[PATCH v5 05/39] ring: use C11 alignas

[Tyler Retzlaff]

On Mon, Feb 26, 2024 at 01:23:35PM +0000, Konstantin Ananyev wrote:
> 
> 
> > 
> > The current location used for __rte_aligned(a) for alignment of types
> > and variables is not compatible with MSVC. There is only a single
> > location accepted by both toolchains.
> > 
> > For variables standard C11 offers alignas(a) supported by conformant
> > compilers i.e. both MSVC and GCC.
> > 
> > For types the standard offers no alignment facility that compatibly
> > interoperates with C and C++ but may be achieved by relocating the
> > placement of __rte_aligned(a) to the aforementioned location accepted
> > by all currently supported toolchains.
> > 
> > To allow alignment for both compilers do the following:
> > 
> > * Move __rte_aligned from the end of {struct,union} definitions to
> >   be between {struct,union} and tag.
> > 
> >   The placement between {struct,union} and the tag allows the desired
> >   alignment to be imparted on the type regardless of the toolchain being
> >   used for all of GCC, LLVM, MSVC compilers building both C and C++.
> > 
> > * Replace use of __rte_aligned(a) on variables/fields with alignas(a).
> > 
> > Signed-off-by: Tyler Retzlaff <roretzla at linux.microsoft.com>
> > Acked-by: Morten Brørup <mb at smartsharesystems.com>
> > ---
> >  lib/ring/rte_ring_core.h    | 16 +++++++++-------
> >  lib/ring/rte_ring_peek_zc.h |  4 ++--
> >  2 files changed, 11 insertions(+), 9 deletions(-)
> > 
> > diff --git a/lib/ring/rte_ring_core.h b/lib/ring/rte_ring_core.h
> > index b770873..497d535 100644
> > --- a/lib/ring/rte_ring_core.h
> > +++ b/lib/ring/rte_ring_core.h
> > @@ -19,6 +19,8 @@
> >   * instead.
> >   */
> > 
> > +#include <stdalign.h>
> > +
> >  #ifdef __cplusplus
> >  extern "C" {
> >  #endif
> > @@ -78,7 +80,7 @@ struct rte_ring_headtail {
> > 
> >  union __rte_ring_rts_poscnt {
> >  	/** raw 8B value to read/write *cnt* and *pos* as one atomic op */
> > -	RTE_ATOMIC(uint64_t) raw __rte_aligned(8);
> > +	alignas(8) RTE_ATOMIC(uint64_t) raw;
> 
> One small question - here and below, would it possible to do 'alignas(sizeof(uint64_t))' instead of 'alignas(8)?
> Or MSVC has some restrictions here?

it should be okay to use a constant expression with alignas(n) even with
MSVC because it is conformant, if it isn't it would be a bug.  it is one benefit
over using __declspec(align(a)).

i'll update the series to use sizeof.

> 
> 
> >  	struct {
> >  		uint32_t cnt; /**< head/tail reference counter */
> >  		uint32_t pos; /**< head/tail position */
> > @@ -94,7 +96,7 @@ struct rte_ring_rts_headtail {
> >