Bug 283576 – REGRESSION: WebGL shader linking error with Metal backend when mixing scalar with vector

• Component: WebGL
• Status: RESOLVED FIXED

Created attachment 473341 [details]
A webpage that reproduces the issue

When subtracting a vector from a scalar in a GLSL shader, the Metal backend throws the following error during the linking stage:

WebGL: INVALID_OPERATION: Error: 0x00000502, in /AppleInternal/Library/BuildRoots/f00160a0-8d47-11ef-b394-4a83e32a47e1/Library/Caches/com.apple.xbs/Sources/ANGLE/Source/ThirdParty/ANGLE/src/libANGLE/renderer/metal/ProgramExecutableMtl.mm, CreateMslShaderLib:381. Internal error compiling shader with Metal backend.
program_source:36:12: error: as_type cast from vector 'metal::make_unsigned_t<int __attribute__((ext_vector_type(4)))>' (aka 'metal::_make_unsigned_impl<int __attribute__((ext_vector_type(4)))>::type') to scalar 'int' of different size
    return as_type<X>(as_type<metal::make_unsigned_t<X>>(x) - as_type<metal::make_unsigned_t<Y>>(y));
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
program_source:128:24: note: in instantiation of function template specialization 'ANGLE_isub<int, int __attribute__((ext_vector_type(4)))>' requested here
  metal::int4 _uorig = ANGLE_isub(1, _usource);


This error was triggered by the following GLSL code:
ivec4 source = ivec4(0);
ivec4 orig = 1 - source;

In GLSL, it is allowed for addition/subtraction to apply between a scalar and a vector. However, it seems that when it converts the GLSL code into Metal, it doesn't automatically convert the scalar into a vector which causes the above error in Metal.

This error is reproducible on Safari 18.1 on both iOS and macOS, and did not happen in older versions of Safari (such as Safari 17.x). However, it's unclear which version of Safari this issue was first introduced. The issue also doesn't happen on other browsers (such as Chrome).

Please see the attachment for an HTML file that reproduces the error. To reproduce:
1. Open the HTML file in Safari 18.1 on macOS or iOS
2. Open the developer console to see the shader compilation result. It should trigger the above error.

[Radar WebKit Bug Importer]

<rdar://problem/140546518>

[Kimmo Kinnunen]

Thanks for the report, I can reproduce the issue.
Until we ship a fix, you should be able to workaround the issue by using "ivec4(1) - source"

[Tadeu Zagallo]

Pull request: https://github.com/WebKit/WebKit/pull/37668

[EWS]

Committed 287588@main (11b5279aec61): <https://commits.webkit.org/287588@main>

Reviewed commits have been landed. Closing PR #37668 and removing active labels.

[EWS]

Committed 283286.583@safari-7620-branch (f936712e6292): <https://commits.webkit.org/283286.583@safari-7620-branch>

Reviewed commits have been landed. Closing PR #2399 and removing active labels.