[Webkit-unassigned] [Bug 226975] iPhone 6S - iOS 15.0 - unable to retrieve WebGL2 context

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=226975

--- Comment #3 from Kimmo Kinnunen <kkinnunen at apple.com> ---
To recap:

Limitation "iOS GPU Family 4 || macOS GPU Family 1 means GLES 3.0" went in with the shadow texture implementation. This implementation was with GLSL -> SPIRV -> MSL code-path.
It is assumed that this intended to check support of non-trivial `sample_compare` `lod_options` parameters `lod` and `gradient`.

This check is errorneous since this is not what the docs say today, at least from the direct-to-msl implementation perspective.

The docs say that
 a) `sample_compare` has `lod_options`
 b) On macOS `sample_compare` does not support non-trivial `lod_options` prior to Metal 2.2 [sic]
 c) On macOS some hardware supports `sample_compare` `lod` and `gradient` Metal 2.3. [sic]


So the doc leaves a bit much for the reader to intrepret about non-trivial `lod_options`. My interpretation is:
 - On iOS it is always supported (unknown if this is true)
 - On macOS it is not supported by default
 - On macOS Metal >= 2.3 it is supported on unspecified GPUs

So the correct check is:
 `iOS || (macOS && Metal >= 2.3 && GPU == unspecified GPU)`

In direct-to-metal translator we already have feature flags for these features:

   ANGLE_FEATURE_CONDITION((&mFeatures), allowSamplerCompareGradient, supportsEitherGPUFamily(3,1));
    ANGLE_FEATURE_CONDITION((&mFeatures), allowSamplerCompareLod, supportsEitherGPUFamily(3,1));

The condition is not consistent with my interpretation of the specification. 
However, it is more convenient condition, since it enables support for all supported iOS devices.

The feature flag is queried in the _shaders_. As an aside: I don't understand why the shaders aren't generated per feature flag.

However, I'm not versed enough in the functions to understand if the `else` branches
are correct. If I understand correctly, they are not, as otherwise we shouldn't have
the `sample_compare` function in the first place.

The plan of action based on my interpretation:
1) To support WebGL 2.0 on iPhone, iPad at the A9, A10 processor level:
 - downgrade the GLES 3.0 GPU family check to 3.
2) To support GLSL shadow samplers correctly on new macOS versions:
 - implement the fallback correctly
 - implement the unspecified gpu check correctly
3) To support GLSL shadow samplers correctly on older macOS versions 
 - implement the Metal version check correctly

The fast path that currently is the only implemented version cannot be supported on older macOSes at the moment due to Metal versions shipped in these OSes.

Reference:

https://developer.apple.com/metal/Metal-Shading-Language-Specification.pdf

! ios-metal2.1: Support the unified graphics and computing language revision 2.1 programs for iOS 12.
! ios-metal2.2: Support the unified graphics and computing language revision 2.2 programs for iOS 13.
! ios-metal2.3: Support the unified graphics and computing language revision 2.3 programs for iOS 14.

! macos-metal2.1: Support the unified graphics and computing language revision 2.1 programs for macOS 10.14.
! macos-metal2.2: Support the unified graphics and computing language revision 2.2 programs for macOS 10.15.
! macos-metal2.3: Support the unified graphics and computing language revision 2.3 programs for macOS 11.0.


> On macOS prior to Metal 2.2, for sample_compare functions, bias and gradient* are not supported, and lod must be a zero constant. In Metal 2.3, this restriction is lifted for some hardware.
> Since Metal 2.2, you can specify a LOD range for a sampler.

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