Merge pull request #6377 from ahaldane/fix_align

BUG: define "uint-alignment", fixes complex64 alignment

1 files changed, 15 insertions, 0 deletions

diff --git a/doc/release/1.16.0-notes.rst b/doc/release/1.16.0-notes.rst
index f85ecb300..72bf96295 100644
--- a/doc/release/1.16.0-notes.rst
+++ b/doc/release/1.16.0-notes.rst
@@ -69,6 +69,21 @@ old behavior, use ``np.isnat`` to explicitly check for NaT or convert
 datetime64/timedelta64 arrays with ``.astype(np.int64)`` before making
 comparisons.
 
+complex64/128 alignment has changed
+-----------------------------------
+The memory alignment of complex types is now the same as a C-struct composed of
+two floating point values, while before it was equal to the size of the type.
+For many users (for instance on x64/unix/gcc) this means that complex64 is now
+4-byte aligned instead of 8-byte aligned. An important consequence is that
+aligned structured dtypes may now have a different size. For instance,
+``np.dtype('c8,u1', align=True)`` used to have an itemsize of 16 (on x64/gcc)
+but now it is 12.
+
+More in detail, the complex64 type now has the same alignment as a C-struct
+``struct {float r, i;}``, according to the compiler used to compile numpy, and
+similarly for the complex128 and complex256 types.
+
+
 C API changes
 =============