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Re: Implicit conversion to a generic vector type


Hi Marc,

Thank you for the reply. So it's a known g++ issue with a candidate
patch. Looking at the patch, I was wondering, what precludes the
generic vector types form being proper arithmetic types?

Regards,
Martin

On 26 May 2016 at 09:02, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Thu, 26 May 2016, martin krastev wrote:
>
>> Hello,
>>
>> I've been scratching my head over an implicit conversion issue,
>> depicted in the following code:
>>
>>
>> typedef __attribute__ ((vector_size(4 * sizeof(int)))) int
>> generic_int32x4;
>>
>> struct Foo {
>>    Foo() {
>>    }
>>    Foo(const generic_int32x4& src) {
>>    }
>>    operator generic_int32x4() const {
>>        return (generic_int32x4){ 42 };
>>    }
>> };
>>
>> struct Bar {
>>    Bar() {
>>    }
>>    Bar(const int src) {
>>    }
>>    operator int() const {
>>        return 42;
>>    }
>> };
>>
>> int main(int, char**) {
>>
>>    const Bar b = Bar() + Bar();
>>    const generic_int32x4 v = (generic_int32x4){ 42 } + (generic_int32x4){
>> 42 };
>>    const Foo e = generic_int32x4(Foo()) + generic_int32x4(Foo());
>>    const Foo f = Foo() + Foo();
>>    const Foo g = (generic_int32x4){ 42 } + Foo();
>>    const Foo h = Foo() + (generic_int32x4){ 42 };
>>    return 0;
>> }
>>
>> In the above, the initialization expression for local 'b' compiles as
>> expected, and so do the expressions for locals 'v' and 'e'. The
>> initializations of locals 'f', 'g' and 'h', though, fail to compile
>> (under g++-6.1.1, likewise under 5.x and 4.x) with:
>>
>> $ g++-6 xxx.cpp
>> xxx.cpp: In function âint main(int, char**)â:
>> xxx.cpp:28:22: error: no match for âoperator+â (operand types are
>> âFooâ and âFooâ)
>>  const Foo f = Foo() + Foo();
>>                ~~~~~~^~~~~~~
>> xxx.cpp:29:40: error: no match for âoperator+â (operand types are
>> âgeneric_int32x4 {aka __vector(4) int}â and âFooâ)
>>  const Foo g = (generic_int32x4){ 42 } + Foo();
>>                                 ~~~~~~~^~~~~~~
>> xxx.cpp:30:22: error: no match for âoperator+â (operand types are
>> âFooâ and âgeneric_int32x4 {aka __vector(4) int}â)
>>  const Foo h = Foo() + (generic_int32x4){ 42 };
>>                ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> Apparently there is some implicit conversion rule that stops g++ from
>> doing the expected implicit conversions, but I can't figure out which
>> rule that is. The fact clang handles the code without an issue does
>> not help either. Any help will be appreciated.
>
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57572
>
> --
> Marc Glisse


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