Vulkan: Featurize shadow buffers

include/platform/FeaturesVk.h

@@ -284,6 +284,14 @@ struct FeaturesVk : FeatureSetBase
         "Fill new allocations with non-zero values to flush out errors.", &members,
         "http://anglebug.com/4384"};
 
+    // Allocate a "shadow" buffer for GL buffer objects. For GPU-read only buffers
+    // glMap* latency can be reduced by maintaining a copy of the buffer which is
+    // writeable only by the CPU. We then return this shadow buffer on glMap* calls.
+    Feature shadowBuffers = {
+        "shadow_buffers", FeatureCategory::VulkanFeatures,
+        "Allocate a shadow buffer for GL buffer objects to reduce glMap* latency.", &members,
+        "http://anglebug.com/4339"};
+
     // Persistently map buffer memory until destroy, saves on map/unmap IOCTL overhead
     // for buffers that are updated frequently.
     Feature persistentlyMappedBuffers = {

src/libANGLE/renderer/vulkan/BufferVk.cpp

@@ -190,6 +190,11 @@ angle::Result BufferVk::initializeShadowBuffer(ContextVk *contextVk,
                                                gl::BufferBinding target,
                                                size_t size)
 {
+    if (!contextVk->getRenderer()->getFeatures().shadowBuffers.enabled)
+    {
+        return angle::Result::Continue;
+    }
+
     // For now, enable shadow buffers only for pixel unpack buffers.
     // If usecases present themselves, we can enable them for other buffer types.
     if (target == gl::BufferBinding::PixelUnpack)

src/libANGLE/renderer/vulkan/RendererVk.cpp

@@ -1706,6 +1706,8 @@ void RendererVk::initFeatures(DisplayVk *displayVk, const ExtensionNameList &dev
     // that can cause OOM and timeouts.
     ANGLE_FEATURE_CONDITION(&mFeatures, allocateNonZeroMemory, false);
 
+    ANGLE_FEATURE_CONDITION(&mFeatures, shadowBuffers, true);
+
     ANGLE_FEATURE_CONDITION(&mFeatures, persistentlyMappedBuffers, true);
 
     ANGLE_FEATURE_CONDITION(