initial commit on this idea

platforms/opencl/src/OpenCLContext.cpp

@@ -88,17 +88,25 @@ OpenCLContext::OpenCLContext(const System& system, int platformIndex, int device
         contextIndex = platformData.contexts.size();
         std::vector<cl::Platform> platforms;
         cl::Platform::get(&platforms);
-        if (platformIndex < 0 || platformIndex >= (int) platforms.size())
-            throw OpenMMException("Illegal value for OpenCL platform index");
-        string platformVendor = platforms[platformIndex].getInfo<CL_PLATFORM_VENDOR>();
-        vector<cl::Device> devices;
-        platforms[platformIndex].getDevices(CL_DEVICE_TYPE_ALL, &devices);
-        const int minThreadBlockSize = 32;
-        if (deviceIndex < 0 || deviceIndex >= (int) devices.size()) {
-            // Try to figure out which device is the fastest.
 
-            int bestSpeed = -1;
+        int bestSpeed = -1;
+        int bestDevice = -1;
+        int bestPlatform = -1;
+        for (j = 0; i < platforms.size(); j++) {
+            // if they supplied a valid platformIndex, we only look through that platform
+            if (j != plaformIndex && platformIndex >= 0 && platformIndex < (int) platforms.size())
+                continue;
+
+            string platformVendor = platforms[j].getInfo<CL_PLATFORM_VENDOR>();
+            vector<cl::Device> devices;
+            platforms[j].getDevices(CL_DEVICE_TYPE_ALL, &devices);
+            const int minThreadBlockSize = 32;
+
             for (int i = 0; i < (int) devices.size(); i++) {
+                // if they supplied a valid deviceIndex, we only look through that one
+                if (i != deviceIndex && deviceIndex >= 0 && deviceIndex < (int) devices.size())
+                    continue;
+
                 if (platformVendor == "Apple" && devices[i].getInfo<CL_DEVICE_VENDOR>() == "AMD")
                     continue; // Don't use AMD GPUs on OS X due to serious bugs.
                 int maxSize = devices[i].getInfo<CL_DEVICE_MAX_WORK_ITEM_SIZES>()[0];
@@ -137,15 +145,24 @@ OpenCLContext::OpenCLContext(const System& system, int platformIndex, int device
                 }
                 int speed = devices[i].getInfo<CL_DEVICE_MAX_COMPUTE_UNITS>()*processingElementsPerComputeUnit*devices[i].getInfo<CL_DEVICE_MAX_CLOCK_FREQUENCY>();
                 if (maxSize >= minThreadBlockSize && speed > bestSpeed) {
-                    deviceIndex = i;
+                    bestDevice = i;
                     bestSpeed = speed;
+                    bestPlatform = j;
                 }
             }
         }
-        if (deviceIndex == -1)
+
+        if (bestPlatform == -1)
+            throw OpenMMException("No compatible OpenCL platform is available");
+
+        if (bestDevice == -1)
             throw OpenMMException("No compatible OpenCL device is available");
-        device = devices[deviceIndex];
-        this->deviceIndex = deviceIndex;
+
+        vector<cl::Device> devices;
+        platforms[bestPlatform].getDevices(CL_DEVICE_TYPE_ALL, &devices);
+        device = devices[bestDevice];
+
+        this->deviceIndex = bestDevice;
         if (device.getInfo<CL_DEVICE_MAX_WORK_GROUP_SIZE>() < minThreadBlockSize)
             throw OpenMMException("The specified OpenCL device is not compatible with OpenMM");
         compilationDefines["WORK_GROUP_SIZE"] = intToString(ThreadBlockSize);

platforms/opencl/src/OpenCLPlatform.cpp

@@ -133,7 +133,7 @@ void OpenCLPlatform::contextDestroyed(ContextImpl& context) const {
 
 OpenCLPlatform::PlatformData::PlatformData(const System& system, const string& platformPropValue, const string& deviceIndexProperty,
         const string& precisionProperty, const string& cpuPmeProperty) : removeCM(false), stepCount(0), computeForceCount(0), time(0.0)  {
-    int platformIndex = 0;
+    int platformIndex = -1;
     if (platformPropValue.length() > 0)
         stringstream(platformPropValue) >> platformIndex;
     vector<string> devices;