diff --git a/Eigen/src/Core/arch/AltiVec/MatrixProduct.h b/Eigen/src/Core/arch/AltiVec/MatrixProduct.h
index 1888347b1..ea7749610 100644
--- a/Eigen/src/Core/arch/AltiVec/MatrixProduct.h
+++ b/Eigen/src/Core/arch/AltiVec/MatrixProduct.h
@@ -17,8 +17,12 @@
 
 #include "MatrixProductCommon.h"
 
+#if !defined(EIGEN_ALTIVEC_DISABLE_MMA)
+#define EIGEN_ALTIVEC_DISABLE_MMA 0
+#endif
+
 // Check for MMA builtin support. 
-#if !defined(EIGEN_ALTIVEC_DISABLE_MMA) && defined(__has_builtin)
+#if !EIGEN_ALTIVEC_DISABLE_MMA && defined(__has_builtin)
 #if __has_builtin(__builtin_mma_assemble_acc)
   #define EIGEN_ALTIVEC_MMA_SUPPORT
 #endif
@@ -27,12 +31,16 @@
 // Check if and how we should actually use MMA if supported.
 #if defined(EIGEN_ALTIVEC_MMA_SUPPORT)
 
-// Use MMA by default if available.
-#if defined(__MMA__)
-#define EIGEN_ALTIVEC_MMA_ONLY 1
-// Otherwise, check if we want to enable dynamic dispatch. Not supported by LLVM.
-#elif defined(EIGEN_ALTIVEC_ENABLE_MMA_DYNAMIC_DISPATCH) && !defined(EIGEN_COMP_LLVM)
+#if !defined(EIGEN_ALTIVEC_ENABLE_MMA_DYNAMIC_DISPATCH)
+#define EIGEN_ALTIVEC_ENABLE_MMA_DYNAMIC_DISPATCH 0
+#endif
+
+// Check if we want to enable dynamic dispatch. Not supported by LLVM.
+#if EIGEN_ALTIVEC_ENABLE_MMA_DYNAMIC_DISPATCH && !EIGEN_COMP_LLVM
 #define EIGEN_ALTIVEC_MMA_DYNAMIC_DISPATCH 1
+// Otherwise, use MMA by default if available.
+#elif defined(__MMA__)
+#define EIGEN_ALTIVEC_MMA_ONLY 1
 #endif
 
 #endif // EIGEN_ALTIVEC_MMA_SUPPORT