[Webkit-unassigned] [Bug 246335] New: 'flat' vertex shader output is extremely slow with 'WebGL via Metal'

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=246335

            Bug ID: 246335
           Summary: 'flat' vertex shader output is extremely slow with
                    'WebGL via Metal'
           Product: WebKit
           Version: Safari 15
          Hardware: Unspecified
                OS: iOS 15
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebGL
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: msokalski at novomatic-tech.com
                CC: dino at apple.com, kbr at google.com, kkinnunen at apple.com

Declaring vertex shader output with 'flat' specifier makes drawing very slow.
It is not suffering from it when drawing POINTS primitive.
Also, problem does not occur when WebGL via Metal is disabled.

in vec2 clip_space_pos;
in vec4 just_color;

flat out vec4 flat_color;

void main()
{
  gl_Position = vec4(clip_space_pos,0.0,1.0);
  flat_color = just_color * clip_space_pos.x;
}

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