Direct2D on top of WARP

In Firefox 38 we introduced the use of WARP for software rasterization on Window 7. Early in the release we ran into an issue where using WARP on top of the builtin VGA driver was ridiculously slow. We fixed this by disabling WARP when the the VGA driver was being used, but I was curious how Internet Explorer avoids this issue. One big difference between Firefox and Internet Explorer is that we're not currently using Direct2D on top of WARP where as they are. It turns out the WARP driver has a private API that is used by Direct2D to avoid having to use the regular D3D11 API.

Profiling Internet Explorer shows the following private API's used by Direct2D
d3d10warp.dll!UMDevice::DrawGlyphRun
d3d10warp.dll!UMDevice::AlphaBlt2
d3d10warp.dll!UMDevice::InternalGetDC
d3d10warp.dll!UMDevice::CreateGeometry
d3d10warp.dll!UMDevice::DrawGeometryInternal

It looks like these turn into fairly traditional 2D graphics operations as shown with the following call stack snippets below:

RasterizationStage::Rasterize_TEXT
DrawGlyphRun6x1_B8G8R8A8_SSE<1>
DrawGlyphRun4x4_B8G8R8A8_SSE

RasterizationStage::Rasterize_GEOMETRY
PixelJITRasterizeGeometry
PixelJITGeometryRasterizer::Rasterize
WarpGeometry::Rasterize
CAntialiasedFiller::RasterizeEdges
CAntialiasedFiller::FillEdges
CAntialiasedFiller::GenerateOutput
PixelJITGeometryRasterizer::RasterizeComplexScan
PixelJITGeometryRasterizer::BeginSpan
InitializeEdges
InitializeInactiveArray
QuickSortEdges

This suggests that using Direct2D on top of WARP is more efficient than expected and might actually make more sense than our current strategy of only using WARP for composition.