Limit pointers and nesting

Question

I tried using limited qualified pointers and I ran into a problem. The program below is just to introduce the problem.

The calc_function function uses three pointers, which are limited, so they "MUST" not be aliases with each other. When compiling this code in visual studio, the function will be built-in, so for no reason Visual Studio 2010 ignores qualifiers. If I turn off inlining, the code will run more than six times faster (from 2200 to 360 ms). But I don’t want to disable the attachment in the whole project and the whole file (because then it will be an overhead for a call, for example, for all getters and setters, which would be terrible).

(Can the only solution disable the attachment of this function only?)

I tried to create time limits for qualified pointers in a function, both on top and in the inner loop, in order to try to tell the compiler that I promise that there are no aliases, but the compiler will not believe me and this will not work. I also tried setting the compiler options, but the only one I found that works is to disable inlining.

I would appreciate help in solving this optimization problem.

To run the program (in realeasemode), remember to use the arguments 0 1000 2000. Why use the arguments of userinput / program to make sure that the compiler cannot know whether or not anti-aliasing exists between the a, b, and c pointers.

#include <cstdlib> #include <cstdio> #include <ctime> // Data-table where a,b,c will point into, so the compiler cant know if they alias. const size_t listSize = 10000; int data[listSize]; //void calc_function(int * a, int * b, int * c){ void calc_function(int *__restrict a, int *__restrict b, int *__restrict c){ for(size_t y=0; y<1000*1000; ++y){ // <- Extra loop to be able to messure the time. for(size_t i=0; i<1000; ++i){ *a += *b; *c += *a; } } } int main(int argc, char *argv[]){ // argv SHALL be "0 1000 2000" (with no quotes) // init for(size_t i=0; i<listSize; ++i) data[i] = i; // get a, b and c from argv(0,1000,2000) int *a,*b,*c; sscanf(argv[1],"%d",&a); sscanf(argv[2],"%d",&b); sscanf(argv[3],"%d",&c); a = data + int(a); // a, b and c will (after the specified argv) be, b = data + int(b); // a = &data[0], b = &data[1000], c = &data[2000], c = data + int(c); // So they will not alias, and the compiler cant know. // calculate and take time time_t start = clock(); funcResticted(a,b,c); time_t end = clock(); time_t t = (end-start); printf("funcResticted %u (microSec)\n", t); system("PAUSE"); return EXIT_SUCCESS; } 

Answer 1

If you declare a function using __declspec(noinline) , this causes it to not be nested:

http://msdn.microsoft.com/en-us/library/kxybs02x%28v=vs.80%29.aspx

You can use this to manually disable embedding based on each function.

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As for restrict , the compiler can use it only when it wants. Thus, messing around with different versions of the same code is somewhat inevitable when trying to “trick” compilers to perform such optimizations.