[Webkit-unassigned] [Bug 232271] REGRESSION (iOS 15): Fragment shader performance is several times worse

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=232271

--- Comment #11 from Kyle Piddington <kpiddington at apple.com> ---
Hi all! Some updates for you. Apologies for the delay.

We've identified the main issue here. Part of our shader transformation passes tries it's best to preserve the original GLSL struct while following Metal alignment best practices. 

What this means for the attached shaders is that we're promoting the vec3 uniforms to vec4 for sending over as uniform data, but are then converting back to vec3 in the MSL shader. This temporary variable is able to be stored in shader registers, up until the arrays get too big. 

We'll have a fix in on our end, but if you need faster performance in the meantime, try changing your vec3 variables to vec4.

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