On Tue, Jul 21, 2020 at 4:48 AM Bruno Haible <br...@clisp.org> wrote: > > In new gnulib code, I need to allocate memory blocks through malloc() but with > a 16-bytes alignment. > > The posix_memalign and memalign functions only help on those platforms where > they exist. It's not possible to emulate posix_memalign or memalign when they > are not present, because when malloc() returned p, we can call free (p) > but not free (p+4) or free (p+8) or similar. > > Gnulib has a module 'pagealign_alloc' but it produces a getpagesize() > alignment > - not useful (very wasteful) for a 16-bytes alignment. > > So here is a module that provides aligned_malloc() and aligned_free(). > > The alignment is given at compile-time, so that when the desired alignment is > <= the alignment guaranteed by malloc(), there is no overhead. (With an > alignment given at runtime, the aligned_free() function would have to fetch > a back-pointer in all cases, and that means wasting sizeof (void *) bytes > in the case where the desired alignment is small.) > > > 2020-07-21 Bruno Haible <br...@clisp.org> > > aligned-malloc: Add tests. > * tests/test-aligned-malloc.c: New file. > * modules/aligned-malloc-tests: New file. > > aligned-malloc: New module. > * lib/aligned-malloc.h: New file. > * m4/malloc-align.m4: New file. > * modules/aligned-malloc: New file. > * doc/posix-functions/posix_memalign.texi: Mention the new module. > * doc/glibc-functions/memalign.texi: Likewise.
Hi Bruno, Checkout https://forum.kde.org/viewtopic.php?p=66274 for some info on detection of posix_memalign. That one bit me several years ago. I think there's also another option, if interested. On x86 you can get aligned allocations with _mm_malloc. And with PPC you can get an aligned block with vec_malloc. As you probably know, Linux aligns heap allocations to 16 by default. If you want to test the code path where you are stashing away the offset at ptr[-1], do so on AIX. AIX aligns to 4 bytes by default (iirc). Jeff
