diff --git a/Eigen/Core b/Eigen/Core
index cbca16640..0306be3a8 100644
--- a/Eigen/Core
+++ b/Eigen/Core
@@ -61,20 +61,45 @@
 
 #ifndef EIGEN_DONT_VECTORIZE
   #if defined (EIGEN_SSE2_BUT_NOT_OLD_GCC) || defined(EIGEN_SSE2_ON_MSVC_2008_OR_LATER)
+
+    // Defines symbols for compile-time detection of which instructions are
+    // used.
+    // EIGEN_VECTORIZE_YY is defined if and only if the instruction set YY is used
     #define EIGEN_VECTORIZE
     #define EIGEN_VECTORIZE_SSE
-    #include <emmintrin.h>
-    #include <xmmintrin.h>
+    #define EIGEN_VECTORIZE_SSE2
+
+    // Detect sse3/ssse3/sse4:
+    // gcc and icc defines __SSE3__, ..,
+    // there is no way to know about this on msvc. You can define EIGEN_VECTORIZE_SSE* if you
+    // want to force the use of those instructions with msvc.
     #ifdef __SSE3__
-      #include <pmmintrin.h>
+      #define EIGEN_VECTORIZE_SSE3
     #endif
     #ifdef __SSSE3__
-      #include <tmmintrin.h>
+      #define EIGEN_VECTORIZE_SSSE3
     #endif
     #ifdef __SSE4_1__
-      #include <smmintrin.h>
+      #define EIGEN_VECTORIZE_SSE4_1
     #endif
     #ifdef __SSE4_2__
+      #define EIGEN_VECTORIZE_SSE4_2
+    #endif
+
+    // include files
+
+    #include <emmintrin.h>
+    #include <xmmintrin.h>
+    #ifdef  EIGEN_VECTORIZE_SSE3
+      #include <pmmintrin.h>
+    #endif
+    #ifdef EIGEN_VECTORIZE_SSSE3
+      #include <tmmintrin.h>
+    #endif
+    #ifdef EIGEN_VECTORIZE_SSE4_1
+      #include <smmintrin.h>
+    #endif
+    #ifdef EIGEN_VECTORIZE_SSE4_2
       #include <nmmintrin.h>
     #endif
   #elif defined __ALTIVEC__
diff --git a/Eigen/src/Core/arch/SSE/PacketMath.h b/Eigen/src/Core/arch/SSE/PacketMath.h
index a5a56f759..f78bf0dd3 100644
--- a/Eigen/src/Core/arch/SSE/PacketMath.h
+++ b/Eigen/src/Core/arch/SSE/PacketMath.h
@@ -122,7 +122,7 @@ template<> EIGEN_STRONG_INLINE Packet4f ei_pmul<Packet4f>(const Packet4f& a, con
 template<> EIGEN_STRONG_INLINE Packet2d ei_pmul<Packet2d>(const Packet2d& a, const Packet2d& b) { return _mm_mul_pd(a,b); }
 template<> EIGEN_STRONG_INLINE Packet4i ei_pmul<Packet4i>(const Packet4i& a, const Packet4i& b)
 {
-#ifdef __SSE4_1__
+#ifdef EIGEN_VECTORIZE_SSE4_1
   return _mm_mullo_epi32(a,b);
 #else
   // this version is slightly faster than 4 scalar products
@@ -269,7 +269,7 @@ template<> EIGEN_STRONG_INLINE Packet2d ei_pabs(const Packet2d& a)
 }
 template<> EIGEN_STRONG_INLINE Packet4i ei_pabs(const Packet4i& a)
 {
-  #ifdef __SSSE3__
+  #ifdef EIGEN_VECTORIZE_SSSE3
   return _mm_abs_epi32(a);
   #else
   Packet4i aux = _mm_srai_epi32(a,31);
@@ -278,7 +278,7 @@ template<> EIGEN_STRONG_INLINE Packet4i ei_pabs(const Packet4i& a)
 }
 
 
-#ifdef __SSE3__
+#ifdef EIGEN_VECTORIZE_SSE3
 // TODO implement SSE2 versions as well as integer versions
 template<> EIGEN_STRONG_INLINE Packet4f ei_preduxp<Packet4f>(const Packet4f* vecs)
 {
@@ -439,7 +439,7 @@ template<> EIGEN_STRONG_INLINE int ei_predux_max<Packet4i>(const Packet4i& a)
 // }
 #endif
 
-#ifdef __SSSE3__
+#ifdef EIGEN_VECTORIZE_SSSE3
 // SSSE3 versions
 template<int Offset>
 struct ei_palign_impl<Offset,Packet4f>