[Cython] [PATCH] make memoryviews work when strides is NULL (unfinished)

[Christoph Groth]

Hi,

Thanks a lot for the recently added generic memoryview support, it's
just what we needed to optimize an important part of our program.

However, I believe that there's a problem with Cython's support for
objects which do not provide "strides" information (because they are
C-contiguous).

The buffer API documentation [1] is a bit ambiguous on this, but it says

"If strides is NULL, the array is interpreted as a standard
n-dimensional C-array."

In any case, numpy happily accepts such buffers and computes the
"strides" itself.

A thread [2] on Python's devel mailing list also supports the view that
setting strides to NULL is OK even if strides were requested.

You might ask: why not simply provide strides when they have been
requested?  The problem is to find space for them.  malloc'ing or
PyMem_Alloc'ing memory for strides each time when a buffer is requested
is slow, and reserving space for strides within the object is wasteful
when "shape" is all that's needed and you have _many_ small objects
which expose the buffer interface.

The provided patch works for our use case, but someone with a good
understanding of Cython's memoryview support should finish it.  I
believe the patch will break more complicated buffer use cases.

[1] http://docs.python.org/dev/c-api/buffer.html
[2] http://thread.gmane.org/gmane.comp.python.devel/134387

Thank you for considering fixing this issue in Cython,

Christoph


---
 Cython/Utility/MemoryView_C.c |   19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/Cython/Utility/MemoryView_C.c b/Cython/Utility/MemoryView_C.c
index 84e55da..c2524b0 100644
--- a/Cython/Utility/MemoryView_C.c
+++ b/Cython/Utility/MemoryView_C.c
@@ -207,12 +207,6 @@ static int __Pyx_ValidateAndInit_memviewslice(
         goto fail;
     }
 
-    if (!buf->strides) {
-        PyErr_SetString(PyExc_ValueError,
-            "buffer does not supply strides necessary for memoryview.");
-        goto fail;
-    }
-
     for(i=0; i<ndim; i++) {
         spec = axes_specs[i];
 
@@ -320,8 +314,19 @@ __Pyx_init_memviewslice(struct __pyx_memoryview_obj *memview,
         goto fail;
     }
 
+    if (buf->strides) {
+        for (i = 0; i < ndim; i++) {
+            memviewslice->strides[i] = buf->strides[i];
+        }
+    } else {
+        Py_ssize_t stride = buf->itemsize;
+        for (i = ndim - 1; i >= 0; i--) {
+            memviewslice->strides[i] = stride;
+            stride *= buf->shape[i];
+        }
+    }
+
     for (i = 0; i < ndim; i++) {
-        memviewslice->strides[i] = buf->strides[i];
         memviewslice->shape[i]   = buf->shape[i];
         if (buf->suboffsets) {
             memviewslice->suboffsets[i] = buf->suboffsets[i];
-- 
1.7.10.4