Richard Henderson <richard.hender...@linaro.org> writes:
> These builtins came in clang 3.8, but are not present in gcc through
> version 11. Even in clang the optimization is not ideal except for
> x86_64, but no worse than the hand-coding that we currently do.
Given this statement....
<snip>
> +/**
> + * uadd64_carry - addition with carry-in and carry-out
> + * @x, @y: addends
> + * @pcarry: in-out carry value
> + *
> + * Computes @x + @y + *@pcarry, placing the carry-out back
> + * into *@pcarry and returning the 64-bit sum.
> + */
> +static inline uint64_t uadd64_carry(uint64_t x, uint64_t y, bool *pcarry)
> +{
> +#if __has_builtin(__builtin_addcll)
> + unsigned long long c = *pcarry;
> + x = __builtin_addcll(x, y, c, &c);
what happens when unsigned long long isn't the same as uint64_t? Doesn't
C99 only specify a minimum?
> + *pcarry = c & 1;
Why do we need to clamp it here? Shouldn't the compiler automatically do
that due to the bool?
> + return x;
> +#else
> + bool c = *pcarry;
> + /* This is clang's internal expansion of __builtin_addc. */
> + c = uadd64_overflow(x, c, &x);
> + c |= uadd64_overflow(x, y, &x);
> + *pcarry = c;
> + return x;
> +#endif
Either way if you aren't super happy with the compilers builtin and you
get equivalent code with the unambigious hand coded version then what is
the point of having a builtin leg?
> +}
> +
> +/**
> + * usub64_borrow - subtraction with borrow-in and borrow-out
> + * @x, @y: addends
> + * @pborrow: in-out borrow value
> + *
> + * Computes @x - @y - *@pborrow, placing the borrow-out back
> + * into *@pborrow and returning the 64-bit sum.
> + */
> +static inline uint64_t usub64_borrow(uint64_t x, uint64_t y, bool *pborrow)
> +{
> +#if __has_builtin(__builtin_subcll)
> + unsigned long long b = *pborrow;
> + x = __builtin_subcll(x, y, b, &b);
> + *pborrow = b & 1;
> + return x;
> +#else
> + bool b = *pborrow;
> + b = usub64_overflow(x, b, &x);
> + b |= usub64_overflow(x, y, &x);
> + *pborrow = b;
> + return x;
> +#endif
> +}
> +
> /* Host type specific sizes of these routines. */
>
> #if ULONG_MAX == UINT32_MAX
--
Alex Bennée
