Only use the atomic version of charge spreading on compute level 2.0 or later,...

Only use the atomic version of charge spreading on compute level 2.0 or later, since it is slower than the other version on earlier GPUs.

platforms/opencl/src/OpenCLContext.cpp

@@ -100,12 +100,12 @@ OpenCLContext::OpenCLContext(int numParticles, int deviceIndex, OpenCLPlatform::
             simdWidth = 32;
             if (device.getInfo<CL_DEVICE_EXTENSIONS>().find("cl_nv_device_attribute_query") != string::npos) {
                 // Compute level 1.2 and later Nvidia GPUs support 64 bit atomics, even though they don't list the
-                // proper extension as supported.
+                // proper extension as supported.  We only use them on compute level 2.0 or later, since they're very
+                // slow on earlier GPUs.
 
-                cl_uint computeCapabilityMajor, computeCapabilityMinor;
+                cl_uint computeCapabilityMajor;
                 clGetDeviceInfo(device(), CL_DEVICE_COMPUTE_CAPABILITY_MAJOR_NV, sizeof(cl_uint), &computeCapabilityMajor, NULL);
-                clGetDeviceInfo(device(), CL_DEVICE_COMPUTE_CAPABILITY_MINOR_NV, sizeof(cl_uint), &computeCapabilityMinor, NULL);
-                if (computeCapabilityMajor > 1 || computeCapabilityMinor > 1)
+                if (computeCapabilityMajor > 1)
                     supports64BitGlobalAtomics = true;
             }
         }