http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54236
--- Comment #7 from Oleg Endo <olegendo at gcc dot gnu.org> ---
(In reply to Oleg Endo from comment #6)
> Created attachment 31144 [details]
> stitching addc insns
>
> The attached patch is an example that shows how widening additions can be
> stitched together. One application would be arithmetic on user defined
> integer types with an arbitrary number of bits.
> For example (requires c++11):
>
> template <unsigned int Bits> class integer
> {
> public:
> typedef unsigned int word_type;
> typedef unsigned long long ext_word_type;
>
> static constexpr unsigned int bit_count = Bits;
> static constexpr unsigned int word_bit_count = sizeof (word_type) * 8;
> static constexpr unsigned int word_count = (bit_count + word_bit_count - 1)
> / word_bit_count;
>
> private:
> word_type word[word_count];
>
> public:
> friend integer
> operator + (const integer& a, const integer& b)
> {
> integer result;
>
> word_type carry = 0;
> for (unsigned int i = 0; i < word_count; ++i)
> {
> auto sum = (ext_word_type)a.word[i] + (ext_word_type)b.word[i] + carry;
> result.word[i] = (word_type)sum;
> carry = (sum >> word_bit_count) == 0 ? 0 : 1;
> }
>
> return result;
> }
> };
>
I forgot to mention that the patch works only if the type of the carry variable
is 'unsigned int' or 'unsigned long long'. If a signed or boolean type is used
some weird things seem to happen and there are some sign extensions somewhere
around the T bit.
