RE: [PATCH v5] eal: fix unaligned loads/stores in rte_memcpy_generic

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Luc,

> > > Calls to rte_memcpy_generic could result in unaligned loads/stores for
> > > 1 < n < 16. This is undefined behavior according to the C standard,
> > > and it gets flagged by the clang undefined behavior sanitizer.
> > >
> > > rte_memcpy_generic is called with unaligned src and dst addresses.
> > > When 1 < n < 16, the code would cast both src and dst to a qword,
> > > dword or word pointer, without verifying the alignment of src/dst. The
> > > code was changed to use a packed structure to perform the unaligned
> > > load/store operations. This results in unaligned load/store operations
> > > to be C standards-compliant.
> >
> > Still not sure we need to fix that:
> > This is x86 specific code-path, and as I remember on x86 there are no
> > penalties for unaligned access to 2/4/8 byte values.
> > Though I like introduction of rte_mov15_or_less() function -t helps
> > with code dedup.
> 
> Thanks for your input Konstantin. Much appreciated. Just to make sure
> I understand, can you please confirm that we do not want to fix the
> fact that unaligned access in C is undefined behaviour?

Yes, I don't think it is a real problem in that particular case.

> I understand
> that X86 allows unaligned accesses but since this isn't assembly code,
> we have to go by what the C standard allows.

> Also, do you have any opinion on the strict aliasing violation (as was
> pointed out by Georg)? I suggested adding __may_alias__ thinking it
> would likely fix the issue, but I'd really like to get confirmation
> from someone who has better knowledge of how the attribute works
> exactly.

Not sure I understand the problem you are referring to.
Are you saying that original rte_memcpy() code breaks strict aliasing? 
If so, could you point where exactly?