Re: [PATCH v2 01/10] eal: add workaround for __builtin_constant_p

[Andre Muezerie <andremue@linux.microsoft.com>]

On Wed, Feb 19, 2025 at 09:10:11AM -0800, Stephen Hemminger wrote:
> On Wed, 19 Feb 2025 08:50:02 -0800
> Andre Muezerie <andremue@linux.microsoft.com> wrote:
> 
> > On Tue, Feb 18, 2025 at 06:12:04PM +0100, Morten Brørup wrote:
> > > > From: Andre Muezerie [mailto:andremue@linux.microsoft.com]
> > > > Sent: Tuesday, 18 February 2025 17.32
> > > > 
> > > > There's no MSVC equivalent for compiler extension __builtin_constant_p.
> > > > EAL already had __rte_constant which was used as a first attempt to
> > > > workaround __builtin_constant_p when using MSVC. However, there are
> > > > pieces of code that would benefit from being able to provide a default
> > > > value to be used instead of it being always 0 like how it was done by
> > > > __rte_constant.
> > > > 
> > > > A new macro is added here allowing such default to be provided by the
> > > > caller.  
> > > 
> > > NAK to the new macro.
> > > 
> > > Please use the sizeof() hack suggested by Stephen to provide an implementation of __rte_constant(e) for MSVC.
> > > Here's a link to an example of the hack:
> > > https://news.ycombinator.com/item?id=16720172  
> > 
> > Thanks for the link. I tried to use the hack as suggested but turns out it does
> > not work with msvc. It results in error C2100: "you cannot dereference an
> > operand of type 'void'".
> > 
> > I'm open to further suggestions.
> 
> Maybe contact the MSVC team? there really ought to be way to do this.
> The compiler knows what is constant and what is not since it needs this for static assertions.

I reached out to the MSVC team and they suggested a similar hack using _Generic, which works with
msvc. I'll send out an updated series.