On 07/08/07, David Cournapeau <[EMAIL PROTECTED]> wrote: > Anne, you said previously that it was easy to allocate buffers for a > given alignment at runtime. Could you point me to a document which > explains how ? For platforms without posix_memalign, I don't see how to > implement a memory allocator with an arbitrary alignment (more > precisely, I don't see how to free it if I cannot assume a fixed > alignement: how do I know where the "real" pointer is ?).
Well, it can be done in Python: just allocate a too-big ndarray and take a slice that's the right shape and has the right alignment. But this sucks. Stephen G. Johnson posted code earlier in this thread that provides a portable aligned-memory allocator - it handles the freeing by (always) storing enough information to recover the original pointer in the padding space. (This means you always need to pad, which is a pain, but there's not much you can do about that.) His implementation stores the original pointer just before the beginning of the aligned data, so _aligned_free is free(((void**)ptr)[-1]). If you were worried about space (rather than time) you could store a single byte just before the pointer whose value indicated how much padding was done, or whatever. These schemes all waste space, but unless malloc's internal structures are the size of the alignment block, it's almost unavoidable to waste some space; the only way around it I can see is if the program also allocates lots of small, odd-shaped, unaligned blocks of memory that can be used to fill the gaps (and even then I doubt any sensible malloc implementation fills in little gaps like this, since it seems likely to lead to memory fragmentation). A posix_memalign that is built into malloc can do better than any implementation that isn't, though, with the possible exception of a specialized pool allocator built with aligned allocation in mind. Anne _______________________________________________ Numpy-discussion mailing list Numpy-discussion@scipy.org http://projects.scipy.org/mailman/listinfo/numpy-discussion
