Contributing to SkJumper

SkJumper is the execution engine of SkRasterPipeline, a system we’ve been using to accelerate CPU-bound work inside Skia, most notably color-space conversions and color-correct drawing.

(This is where I’d put my link to design document if I had one…)

SkJumper is more annoying to contribute to than most Skia code because of its offline compilation step. You’ll need particular tools installed on your machine and to tell GN about them. This document is designed to guide you through this process and ease some of that annoyance.

One-time Setup

To generate stage code you need Clang 5.0, objdump, and ccache. It’s best that Clang is exactly the same version we typically use (as of writing 5.0.0) and you’ll need objdump to be compiled with support for x86-64, ARMv7, and ARMv8.

The easiest way to satisfy these contraints is to get your hands on a Mac and install Homebrew. Once you have brew installed, run this to get the tools you need:

brew install llvm binutils ccache

Setting up GN

With your tools installed, tell GN about them

skia_jumper_clang = path/to/clang-5.0
skia_jumper_objdump = path/to/gobjdump
skia_jumper_ccache = path/to/ccache

then regenerate and build as normal.

If you look in your GN out directory, you should now see a bunch of .o files, and git status should show no changes to src/jumper/SkJumper_generated*.S. That’s good. Those object files are the intermediates we parse to produce the assembly files. We just leave them around in case you want to look at them yourself.

Make A Change

Let’s use the from_srgb stage as a little playground to make a real change. Linearizing sRGB encoded bytes is slow, so let’s pretend we’ve decided to trade quality for speed, approximating the existing implementation with a simple square.

Open up SkJumper_stages.cpp and find the from_srgb stage. It’ll look like

STAGE(from_srgb) {
    r = from_srgb(r);
    g = from_srgb(g);
    b = from_srgb(b);

Let’s replace whatever’s there with our fast approximation:

STAGE(from_srgb) {
    r *= r;
    g *= g;
    b *= b;

When you save and re-Ninja, you should now see changes to src/jumper/SkJumper_generated.S and src/jumper/SkJumper_generated_win.S. If you can’t read assembly, no big deal. If you can, run git diff. You should see the various sk_from_srgb_* functions get dramatically simpler, something like three multiplies and a couple other bookkeeping instructions.

It’s not unusual for isolated changes in one stage to cause seemingly unrelated changes in another. When adding or removing any code you’ll usually see all the comments in branch instructions change a little bit, but the actual instruction on the left won’t change. When adding or removing uses of constants, you’ll often see both the comment and instruction on the left change for other loads of constants from memory, especially on x86-64. You’ll also see some code that looks like garbage change; those are the constants. If any of this worries you, please do go running to someone who knows more for help, but odds are everything is fine.

At this point things should just be business as usual. Any time you change SkJumper_stages.cpp, Ninja ought to notice and regenerate the assembly files.

Adding a new Stage

Adding a new stage is a lot like changing an existing stage. Edit SkJumper_stages.cpp, build Skia, test, repeat until correct.

You’ll just need to also edit SkRasterPipeline.h to add your new stage to the macro listing all the stages. The stage name is the handle normal Skia code uses to refer to the stage abstractly, and the wiring between SkRasterPipeline::foo and STAGE(foo) { ... } should work automatically.