refactor: use local loop variable in copyFromFrameBuffer

This change allows the compiler to keep the loop variable (readPtr) in a
register and therefore avoid cache miss in what is essentially a more general
memcpy.

By analysing the assembly generated by both gcc 6.3.1, gcc 4.8.5 and clang 5.0
I found that these compilers interpret the mutable reference such that it
has to be written back into memory in every iteration.

The performance regression  was when upgrading the compilers for Foundry's
Nuke. Our original build of OpenEXR 2.2.0 built with GCC 4.1.2 did not exhibit
this behaviour. It yielded significant speed up in Nuke's writing speed.

Signed-off-by: Gyula Gubacsi <gyula.gubacsi@foundry.com>

OpenEXR/IlmImf/ImfMisc.cpp

@@ -1448,6 +1448,8 @@ copyFromFrameBuffer (char *& writePtr,
                      Compressor::Format format,
 		     PixelType type)
 {
+    char * localWritePtr = writePtr;
+    const char * localReadPtr = readPtr;
     //
     // Copy a horizontal row of pixels from a frame
     // buffer to an output file's line or tile buffer.
@@ -1463,29 +1465,29 @@ copyFromFrameBuffer (char *& writePtr,
         {
           case OPENEXR_IMF_INTERNAL_NAMESPACE::UINT:
 
-            while (readPtr <= endPtr)
+            while (localReadPtr <= endPtr)
             {
-                Xdr::write <CharPtrIO> (writePtr,
-                                        *(const unsigned int *) readPtr);
-                readPtr += xStride;
+                Xdr::write <CharPtrIO> (localWritePtr,
+                                        *(const unsigned int *) localReadPtr);
+                localReadPtr += xStride;
             }
             break;
 
           case OPENEXR_IMF_INTERNAL_NAMESPACE::HALF:
 
-            while (readPtr <= endPtr)
+            while (localReadPtr <= endPtr)
             {
-                Xdr::write <CharPtrIO> (writePtr, *(const half *) readPtr);
-                readPtr += xStride;
+                Xdr::write <CharPtrIO> (localWritePtr, *(const half *) localReadPtr);
+                localReadPtr += xStride;
             }
             break;
 
           case OPENEXR_IMF_INTERNAL_NAMESPACE::FLOAT:
 
-            while (readPtr <= endPtr)
+            while (localReadPtr <= endPtr)
             {
-                Xdr::write <CharPtrIO> (writePtr, *(const float *) readPtr);
-                readPtr += xStride;
+                Xdr::write <CharPtrIO> (localWritePtr, *(const float *) localReadPtr);
+                localReadPtr += xStride;
             }
             break;
 
@@ -1504,33 +1506,33 @@ copyFromFrameBuffer (char *& writePtr,
         {
           case OPENEXR_IMF_INTERNAL_NAMESPACE::UINT:
 
-            while (readPtr <= endPtr)
+            while (localReadPtr <= endPtr)
             {
                 for (size_t i = 0; i < sizeof (unsigned int); ++i)
-                    *writePtr++ = readPtr[i];
+                    *localWritePtr++ = localReadPtr[i];
 
-                readPtr += xStride;
+                localReadPtr += xStride;
             }
             break;
 
           case OPENEXR_IMF_INTERNAL_NAMESPACE::HALF:
 
-            while (readPtr <= endPtr)
+            while (localReadPtr <= endPtr)
             {
-                *(half *) writePtr = *(const half *) readPtr;
-                writePtr += sizeof (half);
-                readPtr += xStride;
+                *(half *) localWritePtr = *(const half *) localReadPtr;
+                localWritePtr += sizeof (half);
+                localReadPtr += xStride;
             }
             break;
 
           case OPENEXR_IMF_INTERNAL_NAMESPACE::FLOAT:
 
-            while (readPtr <= endPtr)
+            while (localReadPtr <= endPtr)
             {
                 for (size_t i = 0; i < sizeof (float); ++i)
-                    *writePtr++ = readPtr[i];
+                    *localWritePtr++ = localReadPtr[i];
 
-                readPtr += xStride;
+                localReadPtr += xStride;
             }
             break;
 
@@ -1539,6 +1541,9 @@ copyFromFrameBuffer (char *& writePtr,
             throw IEX_NAMESPACE::ArgExc ("Unknown pixel data type.");
         }
     }
+
+    writePtr = localWritePtr;
+    readPtr = localReadPtr;
 }
 
 void