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POWhy is my loop slower when I remove code
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  1. POWhy is my loop slower when I remove code
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    copied!<p>When I remove the tests to compute minimum and maximum from the loop, the execution time is actually longer than with the test. How is that possible ? </p> <p>Edit : After running more test, it seems the runtime is not constant, ie the same code can run in 9 sec or 13 sec.... So it was just a repetable coincidence. Repetable until you do enough tests that is...</p> <p>Some details :</p> <ul> <li>execution time with the min max test : 9 sec</li> <li>execution time without the min max test : 13 sec</li> <li><code>CFLAGS=-Wall -O2 -fPIC -g</code></li> <li>gcc 4.4.3 32 bit Section to remove is now indicated in code</li> </ul> <p>Some guess : bad cache interaction ?</p> <pre><code>void FillFullValues(void) { int i,j,k; double X,Y,Z; double p,q,r,p1,q1,r1; double Ls,as,bs; unsigned long t1, t2; t1 = GET_TICK_COUNT(); MinLs = Minas = Minbs = 1000000.0; MaxLs = Maxas = Maxbs = 0.0; for (i=0;i&lt;256;i++) { for (j=0;j&lt;256;j++) { for (k=0;k&lt;256;k++) { X = 0.4124*CielabValues[i] + 0.3576*CielabValues[j] + 0.1805*CielabValues[k]; Y = 0.2126*CielabValues[i] + 0.7152*CielabValues[j] + 0.0722*CielabValues[k]; Z = 0.0193*CielabValues[i] + 0.1192*CielabValues[j] + 0.9505*CielabValues[k]; p = X * InvXn; q = Y; r = Z * InvZn; if (q&gt;0.008856) { Ls = 116*pow(q,third)-16; } else { Ls = 903.3*q; } if (q&lt;=0.008856) { q1 = 7.787*q+seiz; } else { q1 = pow(q,third); } if (p&lt;=0.008856) { p1 = 7.787*p+seiz; } else { p1 = pow(p,third); } if (r&lt;=0.008856) { r1 = 7.787*r+seiz; } else { r1 = pow(r,third); } as = 500*(p1-q1); bs = 200*(q1-r1); // // cast on short int for reducing array size // FullValuesLs[i][j][k] = (char) (Ls); FullValuesas[i][j][k] = (char) (as); FullValuesbs[i][j][k] = (char) (bs); //// Remove this and get slower code if (MaxLs&lt;Ls) MaxLs = Ls; if ((abs(Ls)&lt;MinLs) &amp;&amp; (abs(Ls)&gt;0)) MinLs = Ls; if (Maxas&lt;as) Maxas = as; if ((abs(as)&lt;Minas) &amp;&amp; (abs(as)&gt;0)) Minas = as; if (Maxbs&lt;bs) Maxbs = bs; if ((abs(bs)&lt;Minbs) &amp;&amp; (abs(bs)&gt;0)) Minbs = bs; //// End of Remove } } } TRACE(_T("LMax = %f LMin = %f\n"),(MaxLs),(MinLs)); TRACE(_T("aMax = %f aMin = %f\n"),(Maxas),(Minas)); TRACE(_T("bMax = %f bMin = %f\n"),(Maxbs),(Minbs)); t2 = GET_TICK_COUNT(); TRACE(_T("WhiteBalance init : %lu ms\n"), t2 - t1); } </code></pre>
 

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