iOS metal seems to require compile-time constant sampler for sample_compare

[cyh36]

Hi, I've raised an issue #529 several days ago. You said "no constexpr samplers are needed" because you tested on OSX, it seems that only iOS metal requires this. I upgraded my iOS system and Xcode to the latest version, but the issue still remains. Is there any way to deal with this problem? Otherwise, maybe I have to use sample instead of sample_compare on iOS.

[HansKristian-Work]

If iOS truly requires this, MoltenVK would need to add emulation support here because SPIRV-Cross would need another interface which lets you set constant sampler state ahead of time. I don't think there is much I can do right now. Please file a bug on MoltenVK if you're using that.

[alek314]

Can you add a new interface while remain backward compatibility? We can not do PCF shadow mapping without this feature in our in house engine, which has nothing to do with MoltenVK.
Thank you.

[HansKristian-Work]

If you're using the API only, I could add something like:

void CompilerMSL::remap_constexpr_sampler(unsigned id, const MSLConstexprSamplerData &)

However, I don't know how this works in MSL, so please paste some example shaders how you'd like the resulting MSL code to look.

[cyh36]

The sampler remapped could be declared at the beginning of the entry point function, and passed to those functions that use this sampler. Just like:

#pragma clang diagnostic ignored "-Wmissing-prototypes"

#include <metal_stdlib>
#include <simd/simd.h>

using namespace metal;

struct main0_in
{
    float3 vUV [[user(locn0)]];
};

struct main0_out
{
    float FragColor [[color(0)]];
};

float Samp(thread const float3& uv, thread depth2d<float> uTex, thread sampler uSamp)
{
    return uTex.sample_compare(uSamp, uv.xy, uv.z);
}

fragment main0_out main0(main0_in in [[stage_in]], depth2d<float> uTex [[texture(0)]], depth2d<float> uTex2 [[texture(1)]], sampler uSamp [[sampler(0)]], sampler uSamp2 [[sampler(1)]])
{
    constexpr sampler uSamp_(address::clamp_to_edge, mip_filter::none, min_filter::linear, mag_filter::linear, compare_func::less);
    constexpr sampler uSamp2_(address::clamp_to_edge, mip_filter::none, min_filter::nearest, mag_filter::nearest, compare_func::less_equal);

    main0_out out = {};
    out.FragColor = uTex.sample_compare(uSamp_, in.vUV.xy, in.vUV.z);
    out.FragColor += Samp(in.vUV, uTex2, uSamp2_);
    return out;
}

[HansKristian-Work]

Ok, that seems reasonable enough.

[HansKristian-Work]

Got something WIP that should solve your problem. Please check out the PR and give feedback if this fixes your issue.

[alek314]

We will test it ASAP

[cyh36]

This seems to resolve the issue we had. Thanks very much! @HansKristian-ARM