PERF: Only compute log(fixedImageMarginalPDFValue once outside loop

A dummy value is used in the case where the log computation is not used
and would otherwise be invalid so that the log() can be precomputed
outside the looping structure.

Modules/Registration/Metricsv4/include/itkMattesMutualInformationImageToImageMetricv4.hxx

@@ -377,21 +377,24 @@ MattesMutualInformationImageToImageMetricv4<TFixedImage,
   {
     const PDFValueType fixedImageMarginalPDFValue = l_FixedImageMarginalPDF[fixedIndex];
 
-    // const bool         isNotNearZerofixedImageMarginalPDFValue = (fixedImageMarginalPDFValue > closeToZero);
-    // if (isNotNearZerofixedImageMarginalPDFValue)
+    const bool isNotNearZerofixedImageMarginalPDFValue = (fixedImageMarginalPDFValue > closeToZero);
+    // NOTE: if isNotNearZerofixedImageMarginalPDFValue is false, logfixedImageMarginalPDFValue is never used
+    //       The common case is that it is used, so only perform std::log(fixedImageMarginalPDFValue one time
+    const PDFValueType logfixedImageMarginalPDFValue = (isNotNearZerofixedImageMarginalPDFValue)
+                                                         ? std::log(fixedImageMarginalPDFValue)
+                                                         : std::numeric_limits<PDFValueType>::max();
     {
       for (unsigned int movingIndex = 0; movingIndex < temp_num_histogram_bins;
            ++movingIndex, ++jointPDFPtr /* GOTO NEXT BIN */)
       {
-        const PDFValueType movingImageMarginalPDF = this->m_MovingImageMarginalPDF[movingIndex];
-        const PDFValueType jointPDFValue = *(jointPDFPtr);
-        // check for non-zero bin contribution
-        if (jointPDFValue > closeToZero && movingImageMarginalPDF > closeToZero)
+        const PDFValueType & movingImageMarginalPDF = this->m_MovingImageMarginalPDF[movingIndex];
+        const PDFValueType & jointPDFValue = *(jointPDFPtr);
+        // check for non-zero bin contribution, if movingImageMarginalPDF <= closeToZero, then so is joinPDFValue
+        if (movingImageMarginalPDF > closeToZero && jointPDFValue > closeToZero)
         {
           const PDFValueType pRatio = std::log(jointPDFValue / movingImageMarginalPDF);
-          if (fixedImageMarginalPDFValue > closeToZero)
+          if (isNotNearZerofixedImageMarginalPDFValue)
           {
-            const PDFValueType logfixedImageMarginalPDFValue = std::log(fixedImageMarginalPDFValue);
             sum += jointPDFValue * (pRatio - logfixedImageMarginalPDFValue);
           }