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Fix WebGL error with MirrorPadPacked on Safari 18.1 #8440

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Fix WebGL error with MirrorPadPacked on Safari 18.1 #8440

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@edmundp edmundp commented Nov 22, 2024

Issue

This resolves a shader compiler error with Safari 18.1 when running the MirrorPadPacked op with the WebGL backend.

The error that occurs is:

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:44:8: warning: constexpr if is a C++17 extension [-Wc++17-extensions]
    if constexpr (metal::is_signed_v<X>)
       ^
program_source:60:12: 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:321:24: note: in instantiation of function template specialization 'ANGLE_isub<int, int __attribute__((ext_vector_type(4)))>' requested here
  metal::int4 _uorig = ANGLE_isub(1, ANGLE__62);

To reproduce the issue, run the following code on Safari 18.1 on iOS or macOS:

<html>
  <head>
    <script src="https://cdn.jsdelivr.net/npm/@tensorflow/tfjs/dist/tf.min.js"> </script>
    <script>
      const x = tf.range(0, 9).reshape([1, 1, 3, 3]);
      x.mirrorPad([[0, 0], [0, 0], [2, 2], [2, 2]], 'reflect').print();
    </script>
  </head>
  <body>
  </body>
</html>

This issue does not happen on older versions of Safari (such as Safari 17.x). It's unclear which version of Safari this issue was first introduced in.

Investigation

When Safari compiles the shader, it converts the GLSL code to Metal but it appears there is a bug with this conversion process. In particular, it seems that applying an add/sub between a scalar and vector causes the shader compile error to happen, which occurs with the following line: orig = 1 - (lt + gte). The three variables are vectors while the 1 is a scalar.

A bug has been filed with WebKit: https://bugs.webkit.org/show_bug.cgi?id=283576

Solution

Casting the scalar to the vector before doing the subtraction resolves the error.

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