Cycles/OpenCL: Remove NULL PTR Workaround

In the current OpenCL implementation we have a work-around for platforms
that didn't support NULL pointers. We used to replace all NULLs and
empty arrays with a pointer to a single byte on the OpenCL Device.

During investigation of {T65924} it was asked to remove this work-around
for testing. This change improves the render times.

    SCENE              | BEFORE | AFTER
   --------------------+--------+-------
   bmw27               | 108    | 89
   barbershop_interior | 867    | 673
   classroom           | 270    | 173
   fishy_cat           | 244    | 196
   koro                | 249    | 207
   pavillon_barcelona  | 582    | 414

Note that this change does not fix T65924 it just improves the
rendering performance for OpenCL. We haven't tested this patch on all
platforms so we should keep an eye out on the tracker.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D6391

intern/cycles/device/opencl/memory_manager.cpp

@@ -251,7 +251,7 @@ void MemoryManager::set_kernel_arg_buffers(cl_kernel kernel, cl_uint *narg)
       device->kernel_set_args(kernel, (*narg)++, *device_buffer.buffer);
     }
     else {
-      device->kernel_set_args(kernel, (*narg)++, device->null_mem);
+      device->kernel_set_args(kernel, (*narg)++, 0);
     }
   }
 }

intern/cycles/device/opencl/opencl.h

@@ -381,7 +381,6 @@ class OpenCLDevice : public Device {
 
   ConstMemMap const_mem_map;
   MemMap mem_map;
-  device_ptr null_mem;
 
   bool device_initialized;
   string platform_name;

intern/cycles/device/opencl/opencl_split.cpp

@@ -611,7 +611,6 @@ OpenCLDevice::OpenCLDevice(DeviceInfo &info, Stats &stats, Profiler &profiler, b
   cdDevice = NULL;
   cxContext = NULL;
   cqCommandQueue = NULL;
-  null_mem = 0;
   device_initialized = false;
   textures_need_update = true;
   use_preview_kernels = !background;
@@ -662,12 +661,6 @@ OpenCLDevice::OpenCLDevice(DeviceInfo &info, Stats &stats, Profiler &profiler, b
     return;
   }
 
-  null_mem = (device_ptr)clCreateBuffer(cxContext, CL_MEM_READ_ONLY, 1, NULL, &ciErr);
-  if (opencl_error(ciErr)) {
-    opencl_error("OpenCL: Error creating memory buffer for NULL");
-    return;
-  }
-
   /* Allocate this right away so that texture_info
    * is placed at offset 0 in the device memory buffers. */
   texture_info.resize(1);
@@ -689,9 +682,6 @@ OpenCLDevice::~OpenCLDevice()
 
   memory_manager.free();
 
-  if (null_mem)
-    clReleaseMemObject(CL_MEM_PTR(null_mem));
-
   ConstMemMap::iterator mt;
   for (mt = const_mem_map.begin(); mt != const_mem_map.end(); mt++) {
     delete mt->second;
@@ -962,7 +952,7 @@ void OpenCLDevice::mem_alloc(device_memory &mem)
     opencl_assert_err(ciErr, "clCreateBuffer");
   }
   else {
-    mem.device_pointer = null_mem;
+    mem.device_pointer = 0;
   }
 
   stats.mem_alloc(size);
@@ -1084,7 +1074,7 @@ void OpenCLDevice::mem_free(device_memory &mem)
   }
   else {
     if (mem.device_pointer) {
-      if (mem.device_pointer != null_mem) {
+      if (mem.device_pointer != 0) {
         opencl_assert(clReleaseMemObject(CL_MEM_PTR(mem.device_pointer)));
       }
       mem.device_pointer = 0;
@@ -1120,7 +1110,7 @@ device_ptr OpenCLDevice::mem_alloc_sub_ptr(device_memory &mem, int offset, int s
 
 void OpenCLDevice::mem_free_sub_ptr(device_ptr device_pointer)
 {
-  if (device_pointer && device_pointer != null_mem) {
+  if (device_pointer != 0) {
     opencl_assert(clReleaseMemObject(CL_MEM_PTR(device_pointer)));
   }
 }
@@ -1239,8 +1229,7 @@ void OpenCLDevice::set_kernel_arg_mem(cl_kernel kernel, cl_uint *narg, const cha
     ptr = CL_MEM_PTR(i->second);
   }
   else {
-    /* work around NULL not working, even though the spec says otherwise */
-    ptr = CL_MEM_PTR(null_mem);
+    ptr = 0;
   }
 
   opencl_assert(clSetKernelArg(kernel, (*narg)++, sizeof(ptr), (void *)&ptr));