Compiler optimization causing the performance to slow down
- by aJ
I have one strange problem. I have following piece of code:
template<clss index, class policy>
inline int CBase<index,policy>::func(const A& test_in, int* srcPtr ,int* dstPtr)
{
int width = test_in.width();
int height = test_in.height();
double d = 0.0; //here is the problem
for(int y = 0; y < height; y++)
{
//Pointer initializations
//multiplication involving y
//ex: int z = someBigNumber*y + someOtherBigNumber;
for(int x = 0; x < width; x++)
{
//multiplication involving x
//ex: int z = someBigNumber*x + someOtherBigNumber;
if(soemCondition)
{
// floating point calculations
}
*dstPtr++ = array[*srcPtr++];
}
}
}
The inner loop gets executed nearly 200,000 times and the entire function takes 100 ms for completion. ( profiled using AQTimer)
I found an unused variable double d = 0.0; outside the outer loop and removed the same. After this change, suddenly the method is taking 500ms for the same number of executions. ( 5 times slower).
This behavior is reproducible in different machines with different processor types.
(Core2, dualcore processors).
I am using VC6 compiler with optimization level O2.
Follwing are the other compiler options used :
-MD -O2 -Z7 -GR -GX -G5 -X -GF -EHa
I suspected compiler optimizations and removed the compiler optimization /O2. After that function became normal and it is taking 100ms as old code.
Could anyone throw some light on this strange behavior?
Why compiler optimization should slow down performance when I remove unused variable ?
Note: The assembly code (before and after the change) looked same.
