Strange C++ performance difference?
- by STingRaySC
I just stumbled upon a change that seems to have counterintuitive performance ramifications. Can anyone provide a possible explanation for this behavior?
Original code:
for (int i = 0; i < ct; ++i) {
// do some stuff...
int iFreq = getFreq(i);
double dFreq = iFreq;
if (iFreq != 0) {
// do some stuff with iFreq...
// do some calculations with dFreq...
}
}
While cleaning up this code during a "performance pass," I decided to move the definition of dFreq inside the if block, as it was only used inside the if. There are several calculations involving dFreq so I didn't eliminate it entirely as it does save the cost of multiple run-time conversions from int to double. I expected no performance difference, or if any at all, a negligible improvement. However, the perfomance decreased by nearly 10%. I have measured this many times, and this is indeed the only change I've made. The code snippet shown above executes inside a couple other loops. I get very consistent timings across runs and can definitely confirm that the change I'm describing decreases performance by ~10%. I would expect performance to increase because the int to double conversion would only occur when iFreq != 0.
Chnaged code:
for (int i = 0; i < ct; ++i) {
// do some stuff...
int iFreq = getFreq(i);
if (iFreq != 0) {
// do some stuff with iFreq...
double dFreq = iFreq;
// do some stuff with dFreq...
}
}
Can anyone explain this? I am using VC++ 9.0 with /O2. I just want to understand what I'm not accounting for here.
