[PATCH v5 01/16] eal: provide pack start macro for MSVC

[Tyler Retzlaff]

On Tue, Nov 19, 2024 at 09:32:07AM +0100, Morten Brørup wrote:
> > From: Andre Muezerie [mailto:andremue at linux.microsoft.com]
> > Sent: Tuesday, 19 November 2024 05.35
> > 
> > From: Tyler Retzlaff <roretzla at linux.microsoft.com>
> > 
> > MSVC struct packing is not compatible with GCC. Provide a macro that
> > can be used to push existing pack value and sets packing to 1-byte.
> > The existing __rte_packed macro is then used to restore the pack value
> > prior to the push.
> > 
> > Instead of providing macros exclusively for MSVC and for GCC the
> > existing macro is deliberately utilized to trigger a warning if no
> > existing packing has been pushed allowing easy identification of
> > locations where the __rte_msvc_pack is missing.
> > 
> > Signed-off-by: Tyler Retzlaff <roretzla at linux.microsoft.com>
> > ---
> >  lib/eal/include/rte_common.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/lib/eal/include/rte_common.h
> > b/lib/eal/include/rte_common.h
> > index 4d299f2b36..409890863e 100644
> > --- a/lib/eal/include/rte_common.h
> > +++ b/lib/eal/include/rte_common.h
> > @@ -103,8 +103,10 @@ typedef uint16_t unaligned_uint16_t;
> >   * Force a structure to be packed
> >   */
> >  #ifdef RTE_TOOLCHAIN_MSVC
> > -#define __rte_packed
> > +#define __rte_msvc_pack __pragma(pack(push, 1))
> > +#define __rte_packed __pragma(pack(pop))
> >  #else
> > +#define __rte_msvc_pack
> >  #define __rte_packed __attribute__((__packed__))
> >  #endif
> > 
> > --
> > 2.47.0.vfs.0.3
> 
> Before proceeding with this, can we please discuss the alternative, proposed here:
> https://inbox.dpdk.org/dev/CAJFAV8yStgiBbe+Nkt9mC30r0+ZP64_kGuRHOzqd90RD2HXZyw@mail.gmail.com/
> 
> The definition of the packing macro in OVS, for reference:
> https://github.com/openvswitch/ovs/blob/main/include/openvswitch/compiler.h#L209
> 
> The current solution requires __rte_packed to be placed at the end of a structure, although __attribute__((packed)) is normally allowed at the beginning (between the "struct" tag and the name of the structure), which introduces a high risk of contributors placing it "incorrectly", thus causing errors.
> 
> I have a strong preference for an __RTE_PACKED(decl) variant.
> 
> Here's a third alternative:
> #ifdef RTE_TOOLCHAIN_MSVC
> #define __rte_msvc_pack_begin __pragma(pack(push, 1))
> #define __rte_msvc_pack_end   __pragma(pack(pop))
> #else
> #define __rte_msvc_pack_begin
> #define __rte_msvc_pack_end
> #endif
> 
> The third alternative is also problematic, e.g. if a contributor forgets the _end after the structure declaration, or adds another structure declaration before the _end.

so just some additional info here.

for the compiler that cares about the end (i.e. msvc) msvc actually
emits a warning (treated as error) if a push pack is not closed with a
pop pack within a header or a tu. that's one of the benefits of the
macros in original proposal.

> 
> -Morten