Only mark unaligned types in buffers (#505)

Previously all types are marked unaligned in buffer format strings,
now we test for alignment before adding the '=' marker.

include/pybind11/numpy.h

@@ -756,6 +756,7 @@ struct field_descriptor {
     const char *name;
     size_t offset;
     size_t size;
+    size_t alignment;
     std::string format;
     dtype descr;
 };
@@ -796,8 +797,10 @@ inline PYBIND11_NOINLINE void register_structured_dtype(
     for (auto& field : ordered_fields) {
         if (field.offset > offset)
             oss << (field.offset - offset) << 'x';
-        // note that '=' is required to cover the case of unaligned fields
-        oss << '=' << field.format << ':' << field.name << ':';
+        // mark unaligned fields with '='
+        if (field.offset % field.alignment)
+            oss << '=';
+        oss << field.format << ':' << field.name << ':';
         offset = field.offset + field.size;
     }
     if (itemsize > offset)
@@ -857,6 +860,7 @@ private:
 #define PYBIND11_FIELD_DESCRIPTOR_EX(T, Field, Name)                                          \
     ::pybind11::detail::field_descriptor {                                                    \
         Name, offsetof(T, Field), sizeof(decltype(std::declval<T>().Field)),                  \
+        alignof(decltype(std::declval<T>().Field)),                                           \
         ::pybind11::format_descriptor<decltype(std::declval<T>().Field)>::format(),           \
         ::pybind11::detail::npy_format_descriptor<decltype(std::declval<T>().Field)>::dtype() \
     }

tests/test_numpy_dtypes.py

@@ -30,13 +30,13 @@ def test_format_descriptors():
     assert re.match('^NumPy type info missing for .*UnboundStruct.*$', str(excinfo.value))
 
     assert print_format_descriptors() == [
-        "T{=?:x:3x=I:y:=f:z:}",
-        "T{=?:x:=I:y:=f:z:}",
-        "T{=T{=?:x:3x=I:y:=f:z:}:a:=T{=?:x:=I:y:=f:z:}:b:}",
-        "T{=?:x:3x=I:y:=f:z:12x}",
-        "T{8x=T{=?:x:3x=I:y:=f:z:12x}:a:8x}",
-        "T{=3s:a:=3s:b:}",
-        'T{=q:e1:=B:e2:}'
+        "T{?:x:3xI:y:f:z:}",
+        "T{?:x:=I:y:=f:z:}",
+        "T{T{?:x:3xI:y:f:z:}:a:T{?:x:=I:y:=f:z:}:b:}",
+        "T{?:x:3xI:y:f:z:12x}",
+        "T{8xT{?:x:3xI:y:f:z:12x}:a:8x}",
+        "T{3s:a:3s:b:}",
+        'T{q:e1:B:e2:}'
     ]