diff --git a/Source/WebCore/ChangeLog b/Source/WebCore/ChangeLog
index e4b808b..9271c0a 100644
--- a/Source/WebCore/ChangeLog
+++ b/Source/WebCore/ChangeLog
@@ -1,3 +1,25 @@
+2019-05-02  Simon Fraser  <simon.fraser@apple.com>
+
+        Add logging for RenderLayer clip rects
+        https://bugs.webkit.org/show_bug.cgi?id=197547
+
+        Reviewed by Zalan Bujtas.
+
+        Add a ClipRects log channel, and stream output for ClipRect and ClipRects.
+
+        The ClipRect code is performance sensitive, even in debug, so guard the log sites
+        with clipRectsLogEnabled() because the macro still evaluates its arguments even if
+        the channel is disabled (we need some better way to log that doesn't do this).
+
+        * platform/Logging.h:
+        * rendering/ClipRect.cpp:
+        (WebCore::operator<<):
+        * rendering/ClipRect.h:
+        * rendering/RenderLayer.cpp:
+        (WebCore::operator<<):
+        (WebCore::RenderLayer::calculateClipRects const):
+        * rendering/RenderLayer.h:
+
 2019-05-12  Simon Fraser  <simon.fraser@apple.com>
 
         Refactor composited backing-sharing code
diff --git a/Source/WebCore/platform/Logging.h b/Source/WebCore/platform/Logging.h
index 0171229..e120a29 100644
--- a/Source/WebCore/platform/Logging.h
+++ b/Source/WebCore/platform/Logging.h
@@ -42,6 +42,7 @@
     M(Animations) \
     M(ApplePay) \
     M(Archives) \
+    M(ClipRects) \
     M(Compositing) \
     M(ContentFiltering) \
     M(ContentObservation) \
diff --git a/Source/WebCore/rendering/ClipRect.cpp b/Source/WebCore/rendering/ClipRect.cpp
index b4db563..3630d16 100644
--- a/Source/WebCore/rendering/ClipRect.cpp
+++ b/Source/WebCore/rendering/ClipRect.cpp
@@ -38,4 +38,17 @@
     return hitTestLocation.intersects(m_rect);
 }
 
+TextStream& operator<<(TextStream& ts, const ClipRect& clipRect)
+{
+    ts << "rect ";
+    if (clipRect.isInfinite())
+        ts << "infinite";
+    else
+        ts << clipRect.rect();
+
+    if (clipRect.affectedByRadius())
+        ts << " affected by radius";
+    return ts;
+}
+
 }
diff --git a/Source/WebCore/rendering/ClipRect.h b/Source/WebCore/rendering/ClipRect.h
index 5ed7a6c..52abb67 100644
--- a/Source/WebCore/rendering/ClipRect.h
+++ b/Source/WebCore/rendering/ClipRect.h
@@ -27,6 +27,10 @@
 
 #include "LayoutRect.h"
 
+namespace WTF {
+class TextStream;
+}
+
 namespace WebCore {
 
 class HitTestLocation;
@@ -103,4 +107,6 @@
     return c;
 }
 
+WTF::TextStream& operator<<(WTF::TextStream&, const ClipRect&);
+
 } // namespace WebCore
diff --git a/Source/WebCore/rendering/RenderLayer.cpp b/Source/WebCore/rendering/RenderLayer.cpp
index 7aa11aa..7a1dc38 100644
--- a/Source/WebCore/rendering/RenderLayer.cpp
+++ b/Source/WebCore/rendering/RenderLayer.cpp
@@ -268,6 +268,24 @@
 #endif
 }
 
+#if !LOG_DISABLED
+static TextStream& operator<<(TextStream& ts, const ClipRects& clipRects)
+{
+    TextStream::GroupScope scope(ts);
+    ts << indent << "ClipRects\n";
+    ts << indent << "  overflow  : " << clipRects.overflowClipRect() << "\n";
+    ts << indent << "  fixed     : " << clipRects.fixedClipRect() << "\n";
+    ts << indent << "  positioned: " << clipRects.posClipRect() << "\n";
+
+    return ts;
+}
+
+static bool clipRectsLogEnabled()
+{
+    return LogClipRects.state == WTFLogChannelState::On;
+}
+#endif
+
 RenderLayer::RenderLayer(RenderLayerModelObject& rendererLayerModelObject)
     : m_isRenderViewLayer(rendererLayerModelObject.isRenderView())
     , m_forcedStackingContext(rendererLayerModelObject.isMedia())
@@ -5602,6 +5620,11 @@
             clipRects.setFixedClipRect(intersection(newPosClip, clipRects.fixedClipRect()));
         }
     }
+
+#if !LOG_DISABLED
+    if (clipRectsLogEnabled())
+        LOG_WITH_STREAM(ClipRects, stream << "RenderLayer " << this << " calculateClipRects " << clipRects);
+#endif
 }
 
 Ref<ClipRects> RenderLayer::parentClipRects(const ClipRectsContext& clipRectsContext) const
@@ -5646,6 +5669,11 @@
     // Note: infinite clipRects should not be scrolled here, otherwise they will accidentally no longer be considered infinite.
     if (parentRects->fixed() && &clipRectsContext.rootLayer->renderer() == &view && !backgroundClipRect.isInfinite())
         backgroundClipRect.moveBy(view.frameView().scrollPositionForFixedPosition());
+
+#if !LOG_DISABLED
+    if (clipRectsLogEnabled())
+        LOG_WITH_STREAM(ClipRects, stream << "RenderLayer " << this << " backgroundClipRect with context " << clipRectsContext << " returning " << backgroundClipRect);
+#endif
     return backgroundClipRect;
 }
 
@@ -6808,6 +6836,21 @@
 #endif
 }
 
+TextStream& operator<<(WTF::TextStream& ts, ClipRectsType clipRectsType)
+{
+    switch (clipRectsType) {
+    case PaintingClipRects: ts << "painting"; break;
+    case RootRelativeClipRects: ts << "root-relative"; break;
+    case AbsoluteClipRects: ts << "absolute"; break;
+    case TemporaryClipRects: ts << "temporary"; break;
+    case NumCachedClipRectsTypes:
+    case AllClipRectTypes:
+        ts << "?";
+        break;
+    }
+    return ts;
+}
+
 TextStream& operator<<(TextStream& ts, const RenderLayer& layer)
 {
     ts << "RenderLayer " << &layer << " " << layer.size();
@@ -6826,6 +6869,15 @@
     return ts;
 }
 
+TextStream& operator<<(TextStream& ts, const RenderLayer::ClipRectsContext& context)
+{
+    ts.dumpProperty("root layer:", context.rootLayer);
+    ts.dumpProperty("type:", context.clipRectsType);
+    ts.dumpProperty("overflow-clip:", context.respectOverflowClip == IgnoreOverflowClip ? "ignore" : "respect");
+    
+    return ts;
+}
+
 } // namespace WebCore
 
 #if ENABLE(TREE_DEBUGGING)
diff --git a/Source/WebCore/rendering/RenderLayer.h b/Source/WebCore/rendering/RenderLayer.h
index 22be9bc..e519f4f 100644
--- a/Source/WebCore/rendering/RenderLayer.h
+++ b/Source/WebCore/rendering/RenderLayer.h
@@ -1384,7 +1384,9 @@
 
 bool compositedWithOwnBackingStore(const RenderLayer&);
 
+WTF::TextStream& operator<<(WTF::TextStream&, ClipRectsType);
 WTF::TextStream& operator<<(WTF::TextStream&, const RenderLayer&);
+WTF::TextStream& operator<<(WTF::TextStream&, const RenderLayer::ClipRectsContext&);
 
 } // namespace WebCore