SIMD predefs depend on compiler options. For example, you will have to add the
option `-msse3` to clang or gcc to enable SSE3. SIMD predefs are also inclusive.
This means that if SSE3 is enabled, then every other extensions with a lower
version number will implicitly be enabled and detected. However, some extensions
are CPU specific, they may not be detected nor enabled when an upper version is
enabled.

[note SSE(1) and SSE2 are automatically enabled by default when using x86-64
architecture.]

To check if any SIMD extension has been enabled, you can use:

``
#include <boost/predef/hardware/simd.h>
#include <iostream>

int main()
{
#if defined(BOOST_HW_SIMD_AVAILABLE)
    std::cout << "SIMD detected!" << std::endl;
#endif
    return 0;
}
``

When writing SIMD specific code, you may want to check if a particular extension
has been detected. To do so you have to use the right architecture predef and
compare it. Those predef are of the form `BOOST_HW_SIMD_"ARCH"` (where `"ARCH"`
is either `ARM`, `PPC`, or `X86`). For example, if you compile code for x86
architecture, you will have to use `BOOST_HW_SIMD_X86`. Its value will be the
version number of the most recent SIMD extension detected for the architecture.

To check if an extension has been enabled:

``
#include <boost/predef/hardware/simd.h>
#include <iostream>

int main()
{
#if BOOST_HW_SIMD_X86 >= BOOST_HW_SIMD_X86_SSE3_VERSION
    std::cout << "This is SSE3!" << std::endl;
#endif
    return 0;
}
``

[note The *_VERSION* defines that map version number to actual real
identifiers. This way it is easier to write comparisons without messing up with
version numbers.]

To *"stricly"* check the most recent detected extension:

``
#include <boost/predef/hardware/simd.h>
#include <iostream>

int main()
{
#if BOOST_HW_SIMD_X86 == BOOST_HW_SIMD_X86_SSE3_VERSION
    std::cout << "This is SSE3 and this is the most recent enabled extension!"
        << std::endl;
#endif
    return 0;
}
``

Because of the version systems of predefs and of the inclusive property of SIMD
extensions macros, you can easily check for ranges of supported extensions:

``
#include <boost/predef/hardware/simd.h>
#include <iostream>

int main()
{
#if BOOST_HW_SIMD_X86 >= BOOST_HW_SIMD_X86_SSE2_VERSION &&\
    BOOST_HW_SIMD_X86 <= BOOST_HW_SIMD_X86_SSSE3_VERSION
    std::cout << "This is SSE2, SSE3 and SSSE3!" << std::endl;
#endif
    return 0;
}
``

[note Unlike gcc and clang, Visual Studio does not allow you to specify precisely
the SSE variants you want to use, the only detections that will take place are
SSE, SSE2, AVX and AVX2. For more informations,
    see [@https://msdn.microsoft.com/en-us/library/b0084kay.aspx here].]

