See https://www.nasdaq.com/articles/pirated-academic-database-sci-hub-is-now-on-the-uncensorable-web-2021-01-11

https://share.firefox.dev/38Cln59

For linear gradients, we are currently bottlenecked by looking up a gradient
table entry, doing interpolation, and converting to pixel formats for every
sample.

We can accelerate this by instead looking for contiguous segments of gradient
within the range of entries we need to sample and then interpolating these
as a single gradient. This also enables us to convert to relevant pixel formats
only when setting up this gradient, which greatly reduces the per-pixel processing
down to essentially a shift and add.

To enable this sort of crawling of the gradient table, the output gradients have
been modified such that each entry's step value will equal an adjacent entry's
step value if and only if they are from same gradient. We can ensure this by, in
the very rare case two segments of gradient have the same step, using the equivalent
of nextafter() to imperceptibly alter the value so that the invariant is maintained.

The same optimization of looking for merged linear gradients can also be
applied to radial gradients by solving the quadratic equation to check
how large a span we can process within a given merged span. This allows
us to save a bunch of table lookup and some other math in the inner loops.

Depends on D105716

Some sites use pixelated image-rendering and/or 1x1 images as color sources.
When we hit these, we fall off the fast-path. Try to handle at least some of
the cases we are finding in the wild.

Depends on D105858

https://hg.mozilla.org/mozilla-central/rev/6e97ee33645a
https://hg.mozilla.org/mozilla-central/rev/395458ef80ae
https://hg.mozilla.org/mozilla-central/rev/01f66dc6aca4
https://hg.mozilla.org/mozilla-central/rev/8f0413b1ad6f

== Change summary for alert #28874 (as of Mon, 22 Feb 2021 21:15:19 GMT) ==

Improvements:

For up to date results, see: https://treeherder.mozilla.org/perfherder/alerts?id=28874

Comment on attachment 9204505 [details]
Bug 1686244 - Avoid zero offset in radial gradient.

Beta/Release Uplift Approval Request

• User impact if declined: Trying to do partial SW-WR rollout on Linux. Would like to avoid people reporting significant performance regressions months down the line.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Only impacts SW-WR/Linux. We can abort the rollout if unstable. Adequate time to deal with any bugs.
• String changes made/needed:

Comment on attachment 9204124 [details]
Bug 1686244 - Accelerate linear gradients in SWGL. r?jrmuizel

Approved for 87.0b3.

https://hg.mozilla.org/releases/mozilla-beta/rev/a35a81c995c0
https://hg.mozilla.org/releases/mozilla-beta/rev/f1551c87765c
https://hg.mozilla.org/releases/mozilla-beta/rev/be0f0e9ba18c
https://hg.mozilla.org/releases/mozilla-beta/rev/86ad69ab8ba3

This just reorders some checks to fail against NaNs and also checks for finiteness.

https://hg.mozilla.org/mozilla-central/rev/7be47f00f8cf

Comment on attachment 9205247 [details]
Bug 1686244 - Handle degenerate linear gradients. r?aosmond

Beta/Release Uplift Approval Request

• User impact if declined: Fixes potential crashes related to degenerate gradients we discovered in local mochitest runs.
• Is this code covered by automated tests?: Yes
• Has the fix been verified in Nightly?: Yes
• Needs manual test from QE?: No
• If yes, steps to reproduce:
• List of other uplifts needed: None
• Risk to taking this patch: Low
• Why is the change risky/not risky? (and alternatives if risky): Just a crash fix for the feature uplift here.
• String changes made/needed:

Comment on attachment 9205247 [details]
Bug 1686244 - Handle degenerate linear gradients. r?aosmond

Approved for 87.0b4.

https://hg.mozilla.org/releases/mozilla-beta/rev/88b1220bf251