Update predef.qbk with SIMD heading/examples

doc/predef.qbk

@@ -143,6 +143,8 @@ macros of a particular prefix:
 * `BOOST_OS_` for the operating system we are compiling to.
 * `BOOST_PLAT_` for platforms on top of operating system or compilers.
 * `BOOST_ENDIAN_` for endianness of the os and architecture combination.
+* `BOOST_HW_` for hardware specific features.
+* `BOOST_HW_SIMD` for SIMD (Single Instruction Multiple Data) detection.
 
 [note The detected definitions are for the configuration one is targeting
 during the compile. In particular in a cross-compile this means the target
@@ -223,6 +225,93 @@ detected. I.e. a definition equivalent to:
 Also for each aspect there is a macro defined with a descriptive
 name of what the detection is.
 
+[heading The `BOOST_HW_SIMD_*` predefs]
+
+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 defined(BOOST_HW_SIMD_X86_SSE3_AVAILABLE)
+    std::cout << "This is SSE3!" << std::endl;
+#endif
+    return 0;
+  }
+``
+
+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
+    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 &&\
+    BOOST_HW_SIMD_X86 <= BOOST_HW_SIMD_X86_SSSE3
+    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].
+
 [heading The `*_EMULATED` macros]
 
 Predef definitions are guaranteed to be uniquely detected within one category.