[llvm-dev] Default alignment for 'malloc'

[Joerg Sonnenberger via llvm-dev]

On Mon, Oct 03, 2016 at 12:23:12PM -0700, Pete Cooper wrote:
> 
> > On Oct 3, 2016, at 11:01 AM, Joerg Sonnenberger via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> > 
> > On Mon, Oct 03, 2016 at 02:43:03PM +0200, Michael Kruse via llvm-dev wrote:
> >> 2016-10-03 13:55 GMT+02:00 Martin J. O'Riordan via llvm-dev
> >> <llvm-dev at lists.llvm.org>:
> >>> I am trying to implement some new alignment based optimisations in our
> >>> target backend, and I am wondering if there a way a target can specify that
> >>> ‘malloc’, ‘realloc’ and ‘calloc’ always return a pointer to memory that is
> >>> aligned to a particular boundary?
> >> 
> >> malloc is guaranteed to be properly aligned for any C type. This would
> >> be 8 bytes on most systems for double. However, I think in practice
> >> most modern implementations return 16-byte aligned pointers. I don't
> >> think there is a way to annotate calls malloc to have some specific
> >> alignment from the backend, that has effect on passes before the
> >> backend.
> > 
> > Note that this only applies to base types. Vector types certainly can
> > require larger alignment in practice and that's why posix_memalign
> > exists.
> Is memalign stil needed for vector types?

If you want to be portable to more than one libc implementation, yes.
Base alignment on i386 is still 32bit only for example in the SysV ABI.
I'm not even sure what the requirements for AVX is, e.g. if they finally
bumped it to 256bit for some things.

Joerg