czw., 2 kwi 2026 o 09:05 Muneem via Std-Proposals
<std-proposals_at_[hidden]> napisał(a):
>
> hi!
> Your point is partly correct, but this issue is quite prevalent, below are my branches from multiple sources:
> this is the updated code:
> #include <variant>
> #include <iostream>
> #include <chrono>
> #include <ctime>
> #include <iomanip>
> #include<array>
> std::array<int, 3> array_1={1,2,3};
>
> struct A { int get() { return array_1[0]; } };
> struct B { int get() { return array_1[1]; } };
> struct C { int get() { return array_1[2]; } };
>
> struct data_accessed_through_visit {
> static std::variant<A, B, C> obj;
>
> inline int operator()(int) {
> return std::visit([](auto&& arg) {
> return arg.get();
> }, obj);
> }
> };
> std::variant<A, B, C> data_accessed_through_visit::obj=C{};
> int user_index = 0;

I can hold you right now, you use a static object for the
`std::variant`, this means you test different thing than in other
cases.
This can affect code gen and what optimizer can see and guarantee.

Besides, why use `variant` here? It was not designed for this but to
store different types in the same memory location.
And none of this is used there, only making it harder to compiler to
see what is going on.

Why not use simple pointers there? And if you need to do some
calculations function pointers?

>
> struct data_ternary {
> inline int operator()(int index) {
> return (index == 0) ? array_1[0] : (index == 1) ? array_1[1] : (index == 1) ? array_1[2] : -1;
> }
> };
>
> struct data_switched {
> inline int operator()(int index) {
> switch(index) {
> case 0: return array_1[0];
> case 1: return array_1[1];
> case 2: return array_1[2];
> default: return -1;
> }
> }
> };
>
> struct data_indexing {
> inline int operator()(int index) {
> return array_1[index];
> }
> };
>
>
>
> volatile int x = 0;
> constexpr uint64_t loop_count=10000;
> static void measure_switch() {
> data_switched obj;
> for (int i=0; i++<loop_count;) {
> x = obj(user_index);
> }
> }
>
> static void measure_visit() {
> data_accessed_through_visit obj;
> for (int i=0; i++<loop_count;) {
> x = obj(user_index);
> }
> }

Why not use:
```
static void measure_visit() {
    data_accessed_through_visit obj
    return std::visit([](auto&& arg) {
        for (int i=0; i++<loop_count;) {
            x = arg.get();
        }
    }, obj);
}
```
And this is more similar to what the compiler sees in other test cases.

>
> static void measure_ternary() {
> data_ternary obj;
> for (int i=0; i++<loop_count;) {
> x = obj(user_index);
> }
> }
> static void measure_indexing() {
> data_indexing obj;
> for (int i=0; i++<loop_count;) {
> x = obj(user_index);
> }
> }
>
> template<typename func_t>
> void call_func(func_t callable_obj, int arg){
> const auto start = std::chrono::steady_clock::now();
>
> constexpr int how_much_to_loop=1000;
> for(int i=0; i++<how_much_to_loop;){
> callable_obj();
> }
> const auto end = std::chrono::steady_clock::now();
> auto result= std::chrono::duration_cast<std::chrono::nanoseconds>(end - start).count()/how_much_to_loop;
> std::cout<<result/how_much_to_loop<<std::endl;
>
> }
>
> int main() {
> std::cout << "Enter index (0 for A, 1 for B, 2 for C): ";
> if (!(std::cin >> user_index)) return 1;
>
> // Set the variant state
> if (user_index == 0) data_accessed_through_visit::obj = A{};
> else if (user_index == 1) data_accessed_through_visit::obj = B{};
> else if (user_index == 2) data_accessed_through_visit::obj = C{};
>
> std::cout << "Time (ns) for switch: ";
> call_func(measure_switch, user_index);
>
> std::cout << "Time (ns) for visit: ";
> call_func(measure_visit, user_index);
>
> std::cout << "Time (ns) for ternary: ";
> call_func(measure_ternary, user_index);
>
> std::cout << "Time (ns) for subscript: ";
> call_func(measure_indexing, user_index);
>
> return 0;
> }
> the bench marks consistently show that these syntax constructs do matter (the smaller the index range is, the more the compiler can flatten it and know how to branch), notice how ternary is outperforming them all even though its nesting, This means that adding new syntax with the sole purpose to give compilers as much information as possible is actually useful.

And what do you propose here exactly? diffrent structures behave differently.
How exactly it will look and work, right now you use lot of vague
statement that do not show anything.

> Consider how templates and instantiation give the compiler extra insight. why? because templates are instantiated at the point of instantiation which can be delayed upto link time. these are the benchmarks:

what? did you confuse it with Link Time Optimization? instantiation is
in the Transition Unit not at link time as this would be too late.
We have Extern Templates but then compiler in TU becomes blind to that
code as it instantiationed in other TU, LTO can help there but
then we have exactly the same case if we use normal functions.

> benchmarks for g++:
> Enter index (0 for A, 1 for B, 2 for C): 2
> Time (ns) for switch: 33
> Time (ns) for visit: 278
> Time (ns) for ternary: 19
> Time (ns) for subscript: 34
> PS C:\Users\drnoo\Downloads> .\a.exe
> Enter index (0 for A, 1 for B, 2 for C): 2
> Time (ns) for switch: 33
> Time (ns) for visit: 296
> Time (ns) for ternary: 20
> Time (ns) for subscript: 35
> PS C:\Users\drnoo\Downloads> .\a.exe
> Enter index (0 for A, 1 for B, 2 for C): 2
> Time (ns) for switch: 34
> Time (ns) for visit: 271
> Time (ns) for ternary: 17
> Time (ns) for subscript: 33
> PS C:\Users\drnoo\Downloads> .\a.exe
> Enter index (0 for A, 1 for B, 2 for C): 2
> Time (ns) for switch: 34
> Time (ns) for visit: 281
> Time (ns) for ternary: 19
> Time (ns) for subscript: 32
> PS C:\Users\drnoo\Downloads> .\a.exe
> Enter index (0 for A, 1 for B, 2 for C): 2
> Time (ns) for switch: 34
> Time (ns) for visit: 282
> Time (ns) for ternary: 20
> Time (ns) for subscript: 34
> I really have to go to sleep now ( I am having some issues with visual studio 2026), I Hope, it would be acceptable for me to send the benchmarks for that tomorrow.
>
> regards, Muneem
>
>
> On Thu, Apr 2, 2026 at 10:54 AM Thiago Macieira via Std-Proposals <std-proposals_at_[hidden]> wrote:
>>
>> On Wednesday, 1 April 2026 21:56:44 Pacific Daylight Time Muneem via Std-
>> Proposals wrote:
>> > /*
>> > Time (ns) for switch: 168100
>> > Time (ns) for visit: 3664100
>> > Time (ns) for ternary: 190900
>> > It keeps on getting worse!
>> > */
>>
>> So far you've maybe shown that one implementation is generating bad code. Have
>> you tried others?
>>
>> You need to prove that this is an inherent and unavoidable problem of the
>> requirements, not that it just happened to be bad for this implementation.
>> Just quickly reading the proposed benchmark code, it would seem there's no
>> such inherent reason and you're making an unfounded and probably incorrect
>> assumption about how things actually work.
>>
>> In fact, I pasted a portion of your code into godbolt just to see what the
>> variant visit code, which you claim to be unnecessarily slow, would look like:
>> https://gcc.godbolt.org/z/WK5bMzcae
>>
>> The first thing to note in the GCC/libstdc++ pane is that it does not use
>> user_index. The compiler thinks it's a constant, meaning this benchmark is
>> faulty. And thus it has constant-propagated this value and is *incredibly*
>> efficient in doing nothing useful. MSVC did likewise.
>>
>> Since MSVC outputs the out-of-line copy of inlined functions, we can see the
>> operator() expansion without the proapagation of the user_index constant. And
>> it's no different than what a ternary or switch would look like.
>>
>> In the Clang/libc++ pane, we see indirect function calls. I don't know why
>> libc++ std::variant is implemented this way, but it could be why it is slow
>> for you if you're using this implementation. If you tell Clang to instead use
>> libstdc++ (remove the last argument of the command-line), the indirect
>> function call disappears and we see an unrolled loop of loading the value 10.
>> That would mean Clang is even more efficient at doing nothing.
>>
>> Conclusion: it looks like your assumption that there is a problem to be solved
>> is faulty. There is no problem.
>>
>> --
>> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
>> Principal Engineer - Intel Data Center - Platform & Sys. Eng.
>> --
>> Std-Proposals mailing list
>> Std-Proposals_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals