Add sk_parallel_for()

This should be a drop-in replacement for most for-loops to make them run in parallel:
   for (int i = 0; i < N; i++) { code... }
   ~~~>
   sk_parallel_for(N, [&](int i) { code... });

This is just syntax sugar over SkTaskGroup to make this use case really easy to write.
There's no more overhead that we weren't already forced to add using an interface like batch(),
and no extra heap allocations.

I've replaced 3 uses of SkTaskGroup with sk_parallel_for:
  1) My unit tests for SkOnce.
  2) Cary's path fuzzer.
  3) SkMultiPictureDraw.
Performance should be the same.  Please compare left and right for readability. :)

BUG=skia:

No public API changes.
TBR=reed@google.com

Review URL: https://codereview.chromium.org/1184373003

1 files changed, 7 insertions, 39 deletions

diff --git a/tools/skpdiff/SkDiffContext.cpp b/tools/skpdiff/SkDiffContext.cpp
index c422636f6e..d14546046c 100644
--- a/tools/skpdiff/SkDiffContext.cpp
+++ b/tools/skpdiff/SkDiffContext.cpp
@@ -209,26 +209,6 @@ void SkDiffContext::addDiff(const char* baselinePath, const char* testPath) {
     }
 }
 
-class SkThreadedDiff : public SkRunnable {
-public:
-    SkThreadedDiff() : fDiffContext(NULL) { }
-
-    void setup(SkDiffContext* diffContext, const SkString& baselinePath, const SkString& testPath) {
-        fDiffContext = diffContext;
-        fBaselinePath = baselinePath;
-        fTestPath = testPath;
-    }
-
-    void run() override {
-        fDiffContext->addDiff(fBaselinePath.c_str(), fTestPath.c_str());
-    }
-
-private:
-    SkDiffContext* fDiffContext;
-    SkString fBaselinePath;
-    SkString fTestPath;
-};
-
 void SkDiffContext::diffDirectories(const char baselinePath[], const char testPath[]) {
     // Get the files in the baseline, we will then look for those inside the test path
     SkTArray<SkString> baselineEntries;
@@ -237,12 +217,8 @@ void SkDiffContext::diffDirectories(const char baselinePath[], const char testPa
         return;
     }
 
-    SkTaskGroup tg;
-    SkTArray<SkThreadedDiff> runnableDiffs;
-    runnableDiffs.reset(baselineEntries.count());
-
-    for (int x = 0; x < baselineEntries.count(); x++) {
-        const char* baseFilename = baselineEntries[x].c_str();
+    sk_parallel_for(baselineEntries.count(), [&](int i) {
+        const char* baseFilename = baselineEntries[i].c_str();
 
         // Find the real location of each file to compare
         SkString baselineFile = SkOSPath::Join(baselinePath, baseFilename);
@@ -250,13 +226,11 @@ void SkDiffContext::diffDirectories(const char baselinePath[], const char testPa
 
         // Check that the test file exists and is a file
         if (sk_exists(testFile.c_str()) && !sk_isdir(testFile.c_str())) {
-            // Queue up the comparison with the differ
-            runnableDiffs[x].setup(this, baselineFile, testFile);
-            tg.add(&runnableDiffs[x]);
+            this->addDiff(baselineFile.c_str(), testFile.c_str());
         } else {
             SkDebugf("Baseline file \"%s\" has no corresponding test file\n", baselineFile.c_str());
         }
-    }
+    });
 }
 
 
@@ -281,15 +255,9 @@ void SkDiffContext::diffPatterns(const char baselinePattern[], const char testPa
         return;
     }
 
-    SkTaskGroup tg;
-    SkTArray<SkThreadedDiff> runnableDiffs;
-    runnableDiffs.reset(baselineEntries.count());
-
-    for (int x = 0; x < baselineEntries.count(); x++) {
-        runnableDiffs[x].setup(this, baselineEntries[x], testEntries[x]);
-        tg.add(&runnableDiffs[x]);
-    }
-    tg.wait();
+    sk_parallel_for(baselineEntries.count(), [&](int i) {
+        this->addDiff(baselineEntries[i].c_str(), testEntries[i].c_str());
+    });
 }
 
 void SkDiffContext::outputRecords(SkWStream& stream, bool useJSONP) {