Add doc + _AVAILABLE for simd.h

include/boost/predef/hardware/simd.h

@@ -9,8 +9,51 @@ http://www.boost.org/LICENSE_1_0.txt)
 #ifndef BOOST_PREDEF_HARDWARE_SIMD_H
 #define BOOST_PREDEF_HARDWARE_SIMD_H
 
+/*`
+ [heading `BOOST_HW_SIMD`]
+
+ The SIMD extension detected for a specific architectures.
+ Version number depends on the detected extension.
+
+ [table
+     [[__predef_symbol__] [__predef_version__]]
+
+     [[`BOOST_HW_SIMD_ARM_AVAILABLE`] [__predef_detection__]]
+     [[`BOOST_HW_SIMD_PPC_AVAILABLE`] [__predef_detection__]]
+     [[`BOOST_HW_SIMD_X86_AVAILABLE`] [__predef_detection__]]
+     ]
+ */
+
 #include <boost/predef/hardware/simd/x86.h>
 #include <boost/predef/hardware/simd/arm.h>
 #include <boost/predef/hardware/simd/ppc.h>
 
+// We check if SIMD extension of multiples architectures have been detected,
+// if yes, then this is an error!
+#if \
+    BOOST_HW_SIMD_ARM && BOOST_HW_SIMD_PPC ||\
+    BOOST_HW_SIMD_ARM && BOOST_HW_SIMD_X86 ||\
+    BOOST_HW_SIMD_PPC && BOOST_HW_SIMD_X86
+#   error "Multiple SIMD architectures detected, this cannot happen!"
+#endif
+
+#if defined(BOOST_HW_SIMD_X86_AVAILABLE)
+#   define BOOST_HW_SIMD BOOST_HW_SIMD_X86
+#   define BOOST_HW_SIMD_NAME BOOST_HW_SIMD_X86_NAME
+#endif
+
+#if defined(BOOST_HW_SIMD_ARM_AVAILABLE)
+#   define BOOST_HW_SIMD BOOST_HW_SIMD_ARM
+#   define BOOST_HW_SIMD_NAME BOOST_HW_SIMD_ARM_NAME
+#endif
+
+#if defined(BOOST_HW_SIMD_PPC_AVAILABLE)
+#   define BOOST_HW_SIMD BOOST_HW_SIMD_PPC
+#   define BOOST_HW_SIMD_NAME BOOST_HW_SIMD_PPC_NAME
+#endif
+
+#if defined(BOOST_HW_SIMD)
+#   define BOOST_HW_SIMD_AVAILABLE
+#endif
+
 #endif