Re: [lmi] [lmi-commits] valyuta/005 4483b09 1/9: Avoid passing 'currency' by const reference

[Vadim Zeitlin]

On Wed, 20 Jan 2021 18:09:02 +0000 Greg Chicares <gchicares@sbcglobal.net> 
wrote:

GC> > so I'm surprised that the results are not exactly the same, but I just
GC> > can't explain how could they differ in this direction. I may look at the
GC> > generated assembly, if you'd like, because this is just too strange.
GC> 
GC> Good idea--that's better than measuring how long even a tiny
GC> unit test takes.

 In my simple (but still contrived enough to prevent the compiler from just
computing the value at compile-time and generating a single line of
assembly on output) example the generated assembly is exactly the same:

---------------------------------- >8 --------------------------------------
#include <cstdlib>

#ifndef PASS_BY
    #define PASS_BY
#endif

struct value_holder
{
    explicit value_holder(double PASS_BY x) : x_{x} {}

    value_holder& operator+=(value_holder PASS_BY z) {x_ += z.x_; return *this; 
}

    double x_;
};

inline bool operator==(value_holder PASS_BY lhs, value_holder PASS_BY rhs)
    { return lhs.x_ == rhs.x_; }

int main(int argc, char* argv[])
{
    auto const count = std::atoi(argc == 1 ? "10" : argv[1]);

    value_holder v0{0.0};
    value_holder v1{0.1};
    for (int n = 0; n < count; ++n)
        v0 += v1;

    return v0 == value_holder{1.0} ? EXIT_SUCCESS : EXIT_FAILURE;
}
---------------------------------- >8 --------------------------------------

 Compiling with -DPASS_BY='const&' and without it yields absolutely
identical .s files with -O2. But the code is quite different (and shorter
for the pass-by-value version) when using -O0.

GC> Provided that the generated machine code is the same, would you
GC> favor retaining this change, e.g.:
GC>   +inline bool operator==(currency lhs, currency rhs)
GC>   -inline bool operator==(currency const& lhs, currency const& rhs)

 I think passing by value is preferable as it's at worst as efficient (and
I often use debug, i.e. unoptimized, build of lmi in which it would be a
bit more efficient), shorter, and indicates that currency is a simple,
small value-semantics type, which might be not that obvious otherwise.

 Regards,
VZ

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