On Tue, Apr 9, 2013 at 5:36 PM, Lawrence Crowl <cr...@google.com> wrote: > > On Apr 9, 2013 2:02 AM, "Richard Biener" <richard.guent...@gmail.com> wrote: >> >> On Mon, Apr 8, 2013 at 10:39 PM, Lawrence Crowl <cr...@google.com> wrote: >> > On 4/8/13, Kenneth Zadeck <zad...@naturalbridge.com> wrote: >> >> The other problem, which i invite you to use the full power of >> >> your c++ sorcery on, is the one where defining an operator so >> >> that wide-int + unsigned hwi is either rejected or properly >> >> zero extended. If you can do this, I will go along with >> >> your suggestion that the internal rep should be sign extended. >> >> Saying that constants are always sign extended seems ok, but there >> >> are a huge number of places where we convert unsigned hwis as >> >> the second operand and i do not want that to be a trap. I went >> >> thru a round of this, where i did not post the patch because i >> >> could not make this work. And the number of places where you >> >> want to use an hwi as the second operand dwarfs the number of >> >> places where you want to use a small integer constant. >> > >> > You can use overloading, as in the following, which actually ignores >> > handling the sign in the representation. >> > >> > class number { >> > unsigned int rep1; >> > int representation; >> > public: >> > number(int arg) : representation(arg) {} >> > number(unsigned int arg) : representation(arg) {} >> > friend number operator+(number, int); >> > friend number operator+(number, unsigned int); >> > friend number operator+(int, number); >> > friend number operator+(unsigned int, number); >> > }; >> > >> > number operator+(number n, int si) { >> > return n.representation + si; >> > } >> > >> > number operator+(number n, unsigned int ui) { >> > return n.representation + ui; >> > } >> > >> > number operator+(int si, number n) { >> > return n.representation + si; >> > } >> > >> > number operator+(unsigned int ui, number n) { >> > return n.representation + ui; >> > } >> >> That does not work for types larger than int/unsigned int as HOST_WIDE_INT >> usually is (it's long / unsigned long). When you pass an int or unsigned >> int >> to >> >> number operator+(unsigned long ui, number n); >> number operator+(long ui, number n) >> >> you get an ambiguity. You can "fix" that by relying on template argument >> deduction and specialization instead of on overloading and integer >> conversion >> rules. > > Ah, I hadn't quite gotten the point. This problem is being fixed in the > standard, but that won't help GCC anytime soon. > >> >> > If the argument type is of a template type parameter, then >> > you can test the template type via >> > >> > if (std::is_signed<T>::value) >> > .... // sign extend >> > else >> > .... // zero extend >> > >> > See http://www.cplusplus.com/reference/type_traits/is_signed/. >> >> Yes, if we want to use the standard library. For what integer types >> is std::is_signed required to be implemented in C++98 (long long?)? > > It is in C++03/TR1, which is our base requirement. Otherwise, we can test > ~(T)0<(T)0.
Yeah, I think we want to test ~(T)0<(T)0 here. Relying on C++03/TR1 is too obscure if there is an easy workaround. Richard. >> Consider non-GCC host compilers. >> >> Richard. >> >> > If you want to handle non-builtin types that are asigne dor unsigned, >> > then you need to add a specialization for is_signed. >> > >> > -- >> > Lawrence Crowl