https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102974
--- Comment #13 from cqwrteur <unlvsur at live dot com> ---
Hi, the problem comes out GCC does not do a very good job to deal with crypto
computations that usually exploit all sorts of patterns.
template<typename T>
inline constexpr T add_carry_no_carry_in(T a,T b,T& carryout) noexcept
{
T res{a+b};
carryout=res<a;
return res;
}
template<typename T>
inline constexpr T add_carry(T a,T b,T carryin,T& carryout) noexcept
{
assume(carryout==0||carryout==1);
a=add_carry_no_carry_in(carryin,a,carryout);
a=add_carry_no_carry_in(a,b,carryin);
carryout+=carryin;
return a;
}
See this pattern
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106865
I suggest just adding this addc pattern to GCC instead of adding a built-in
like clang. This can improve the existing code. It is, however, needed for
adding a backend hook for dealing with it.
