use built in alloca with align if available

Eigen/src/Core/util/Memory.h

diff --git a/Eigen/src/Core/util/Memory.h b/Eigen/src/Core/util/Memory.h
index 4409d64..7edd0a1 100644
--- a/Eigen/src/Core/util/Memory.h
+++ b/Eigen/src/Core/util/Memory.h
@@ -765,6 +765,9 @@
 // We always manually re-align the result of EIGEN_ALLOCA.
 // If alloca is already aligned, the compiler should be smart enough to optimize away the re-alignment.
 
+#if (EIGEN_COMP_GNUC || EIGEN_COMP_CLANG)
+#define EIGEN_ALIGNED_ALLOCA(SIZE) __builtin_alloca_with_align(SIZE, CHAR_BIT* EIGEN_DEFAULT_ALIGN_BYTES)
+#else
 EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE void* eigen_aligned_alloca_helper(void* ptr) {
   constexpr std::uintptr_t mask = EIGEN_DEFAULT_ALIGN_BYTES - 1;
   std::uintptr_t ptr_int = std::uintptr_t(ptr);
@@ -772,8 +775,9 @@
   std::uintptr_t offset = aligned_ptr_int - ptr_int;
   return static_cast<void*>(static_cast<uint8_t*>(ptr) + offset);
 }
-
 #define EIGEN_ALIGNED_ALLOCA(SIZE) eigen_aligned_alloca_helper(EIGEN_ALLOCA(SIZE + EIGEN_DEFAULT_ALIGN_BYTES - 1))
+#endif
+
 #else
 #define EIGEN_ALIGNED_ALLOCA(SIZE) EIGEN_ALLOCA(SIZE)
 #endif