1 Contributing to SkJumper 2 ======================== 3 4 SkJumper is the execution engine of SkRasterPipeline, a system we've been using 5 to accelerate CPU-bound work inside Skia, most notably color-space conversions 6 and color-correct drawing. 7 8 (This is where I'd put my link to design document if I had one...) 9 10 SkJumper is more annoying to contribute to than most Skia code because of its 11 offline compilation step. You'll need particular tools installed on your 12 machine and to tell GN about them. This document is designed to guide you 13 through this process and ease some of that annoyance. 14 15 One-time Setup 16 -------------- 17 18 To generate stage code you need Clang 5.0, objdump, and ccache. It's best that 19 Clang is exactly the same version we typically use (as of writing 5.0.0) and 20 you'll need objdump to be compiled with support for x86-64, ARMv7, and ARMv8. 21 22 The easiest way to satisfy these contraints is to get your hands on a Mac and 23 install [Homebrew](https://brew.sh). Once you have `brew` installed, run this 24 to get the tools you need: 25 26 <!--?prettify lang=sh?--> 27 28 brew install llvm binutils ccache 29 30 Setting up GN 31 ------------------------- 32 33 With your tools installed, tell GN about them 34 35 skia_jumper_clang = path/to/clang-5.0 36 skia_jumper_objdump = path/to/gobjdump 37 skia_jumper_ccache = path/to/ccache 38 39 then regenerate and build as normal. 40 41 If you look in your GN out directory, you should now see a bunch of `.o` files, 42 and `git status` should show no changes to `src/jumper/SkJumper_generated*.S`. 43 That's good. Those object files are the intermediates we parse to produce 44 the assembly files. We just leave them around in case you want to look at 45 them yourself. 46 47 Make A Change 48 ------------- 49 50 Let's use the `from_srgb` stage as a little playground to make a real change. 51 Linearizing sRGB encoded bytes is slow, so let's pretend we've decided to trade 52 quality for speed, approximating the existing implementation with a simple square. 53 54 Open 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 64 Let'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 74 When you save and re-Ninja, you should now see changes to 75 `src/jumper/SkJumper_generated.S` and `src/jumper/SkJumper_generated_win.S`. 76 If you can't read assembly, no big deal. If you can, run `git diff`. You 77 should see the various `sk_from_srgb_*` functions get dramatically simpler, 78 something like three multiplies and a couple other bookkeeping instructions. 79 80 It's not unusual for isolated changes in one stage to cause seemingly unrelated 81 changes in another. When adding or removing any code you'll usually see all 82 the comments in branch instructions change a little bit, but the actual 83 instruction on the left won't change. When adding or removing uses of 84 constants, you'll often see both the comment and instruction on the left change 85 for other loads of constants from memory, especially on x86-64. You'll also 86 see some code that looks like garbage change; those are the constants. If 87 any of this worries you, please do go running to someone who knows more for 88 help, but odds are everything is fine. 89 90 At 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 93 Adding a new Stage 94 ------------------ 95 96 Adding a new stage is a lot like changing an existing stage. Edit 97 `SkJumper_stages.cpp`, build Skia, test, repeat until correct. 98 99 You'll just need to also edit `SkRasterPipeline.h` to add your new stage to the 100 macro listing all the stages. The stage name is the handle normal Skia code 101 uses to refer to the stage abstractly, and the wiring between 102 `SkRasterPipeline::foo` and `STAGE(foo) { ... }` should work automatically. 103
