Drop alignment requirements for Jets.

Change-Id: I7cd1e8f73c6d3e0b9369e567149205075bfbcd82

include/ceres/internal/fixed_array.h

diff --git a/include/ceres/internal/fixed_array.h b/include/ceres/internal/fixed_array.h
index e11b71b..ce777d2 100644
--- a/include/ceres/internal/fixed_array.h
+++ b/include/ceres/internal/fixed_array.h
@@ -139,7 +139,6 @@
   // of this code will be broken.
   struct InnerContainer {
     T element;
-    EIGEN_MAKE_ALIGNED_OPERATOR_NEW
   };
 
   // How many elements should we store inline?
@@ -158,7 +157,7 @@
 
   // Allocate some space, not an array of elements of type T, so that we can
   // skip calling the T constructors and destructors for space we never use.
-  ManualConstructor<InnerContainer> CERES_ALIGN_ATTRIBUTE(16) inline_space_[kInlineElements];
+  ManualConstructor<InnerContainer> inline_space_[kInlineElements];
 };
 
 // Implementation details follow

include/ceres/jet.h

diff --git a/include/ceres/jet.h b/include/ceres/jet.h
index e94df5d..49a9098 100644
--- a/include/ceres/jet.h
+++ b/include/ceres/jet.h
@@ -211,20 +211,25 @@
     return *this;
   }
 
-  // The infinitesimal part comes before the scalar part to ensure
-  // alignment. Jets get allocated as an array of type FixedArray
-  // which can allocate memory on the stack or on the heap depending
-  // upon the size of the array. We force the memory allocated on the
-  // stack to be 16 byte boundary aligned, but we also need to ensure
-  // that the elements of the struct are themselves aligned.
-  Eigen::Matrix<T, N, 1> v;  // The infinitesimal part.
-  T a;  // The scalar part.
+  // The scalar part.
+  T a;
 
-  // Needed to make sure that new instances of Jets are properly
-  // aligned. For more details see
+  // The infinitesimal part.
   //
-  // http://eigen.tuxfamily.org/dox/TopicStructHavingEigenMembers.html
-  EIGEN_MAKE_ALIGNED_OPERATOR_NEW
+  // Note the Eigen::DontAlign bit is needed here because this object
+  // gets allocated on the stack and as part of other arrays and
+  // structs. Forcing the right alignment there is the source of much
+  // pain and suffering. Even if that works, passing Jets around to
+  // functions by value has problem because the C++ ABI does not
+  // guarantee alignment for function arguments.
+  //
+  // Setting the DontAlign bit prevents Eigen from using SSE for the
+  // various operations on Jets. This is a small performance penalty
+  // since the AutoDiff code will still expose much of the code as
+  // statically sized loops to the compiler. But given the subtle
+  // issues that arise due to alignment, especially when dealing with
+  // multiple platforms, it seems to be a trade off worth making.
+  Eigen::Matrix<T, N, 1, Eigen::DontAlign> v;
 };
 
 // Unary +