[PATCH v2 2/5] lib/eal: add portable version of __builtin_add_overflow

[Morten Brørup]

> From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> Sent: Monday, 6 January 2025 12.07
> 
> On Fri, Jan 03, 2025 at 12:39:38PM -0800, Andre Muezerie wrote:
> > __builtin_add_overflow is gcc specific. There's a need for a portable
> > version that can also be used with other compilers.
> >
> > This patch introduces rte_add_overflow.
> >
> > +/*
> > + * Function that allows performing simple arithmetic operations
> together with
> > + * checking whether the operation overflowed.
> > + * Example of usage:
> > + *     uint8_t overflow;
> > + *     uint16_t a, b, result;
> > + *     a = 1;
> > + *     b = 2;
> > + *     overflow = rte_add_overflow(a, b, &result);
> > + */
> > +#ifdef RTE_TOOLCHAIN_MSVC
> > +#define rte_add_overflow(a, b, res) _Generic((a), \
> > +	uint8_t : _addcarry_u8, \
> > +	uint16_t : _addcarry_u16, \
> > +	uint32_t : _addcarry_u32, \
> > +	uint64_t : _addcarry_u64)(0, a, b, res)
> > +#else
> > +#define rte_add_overflow(a, b, res) _Generic((a), \
> > +	uint8_t : __builtin_add_overflow, \
> > +	uint16_t : __builtin_add_overflow, \
> > +	uint32_t : __builtin_add_overflow, \
> > +	uint64_t : __builtin_add_overflow)(a, b, res)
> > +#endif
> 
> For the gcc version, can you just simplify to the one-line below?
> 
> #define rte_add_overflow __builtin_add_overflow

Yes, but then GCC compilation would not fail if "a" has some other type than the four types explicitly supported.
I prefer keeping the method used this v2 patch.