make the number of registers easier to configure per architectures

Eigen/src/Core/arch/Default/Settings.h

@@ -62,4 +62,17 @@
 #define EIGEN_TUNE_TRIANGULAR_PANEL_WIDTH 8
 #endif
 
+
+/** Defines the default number of registers available for that architecture.
+  * Currently it must be 8 or 16. Other values will fail.
+  */
+#ifndef EIGEN_ARCH_DEFAULT_NUMBER_OF_REGISTERS
+#if (defined __i386__)
+#define EIGEN_ARCH_DEFAULT_NUMBER_OF_REGISTERS 8
+#else
+#define EIGEN_ARCH_DEFAULT_NUMBER_OF_REGISTERS 16
+#endif
+
+#endif
+
 #endif // EIGEN_DEFAULT_SETTINGS_H

Eigen/src/Core/arch/NEON/PacketMath.h

@@ -35,6 +35,12 @@
 #define EIGEN_TUNE_FOR_CPU_CACHE_SIZE 4*96*96
 #endif
 
+// FIXME NEON has 16 quad registers, but since the current register allocator
+// is so bad, it is much better to reduce it to 8
+#ifndef EIGEN_ARCH_DEFAULT_NUMBER_OF_REGISTERS
+#define EIGEN_ARCH_DEFAULT_NUMBER_OF_REGISTERS 8
+#endif
+
 typedef float32x4_t Packet4f;
 typedef int32x4_t   Packet4i;
 

Eigen/src/Core/util/BlasUtil.h

@@ -130,14 +130,10 @@ struct ei_product_blocking_traits
   typedef typename ei_packet_traits<Scalar>::type PacketType;
   enum {
     PacketSize = sizeof(PacketType)/sizeof(Scalar),
-    #if (defined __i386__)
-    HalfRegisterCount = 4,
-    #else
-    HalfRegisterCount = 8,
-    #endif
+    NumberOfRegisters = EIGEN_ARCH_DEFAULT_NUMBER_OF_REGISTERS,
 
     // register block size along the N direction (must be either 2 or 4)
-    nr = HalfRegisterCount/2,
+    nr = NumberOfRegisters/4,
 
     // register block size along the M direction (currently, this one cannot be modified)
     mr = 2 * PacketSize,