make SkJumper stages normal Skia code

Enough clients are using Clang now that we can say, use Clang to build
if you want these software pipeline stages to go fast.

This lets us drop the offline build aspect of SkJumper stages, instead
building as part of Skia using the SkOpts framework.

I think everything should work, except I've (temporarily) removed
AVX-512 support.  I will put this back in a follow up.

I have had to drop Windows down to __vectorcall and our narrower
stage calling convention that keeps the d-registers on the stack.
I tried forcing sysv_abi, but that crashed Clang.  :/

Added a TODO to up the same narrower stage calling convention
for lowp stages... we just *don't* today, for no good reason.

Change-Id: Iaaa792ffe4deab3508d2dc5d0008c163c24b3383
Reviewed-on: https://skia-review.googlesource.com/110641
Commit-Queue: Mike Klein <mtklein@chromium.org>
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/site/user/build.md b/site/user/build.md
index 1ddcfd812f..c9d17b4ec1 100644
--- a/site/user/build.md
+++ b/site/user/build.md
@@ -33,6 +33,18 @@ link Skia against the headers and libaries found on the system paths.
 use `extra_cflags` and `extra_ldflags` to add include or library paths if
 needed.
 
+A note on software backend performance
+--------------------------------------
+
+A number of routines in Skia's software backend have been written to run
+fastest when compiled by Clang.  If you depend on software rasterization, image
+decoding, or color space conversion and compile Skia with GCC, MSVC or another
+compiler, you will see dramatically worse performance than if you use Clang.
+
+This choice was only a matter of prioritization; there is nothing fundamentally
+wrong with non-Clang compilers.  So if this is a serious issue for you, please
+let us know on the mailing list.
+
 Quickstart
 ----------