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The same operations performing different times

I am optimizing my code for my n-body simulator, and when profiling my code I saw this:

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These two lines,

float diffX = (pNode->CenterOfMassx - pBody->posX);
float diffY = (pNode->CenterOfMassy - pBody->posY);

Where pNodeis a pointer to an object of the type Nodethat I defined and contains (with other things) 2 floats, CenterOfMassxandCenterOfMassy

Where pBodyis a pointer to an object of the type Bodythat I defined and contains (with other things) 2 floats, posXand posY.

It should take the same amount of time, but not do it. In fact, the first row takes into account 0.46% of the sample of functions, and the second - 5.20%.

Now I see that the second line contains 3 commands, and the first - only one.

My question is: why do they seem to do the same thing, but in practice different things?

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movq xmm0, [esi+30h]   # or movlps, but that wouldn't break the dep
movq xmm1, [edi]       # on the old value of xmm0 / xmm1
subps xmm0, xmm1

diffX diffY 0 1 xmm0, regs, .

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