Bug: skia:7633 Change-Id: I662cfb793924f03f65f2076bc150ca97525b1843 Reviewed-on: https://skia-review.googlesource.com/107780 Reviewed-by: Yuqian Li <liyuqian@google.com> Commit-Queue: Yuqian Li <liyuqian@google.com>
4.0 KiB
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 Xcode, Xcode command line tools, and Homebrew. Once
you have brew
installed, run these commands to get the tools you need:
ls -d /usr/include >/dev/null || xcode-select --install
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.