1Contributing to SkJumper 2======================== 3 4SkJumper is the execution engine of SkRasterPipeline, a system we've been using 5to accelerate CPU-bound work inside Skia, most notably color-space conversions 6and color-correct drawing. 7 8(This is where I'd put my link to design document if I had one...) 9 10SkJumper is more annoying to contribute to than most Skia code because of its 11offline compilation step. You'll need particular tools installed on your 12machine and to tell GN about them. This document is designed to guide you 13through this process and ease some of that annoyance. 14 15One-time Setup 16-------------- 17 18To generate stage code you need Clang 4.0, objdump, and ccache. It's best that 19Clang is exactly the same version we typically use (as of writing 4.0.0) and 20you'll need objdump to be compiled with support for x86-64, ARMv7, and ARMv8. 21 22The easiest way to satisfy these contraints is to get your hands on a Mac and 23install [Homebrew](https://brew.sh). Once you have `brew` installed, run this 24to get the tools you need: 25 26<!--?prettify lang=sh?--> 27 28 brew install llvm binutils ccache 29 30Setting up GN 31------------------------- 32 33With your tools installed, tell GN about them 34 35 skia_jumper_clang = path/to/clang-4.0 36 skia_jumper_objdump = path/to/gobjdump 37 skia_jumper_ccache = path/to/ccache 38 39then regenerate and build as normal. 40 41If you look in your GN out directory, you should now see a bunch of `.o` files, 42and `git status` should show no changes to `src/jumper/SkJumper_generated*.S`. 43That's good. Those object files are the intermediates we parse to produce 44the assembly files. We just leave them around in case you want to look at 45them yourself. 46 47Make A Change 48------------- 49 50Let's use the `from_srgb` stage as a little playground to make a real change. 51Linearizing sRGB encoded bytes is slow, so let's pretend we've decided to trade 52quality for speed, approximating the existing implementation with a simple square. 53 54Open up `SkJumper_stages.cpp` and find the `from_srgb` stage. It'll look like 55 56<!--?prettify lang=cc?--> 57 58 STAGE(from_srgb) { 59 r = from_srgb(r); 60 g = from_srgb(g); 61 b = from_srgb(b); 62 } 63 64Let's replace whatever's there with our fast approximation: 65 66<!--?prettify lang=cc?--> 67 68 STAGE(from_srgb) { 69 r *= r; 70 g *= g; 71 b *= b; 72 } 73 74When you save and re-Ninja, you should now see changes to 75`src/jumper/SkJumper_generated.S` and `src/jumper/SkJumper_generated_win.S`. 76If you can't read assembly, no big deal. If you can, run `git diff`. You 77should see the various `sk_from_srgb_*` functions get dramatically simpler, 78something like three multiplies and a couple other bookkeeping instructions. 79 80It's not unusual for isolated changes in one stage to cause seemingly unrelated 81changes in another. When adding or removing any code you'll usually see all 82the comments in branch instructions change a little bit, but the actual 83instruction on the left won't change. When adding or removing uses of 84constants, you'll often see both the comment and instruction on the left change 85for other loads of constants from memory, especially on x86-64. You'll also 86see some code that looks like garbage change; those are the constants. If 87any of this worries you, please do go running to someone who knows more for 88help, but odds are everything is fine. 89 90At this point things should just be business as usual. Any time you change 91`SkJumper_stages.cpp`, Ninja ought to notice and regenerate the assembly files. 92 93Adding a new Stage 94------------------ 95 96Adding a new stage is a lot like changing an existing stage. Edit 97`SkJumper_stages.cpp`, build Skia, test, repeat until correct. 98 99You'll just need to also edit `SkRasterPipeline.h` to add your new stage to the 100macro listing all the stages. The stage name is the handle normal Skia code 101uses to refer to the stage abstractly, and the wiring between 102`SkRasterPipeline::foo` and `STAGE(foo) { ... }` should work automatically. 103