Apply [[no_unique_address]] to predicates

[morinmorin]

This PR applies RANGES_NO_UNIQUE_ADDRESS to functions/predicates in

• transform, take_while, drop_while views
• logical_negate

The latter allows to EBO out empty predicates in filter_view.

[ericniebler]

Does any compiler yet do anything meaningful with this attribute? I'm just curious.

[morinmorin]

GCC 9 has [[no_unique_address]]. Clang trunk also has the attribute, though I didn't test on that compiler.

Here are the results of differences in size of views (before and after applying this patch, tested on GCC 9.2):

views	before	after
arr | transform	24	16
arr | transform | transform	40	24
arr | transform | transform | transform	56	32
transform(rng1, rng2, f)	24	24
arr | take_while	24	16
arr | drop_while	40	32
arr | filter	40	32
arr | remove_if	32	32

Source code:

#include <iostream>
#include <range/v3/core.hpp>
#include <range/v3/view.hpp>

struct F
{
    bool operator()(int x) const { return x % 2 == 0; }
    int operator()(int x, int y) const { return x + y; }
};

int main()
{
    using namespace ranges::views;

    int arr[3] = {4, 5, 6};
    
    std::cout << "arr | transform" << "\n";
    std::cout << sizeof(arr | transform(F{})) << "\n";

    std::cout << "arr | transform | transform" << "\n";
    std::cout << sizeof(arr | transform(F{}) | transform(F{})) << "\n";

    std::cout << "arr | transform | transform" << "\n";
    std::cout << sizeof(arr | transform(F{}) | transform(F{}) | transform(F{})) << "\n";

    std::cout << "transform (binary)" << "\n";
    std::cout << sizeof(transform(arr, arr, F{})) << "\n";

    std::cout << "arr | take_while" << "\n";
    std::cout << sizeof(arr | take_while(F{})) << "\n";

    std::cout << "arr | drop_while" << "\n";
    std::cout << sizeof(arr | drop_while(F{})) << "\n";

    std::cout << "arr | filter" << "\n";
    std::cout << sizeof(arr | filter(F{})) << "\n";

    std::cout << "arr | remove_if" << "\n";
    std::cout << sizeof(arr | remove_if(F{})) << "\n";
}

[ericniebler]

So cool! Thanks!