Drop alignment requirements for Jets.
Change-Id: I7cd1e8f73c6d3e0b9369e567149205075bfbcd82
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 +
