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(...);
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