Supported an OPENCL_VISIBLE_DEVICES environment variable inspired from CUDA_VISIBLE_DEVICES.

OPENCL_VISIBLE_DEVICES=descriptor_list
descriptor_list -> descriptor | descriptor::descriptor_list
descriptor -> driver,platform | driver,platform,type | driver,platform,type,device_list
device_list -> device_id | device_id,device_list
driver: client driver (foo.icd or foo.so)
platform: platform ID
type: gpu, cpu, accelerator, custom, or any
device_id: device ID in the platform

This patch is to land KhronosGroup#45 again.
Most of the code is from pull request 45.

e.g., OPENCL_VISIBLE_DEVICES=foo.icd,0,gpu,0,1::bar.icd,1,cpu,0 only shows
- GPU 0, 1 in the first platform of foo.icd
- CPU 0 in the second platform of bar.icd

Author: cdai2

CMakeLists.txt

@@ -50,6 +50,8 @@ configure_file(${CMAKE_CURRENT_SOURCE_DIR}/loader/icd_cmake_config.h.in
 set (OPENCL_ICD_LOADER_SOURCES
     loader/icd.c
     loader/icd.h
+    loader/icd_device_visible.h
+    loader/icd_device_visible.c
     loader/icd_dispatch.c
     loader/icd_dispatch.h
     loader/icd_dispatch_generated.c

loader/icd.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2020 The Khronos Group Inc.
+ * Copyright (c) 2016-2021 The Khronos Group Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,6 +19,7 @@
 #include "icd.h"
 #include "icd_dispatch.h"
 #include "icd_envvars.h"
+#include "icd_device_visible.h"
 #if defined(CL_ENABLE_LAYERS)
 #include <CL/cl_layer.h>
 #endif // defined(CL_ENABLE_LAYERS)
@@ -122,6 +123,12 @@ void khrIcdVendorAdd(const char *libraryName)
         {
             continue;
         }
+
+        if (!khrIcdCheckPlatformVisible(libraryName, i))
+        {
+            continue;
+        }
+
         result = platforms[i]->dispatch->clGetPlatformInfo(
             platforms[i],
             CL_PLATFORM_ICD_SUFFIX_KHR,
@@ -179,8 +186,9 @@ void khrIcdVendorAdd(const char *libraryName)
             *prevNextPointer = vendor;
         }
 
-        KHR_ICD_TRACE("successfully added vendor %s with suffix %s\n", libraryName, suffix);
+        khrIcdVisibilitySetPlatform(libraryName, i, platforms[i]);
 
+        KHR_ICD_TRACE("successfully added vendor %s with suffix %s\n", libraryName, suffix);
     }
 
 Done: