Re: Alignment of small memory allocations returned by malloc(3)

[David Sainty <dave%dtsp.co.nz@localhost>]

On 01/02/14 09:48, Joerg Sonnenberger wrote:
> On Thu, Jan 30, 2014 at 12:10:54PM +0000, Nick Hudson wrote:
> > Standards and modern tools, e.g. gcc[1] 4.8, expect malloc to return
> > memory with alignment that is different to our current
> > jemalloc.Attached is a suggested diff to fix the problem for most
> > platforms.
> I still don't see the point of the GCC behavior and I don't agree with
> the standard interpretation. The referenced DR covers a quite different
> problem (pointer casts). It doesn't make sense to justify the larger
> alignment if accessing the storage with any such type is UB because it
> is an access beyond the end of the allocation. As such, I do strongly
> consider this an overeager optimisation.

Surely it makes (the most) sense on any platform that can't write a unit 
that small without writing to the surrounding bytes anyway. Isn't that 
the case with the (original) Alpha?

I.e. If a two byte write has to be implemented in terms of an aligned 4 
byte read-modify-write, then you wouldn't ever want to allocate a pair 
of two byte allocations contiguous in the same four bytes anyway - at 
least not if the code might be multi-threaded.  So you may as well both 
align and assume everything aligned on 4 bytes.