Software backend

llvmpipe vs swiftshader current performance investigation

Possible next steps

• Try to figure out the per pixel shading times to compare between llvmpipe and swiftshader
• Fix correctness bugs in swiftshader
• Determine the performance difference between llvmpipe and basic
• Fix low hanging performance fruit when running on llvmpipe. i.e. I believe picture caching is still blitting tiles using draws and shaders instead of just using blits.

Architecture

llvmpipe and swiftshader both run the pixel shader on 2x2 blocks of pixels. A pixel is represented as something like Pixel{ a[f32; 4], r: [f32; 4], g: [f32; 4], b: [f32; 4]} or specifically Vector4f in SwiftShader. This way, all "a" for a block are placed in a SIMD word and processed together, same for "r"s, "g"s, and "b"s.

The vertex shader is similarly run on 4 vertices at a time.

Z buffer

Both llvmpipe and swiftshader seem to use a linear z buffer. i.e. no hierarchical-Z. This probably isn't so bad for llvmpipe because it does pixel shading a tile at a time and so we're more likely to hit the cache for subsequent draws. It does seem like it would be more of a problem for SwiftShader.

Custom

I'm experimenting with compiling the WebRender shaders to C++. We could potentially do this at build time which would avoid the need to ship a jit which is good for code size and security. (We can opt out of being able to make pages executable in the gpu process)