Re: G++ 4.0.3 - NRVO (named return value optimization) does not work if destructor/copy ctor is implicit

[Vlad]

On Jul 12, 12:24 pm, Michael Kilburn <crusader.m...@gmail.com> wrote:
> Hi,
>
> I have got a problem with GCC 4.0.3 -- if class does not have both
> explicit dtor and cctor, NRVO does not work for it. I suspect this
> problem applies to other GCC versions. This (of course) adversely
> affects compiled code performance.
> I can not explain why it happens and looking for:
> - an explanation how implicit cctor/dtor affect NRVO-related decisions
> made by compiler
> - some kind of fix/workaround
>
> Thanks!
>
> (Btw, I checked this in MSVC 2005 -- everything is fine there)
>
> Here is test code (uncomment either 1. or 2. or both and first test
> will become as fast as second one):
>
> -------- code start -------
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <time.h>
> #include <string.h>
>
> struct TestX
> {
>     /************* TEST **************/
>     // if you add any of these we get fast:
>
>     // 1.
>     //TestX() {}
>     //TestX(const TestX& rhs) { memcpy(data, rhs.data, sizeof
> (data)); }
>
>     // 2.
>     //~TestX() {}
>     /************ END TEST **************/
>
>     unsigned data[1000];
>
>     void run_test()
>     {
>         memset(data, 0, sizeof(data));
>     }
>
> };
>
> class RVO_Test
> {
> public:
>     TestX fill(unsigned i)
>     {
>         TestX data;
>         data.run_test();
>         return data;
>     }
>
>     void fill(unsigned i, TestX &data)
>     {
>         data.run_test();
>     }
>
> };
>
> int main()
> {
>     const unsigned MAX = 500000;
>
>     // test #1
>     {
>         printf("Running Option 1\n");
>         clock_t start = clock();
>
>         for(int k = 0; k < 10; k++)
>         {
>             // slow or fast depending on cc.h TEST changes
>             unsigned count = 0;
>             RVO_Test a;
>             for ( unsigned i = 0; i < MAX; ++i ) {
>                 TestX x = a.fill( MAX );
>                 count += x.data[5];
>             }
>             printf("Count: %i\n", count);
>         }
>
>         printf("Time: %f\n", double(clock() - start)/CLOCKS_PER_SEC);
>     }
>
>     // test #2
>     {
>         printf("Running Option 2\n");
>         clock_t start = clock();
>
>         for(int k = 0; k < 10; k++)
>         {
>             // fast
>             unsigned count = 0;
>             RVO_Test a;
>             for ( unsigned i = 0; i < MAX; ++i ) {
>                 TestX x;
>                 a.fill( MAX, x );
>                 count += x.data[5];
>             }
>             printf("Count: %i\n", count);
>         }
>
>         printf("Time: %f\n", double(clock() - start)/CLOCKS_PER_SEC);
>     }
>
>     return 0;
>
> }
>
> -------- code end -------
>
> Sincerely yours,
> Michael.

I'll check asm later but from what I see all your functions are
inline. So it is possible there is no NRVO at all.
It could be simple loop optimization (moving out of loop). ie
for(...) char a;
vs
char a;
for(...);