

    return true;
#else
    return false;
#endif
}

SSE and SSE2 are required for all processors capable of x64 native, so they are good baseline assumptions for all code. Windows 8.0, Windows 8.1, and Windows 10 explicitly require SSE and SSE2 support even for x86 architectures so those instruction sets are pretty ubiquitous. In other words, if you fail a check for SSE or SSE2, just exit the app with a fatal error.

#include <windows.h>

inline bool IsSSESupported()
{
#if defined(_M_IX86 ) || defined(_M_X64)
   return ( IsProcessorFeaturePresent( PF_XMMI_INSTRUCTIONS_AVAILABLE ) != 0 && IsProcessorFeaturePresent( PF_XMMI64_INSTRUCTIONS_AVAILABLE ) != 0 );
#else
    return false;
#endif
}

-or-

#include <intrin.h>

inline bool IsSSESupported()
{
#if defined(_M_IX86 ) || defined(_M_X64)
   int CPUInfo[4] = {-1};
   __cpuid( CPUInfo, 0 );

   if ( CPUInfo[0] < 1  )
       return false;

    __cpuid(CPUInfo, 1 );

    int edx = 0x4000000 // SSE2
              | 0x2000000; // SSE

    if ( ( CPUInfo[3] & edx ) != edx )
        return false;

    return true;
#else
    return false;
#endif
}

Also, keep in mind that MMX, x87 FPU, and AMD 3DNow!* are all deprecated instruction sets for x64 native, so you shouldn't be using them actively anymore in newer code. A good rule of thumb is to avoid using any intrinsic that returns a __m64 or takes a __m64 data type.

You may want to check out this DirectXMath blog series with notes on many of these instruction sets and the relevant processor support requirements.

Note (*) - All the AMD 3DNow! instructions are deprecated except for PREFETCH and PREFETCHW which were carried forward. First generation Intel64 processors lacked support for these instructions, but they were later added as they are considered part of the core X64 instruction set. Windows 8.1 and Windows 10 x64 require PREFETCHW in particular, although the test is a little odd. Most Intel CPUs prior to Broadwell do not in fact report support for PREFETCHW through CPUID, but they treat the opcode as a no-op rather than throw an 'illegal instruction' exception. As such, the test here is (a) is it supported by CPUID, and (b) if not, does PREFETCHW at least not throw an exception.

Here's some test code for Visual Studio that demonstrates the PREFETCHW test as well as many other CPUID bits for the x86 and x64 platforms.

#include <intrin.h>
#include <stdio.h>
#include <windows.h>
#include <excpt.h>

void main()
{
   unsigned int x = _mm_getcsr();
   printf("%08X\n", x );

   bool prefetchw = false;

   // See http://msdn.microsoft.com/en-us/library/hskdteyh.aspx
   int CPUInfo[4] = {-1};
   __cpuid( CPUInfo, 0 );

   if ( CPUInfo[0] > 0 )
   {
       __cpuid(CPUInfo, 1 );

       // EAX
       {
           int stepping = (CPUInfo[0] & 0xf);
           int basemodel = (CPUInfo[0] >> 4) & 0xf;
           int basefamily = (CPUInfo[0] >> 8) & 0xf;
           int xmodel = (CPUInfo[0] >> 16) & 0xf;
           int xfamily = (CPUInfo[0] >> 20) & 0xff;

           int family = basefamily + xfamily;
           int model = (xmodel << 4) | basemodel;

           printf("Family %02X, Model %02X, Stepping %u\n", family, model, stepping );
       }

       // ECX
       if ( CPUInfo[2] & 0x20000000 ) // bit 29
          printf("F16C\n");

       if ( CPUInfo[2] & 0x10000000 ) // bit 28
          printf("AVX\n");

       if ( CPUInfo[2] & 0x8000000 ) // bit 27
          printf("OSXSAVE\n");

       if ( CPUInfo[2] & 0x400000 ) // bit 22
          printf("MOVBE\n");

       if ( CPUInfo[2] & 0x100000 ) // bit 20
          printf("SSE4.2\n");

       if ( CPUInfo[2] & 0x80000 ) // bit 19
          printf("SSE4.1\n");

       if ( CPUInfo[2] & 0x2000 ) // bit 13
          printf("CMPXCHANG16B\n");

       if ( CPUInfo[2] & 0x1000 ) // bit 12
          printf("FMA3\n");

       if ( CPUInfo[2] & 0x200 ) // bit 9
          printf("SSSE3\n");

       if ( CPUInfo[2] & 0x1 ) // bit 0
          printf("SSE3\n");

       // EDX
       if ( CPUInfo[3] & 0x4000000 ) // bit 26
           printf("SSE2\n");

       if ( CPUInfo[3] & 0x2000000 ) // bit 25
           printf("SSE\n");

       if ( CPUInfo[3] & 0x800000 ) // bit 23
           printf("MMX\n");
   }
   else
       printf("CPU doesn't support Feature Identifiers\n");

   if ( CPUInfo[0] >= 7 )
   {
       __cpuidex(CPUInfo, 7, 0);

       // EBX
       if ( CPUInfo[1] & 0x100 ) // bit 8
         printf("BMI2\n");

       if ( CPUInfo[1] & 0x20 ) // bit 5
         printf("AVX2\n");

       if ( CPUInfo[1] & 0x8 ) // bit 3
         printf("BMI\n");
   }
   else
       printf("CPU doesn't support Structured Extended Feature Flags\n");

   // Extended features
   __cpuid( CPUInfo, 0x80000000 );

   if ( CPUInfo[0] > 0x80000000 )
   {
       __cpuid(CPUInfo, 0x80000001 );

       // ECX
       if ( CPUInfo[2] & 0x10000 ) // bit 16
           printf("FMA4\n");

       if ( CPUInfo[2] & 0x800 ) // bit 11
           printf("XOP\n");

       if ( CPUInfo[2] & 0x100 ) // bit 8
       {
           printf("PREFETCHW\n");
           prefetchw = true;
       }

       if ( CPUInfo[2] & 0x80 ) // bit 7
           printf("Misalign SSE\n");

       if ( CPUInfo[2] & 0x40 ) // bit 6
           printf("SSE4A\n");

       if ( CPUInfo[2] & 0x1 ) // bit 0
           printf("LAHF/SAHF\n");

       // EDX
       if ( CPUInfo[3] & 0x80000000 ) // bit 31
           printf("3DNow!\n");

       if ( CPUInfo[3] & 0x40000000 ) // bit 30
           printf("3DNowExt!\n");

       if ( CPUInfo[3] & 0x20000000 ) // bit 29
           printf("x64\n");

       if ( CPUInfo[3] & 0x100000 ) // bit 20
           printf("NX\n");
   }
   else
       printf("CPU doesn't support Extended Feature Identifiers\n");

   if ( !prefetchw )
   {
       bool illegal = false;

       __try
       {
           static const unsigned int s_data = 0xabcd0123;

           _m_prefetchw(&s_data);
       }
       __except (EXCEPTION_EXECUTE_HANDLER)
       {
           illegal = true;
       }

       if (illegal)
       {
           printf("PREFETCHW is an invalid instruction on this processor\n");
       }
   }
}

UPDATE: The fundamental challenge, of course, is how do you handle systems that lack support for AVX? While the instruction set is useful, the biggest benefit of having an AVX-capable processor is the ability to use the /arch:AVX build switch which enables the global use of the VEX prefix for better SSE/SSE2 code-gen. The only problem is the resulting code DLL/EXE is not compatible with systems that lack AVX support.

As such, for Windows, ideally you should build one EXE for non-AVX systems (assuming SSE/SSE2 only so use /arch:SSE2 instead for x86 code; this setting is implicit for x64 code), a different EXE that is optimized for AVX (using /arch:AVX), and then use CPU detection to determine which EXE to use for a given system.

Luckily with Xbox One, we can just always build with /arch::AVX since it's a fixed platform...

回答2:

As a general rule - don't mix different generations of SSE / AVX unless you have to. If you do, make sure you use vzeroupper or similar state clearing instructions, otherwise you may drag partial values and unknowingly create false dependencies, since most of the registers are shared between the modes Even when clearing, switching between modes may cause penalties, depending on the exact micro architecture.

Further reading - https://software.intel.com/sites/default/files/m/d/4/1/d/8/11MC12_Avoiding_2BAVX-SSE_2BTransition_2BPenalties_2Brh_2Bfinal.pdf

回答3:

See Chuck's answer for good advice on what you should do. See this answer for a literal answer to the question asked, in case you're curious.

---

AVX support absolutely guarantees support for all Intel SSE* instruction sets, since it includes VEX-encoded versions of all of them. As Chuck points out, you can check for previous ones at the same time with a bitmask, without bloating your code, but don't sweat it.

Note that POPCNT, TZCNT, and stuff like that are not part of SSE-anything. POPCNT has its own feature bit. LZCNT has its own feature bit, too, since AMD introduced it separately from BMI1. TZCNT is just part of BMI1, though. Since some BMI1 instructions use VEX encodings, even latest-generation Pentium/Celeron CPUs (like Skylake Pentium) don't have BMI1. :( I think Intel just wanted to omit AVX/AVX2, probably so they could sell CPUs with faulty upper-lanes of execution units as Pentiums, and they do this by disabling VEX support in the decoders.

---

Intel SSE support has been incremental in all CPUs released so far. SSE4.1 implies SSSE3, SSE3, SSE2, and SSE. And SSE4.2 implies all of the preceding. I'm not sure if any official x86 documentation precludes the possibility of a CPU with SSE4.1 support but not SSSE3. (i.e. leave out PSHUFB, which is possibly expensive-to-implement.) It's extremely unlikely in practice, though, since this would violate many people's assumptions. As I said, it might even be officially forbidden, but I didn't check carefully.

---

AVX does not include AMD SSE4a or AMD XOP. AMD extensions have to be checked-for specially. Also note that the newest AMD CPUs are dropping XOP support. (Intel never adopted it, so most people don't write code to take advantage of it, so for AMD those transistors are mostly wasted. It does have some nice stuff, like a 2-source byte permute, allowing a byte LUT twice as wide as PSHUFB, without the in-lane limitation of AVX2's VPSHUFB ymm).

---

SSE2 is baseline for the x86-64 architecture. You do not have to check for SSE or SSE2 support in 64-bit builds. I forget if MMX is baseline, too. Almost certainly.

The SSE instruction set includes some instructions that operate on MMX registers. (e.g. PMAXSW mm1, mm2/m64 was new with SSE. The XMM version is part of SSE2.) Even a 32-bit CPU supporting SSE needs to have MMX registers. It would be madness to have MMX registers but only support the SSE instructions that use them, not the original MMX instructions (e.g. movq mm0, [mem]). However, I haven't found anything definitive that rules out the possibility of an x86-based Deathstation 9000 with SSE but not MMX CPUID feature bits, but I didn't wade into Intel's official x86 manuals. (See the x86 tag wiki for links).

Don't use MMX anyway, it's generally slower even if you only have 64 bits at a time to work on, in the low half of an XMM register. The latest CPUs (like Intel Skylake) have lower throughput for the MMX versions of some instructions than for the XMM version. In some cases, even worse latency. For example, according to Agner Fog's testing, PACKSSWB mm0, mm1 is 3 uops, with 2c latency, on Skylake. The 128b and 256b XMM / YMM versions are 1 uop, with 1c latency.