Re: [PATCH v5] eal/x86: optimize memcpy of small sizes

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 12 Jan 2026 11:00:36 -0500
Scott Mitchell <scott.k.mitch1@gmail.com> wrote:

> >
> > The discussion about the optimized checksum function [1] has shown us that memcpy() sometimes prevents Clang from optimizing (loop unrolling and vectorizing) and potentially causes strict aliasing bugs with GCC, so I will work on a new patch version that keeps using the above types, instead of introducing memcpy() inside rte_memcpy().
> >
> > [1]: https://inbox.dpdk.org/dev/CAFn2buBzBLFLVN-K=u3MgBEbQ-hqbgJLVpDx3vSXVKJpa0yPNg@mail.gmail.com/
> >  
> 
> Great timing for this thread :)
> 
> My observation:
> - clang is unable to apply optimizations with RTE_PTR_[ADD,SUB]
> like loop unrolling and vectorization (e.g. cksum)
> - Even when clang/gcc do apply optimizations the assembly can be non-optimal
> - direct usage of unaligned_NN_t types can cause incorrect results
> (due to gcc bugs)
> 
> I don't think "rte_NN_alias" structs are safe on architectures that don't allow
> unaligned access bcz the inner "val" needs to indicate it maybe for
> unaligned access.
> 
> My suggestion:
> 1. Fix unaligned_NN_t types to ensure compiler doesn't aggressively
> apply strict-alias
> optimizations resulting in incorrect results
> (https://patches.dpdk.org/project/dpdk/patch/20260112120411.27314-2-scott.k.mitch1@gmail.com/).
> Intermediate structs rte_NN_alias are then unnecessary and we can directly use
> unaligned_NN_t instead (e.g.
> https://patches.dpdk.org/project/dpdk/patch/20260112120411.27314-3-scott.k.mitch1@gmail.com/)
> 
> 2. Improve RTE_PTR_[ADD,SUB] to be more compiler friendly
> (https://patches.dpdk.org/project/dpdk/patch/20260112154059.36879-1-scott.k.mitch1@gmail.com/)

FYI the Linux kernel avoids the memcpy silliness.
Mostly by identifying architectures where unaligned access is non-issue.
On x86, unaligned access works fine. As I remember it works on ARM as well.
The only place where unaligned can break badly is when this is an atomic operation.