add initial support ofr multi-threaded execution

tools/skpdiff/skpdiff_main.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #if SK_SUPPORT_OPENCL
 
 #define __NO_STD_VECTOR // Uses cl::vectpr instead of std::vectpr
@@ skipping 22 matching lines @@
 __SK_FORCE_IMAGE_DECODER_LINKING;
 
 // Command line argument definitions go here
 DEFINE_bool2(list, l, false, "List out available differs");
 DEFINE_string2(differs, d, "", "The names of the differs to use or all of them by default");
 DEFINE_string2(folders, f, "", "Compare two folders with identical subfile names: <baseline folder> <test folder>");
 DEFINE_string2(patterns, p, "", "Use two patterns to compare images: <baseline> <test>");
 DEFINE_string2(output, o, "", "Writes the output of these diffs to output: <output>");
 DEFINE_bool(jsonp, true, "Output JSON with padding");
 DEFINE_string(csv, "", "Writes the output of these diffs to a csv file");
+DEFINE_int32(threads, -1, "run N threads in parallel [default is derived from CPUs available]");
 
 #if SK_SUPPORT_OPENCL
 /// A callback for any OpenCL errors
 static void CL_CALLBACK error_notify(const char* errorInfo, const void* privateInfoSize, ::size_t cb, void* userData) {
     SkDebugf("OpenCL error notify: %s\n", errorInfo);
     exit(1);
 }
 
 /// Creates a device and context with OpenCL
 static bool init_device_and_context(cl::Device* device, cl::Context* context) {
@@ skipping 128 matching lines @@
     if (!FLAGS_csv.isEmpty()) {
         if (1 != FLAGS_csv.count()) {
             SkDebugf("csv flag expects one argument: <csv file>\n");
             return 1;
         }
     }
 
     SkDiffContext ctx;
     ctx.setDiffers(chosenDiffers);
 
+    if (FLAGS_threads >= 0) {
+        ctx.setThreadCount(FLAGS_threads);
+    }
+
     // Perform a folder diff if one is requested
     if (!FLAGS_folders.isEmpty()) {
         ctx.diffDirectories(FLAGS_folders[0], FLAGS_folders[1]);
     }
 
     // Perform a pattern diff if one is requested
     if (!FLAGS_patterns.isEmpty()) {
         ctx.diffPatterns(FLAGS_patterns[0], FLAGS_patterns[1]);
     }
 
     // Output to the file specified
     if (!FLAGS_output.isEmpty()) {
         SkFILEWStream outputStream(FLAGS_output[0]);
         ctx.outputRecords(outputStream, FLAGS_jsonp);
     }
 
     if (!FLAGS_csv.isEmpty()) {
         SkFILEWStream outputStream(FLAGS_csv[0]);
         ctx.outputCsv(outputStream);
     }
 
     return 0;
 }
 
 #if !defined(SK_BUILD_FOR_IOS) && !defined(SK_BUILD_FOR_NACL)
 int main(int argc, char * argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif