Bug 1409440. r=tnikkel

image/Downscaler.cpp

@@ -195,13 +195,14 @@ Downscaler::CommitRow()
     int32_t inLineToRead = filterOffset + mLinesInBuffer;
     MOZ_ASSERT(mCurrentInLine <= inLineToRead, "Reading past end of input");
     if (mCurrentInLine == inLineToRead) {
+      MOZ_RELEASE_ASSERT(mLinesInBuffer < mWindowCapacity, "Need more rows than capacity!");
       mXFilter.ConvolveHorizontally(mRowBuffer.get(), mWindow[mLinesInBuffer++], mHasAlpha);
     }
 
     MOZ_ASSERT(mCurrentOutLine < mTargetSize.height,
                "Writing past end of output");
 
-    while (mLinesInBuffer == filterLength) {
+    while (mLinesInBuffer >= filterLength) {
       DownscaleInputLine();
 
       if (mCurrentOutLine == mTargetSize.height) {
@@ -297,9 +298,14 @@ Downscaler::DownscaleInputLine()
 
   // Shift the buffer. We're just moving pointers here, so this is cheap.
   mLinesInBuffer -= diff;
-  mLinesInBuffer = max(mLinesInBuffer, 0);
-  for (int32_t i = 0; i < mLinesInBuffer; ++i) {
-    swap(mWindow[i], mWindow[filterLength - mLinesInBuffer + i]);
+  mLinesInBuffer = min(max(mLinesInBuffer, 0), mWindowCapacity);
+
+  // If we already have enough rows to satisfy the filter, there is no need
+  // to swap as we won't be writing more before the next convolution.
+  if (filterLength > mLinesInBuffer) {
+    for (int32_t i = 0; i < mLinesInBuffer; ++i) {
+      swap(mWindow[i], mWindow[filterLength - mLinesInBuffer + i]);
+    }
   }
 }
 

image/DownscalingFilter.h

@@ -232,13 +232,14 @@ protected:
     int32_t inputRowToRead = filterOffset + mRowsInWindow;
     MOZ_ASSERT(mInputRow <= inputRowToRead, "Reading past end of input");
     if (mInputRow == inputRowToRead) {
+      MOZ_RELEASE_ASSERT(mRowsInWindow < mWindowCapacity, "Need more rows than capacity!");
       mXFilter.ConvolveHorizontally(mRowBuffer.get(), mWindow[mRowsInWindow++], mHasAlpha);
     }
 
     MOZ_ASSERT(mOutputRow < mNext.InputSize().height,
                "Writing past end of output");
 
-    while (mRowsInWindow == filterLength) {
+    while (mRowsInWindow >= filterLength) {
       DownscaleInputRow();
 
       if (mOutputRow == mNext.InputSize().height) {
@@ -297,9 +298,14 @@ private:
 
     // Shift the buffer. We're just moving pointers here, so this is cheap.
     mRowsInWindow -= diff;
-    mRowsInWindow = std::max(mRowsInWindow, 0);
-    for (int32_t i = 0; i < mRowsInWindow; ++i) {
-      std::swap(mWindow[i], mWindow[filterLength - mRowsInWindow + i]);
+    mRowsInWindow = std::min(std::max(mRowsInWindow, 0), mWindowCapacity);
+
+    // If we already have enough rows to satisfy the filter, there is no need
+    // to swap as we won't be writing more before the next convolution.
+    if (filterLength > mRowsInWindow) {
+      for (int32_t i = 0; i < mRowsInWindow; ++i) {
+        std::swap(mWindow[i], mWindow[filterLength - mRowsInWindow + i]);
+      }
     }
   }