[spv-in] Spirv Shader with Uniform Sampling/TextureGrad Fails Uniformity Analysis

[cwfitzgerald]

A shader which passed validation in 0.6.3 fails to validate in naga master due to:

Function [1] 'main' is invalid:
        Required uniformity of control flow for IMPLICIT_LEVEL in [419] is not fulfilled because of Expression([390])

This shader uses either textureGrad or sampling inside of uniform control flow. and shouldn't fail.

opaque.frag.cpu.zip

---

This was found because I found a hlsl compile error in naga 0.6.3 that I wondered might be fixed by the new spirv control flow stuff. If you compile this shader to hlsl with 0.6.3, it'll try to use a inside-scope variable outside of that scope, causing a compile error.

[kvark]

It's a problem with SPV-in, the generated IR doesn't have anything about grads:

                [419]: ImageSample {
                    image: [19],
                    sampler: [2],
                    coordinate: [416],
                    array_index: Some(
                        [418],
                    ),
                    offset: None,
                    level: Auto,
                    depth_ref: Some(
                        [412],
                    ),
                },

[cwfitzgerald]

Interesting, the grad code is there 🤔

[kvark]

It seems that it should been spewing zero:

                    level = if options.compare {
                        log::debug!(
                            "Assuming gradients {:?} and {:?} are not greater than 1",
                            grad_x_handle,
                            grad_y_handle
                        );
                        crate::SampleLevel::Zero
                    } else {
                        crate::SampleLevel::Gradient {
                            x: grad_x_handle,
                            y: grad_y_handle,
                        }
                    };

But the resulting IR has Auto 🤯

[JCapucho]

Your shader does have an implicit sample

Spir-v disassembly

       %1145 = OpLoad %1142 %shadow
       %1147 = OpLoad %69 %shadow_sampler
       %1149 = OpSampledImage %1148 %1145 %1147
       %1152 = OpImageSampleDrefImplicitLod %float %1149 %1140 %1139
               OpLine %1 76 0

You have this in the debug data

    if (MATERIAL_FLAG(FLAGS_UNLIT)) {
        o_color = pixel.albedo;
        // o_normal = vec4(i_normal, 0.0);
    } else {
        vec3 v = -normalize(i_view_position.xyz);

        vec3 color = vec3(pixel.emissive);
        for (uint i = 0; i < directional_light_header.total_lights; ++i) {
            DirectionalLight light = directional_lights[i];

            vec3 shadow_ndc = (directional_lights[i].view_proj * uniforms.inv_view * i_view_position).xyz;
            vec2 shadow_flipped = (shadow_ndc.xy * 0.5) + 0.5;
            vec4 shadow_shadow_coords = vec4(shadow_flipped.x, 1 - shadow_flipped.y, float(i), shadow_ndc.z);

            float shadow_value;
            shadow_value = texture(sampler2DArrayShadow(shadow, shadow_sampler), shadow_shadow_coords);
            if (shadow_shadow_coords.x < 0 || shadow_shadow_coords.x > 1 || shadow_shadow_coords.y < 0 || shadow_shadow_coords.y > 1 || shadow_ndc.z < -1 || shadow_ndc.z > 1) {
                shadow_value = 1.0;
            }

            color += surface_shading(directional_lights[i], pixel, v, shadow_value * pixel.ambient_occlusion);
        }

        o_color = max(vec4(color, pixel.albedo.a), uniforms.ambient * pixel.albedo);
        // o_normal = vec4(pixel.normal, 0.0);
    }

Clearly there's a texture call in a If statement

[kvark]

Having the texture call in an If statement could still be unconditional if we statically know that this branch is taken either 100% of time or 0% of time.

[cwfitzgerald]

Yeah the if and the for loop are both based on uniform values, and as such should be valid to use implicit derivatives there. Incidentally, I'm only using implicit derivatives there because textureLod doesn't exist for sampler2DArrayShadow in GLSL :|

[JCapucho]

This is related to #4320

Also had this problem, textureLod isn't available for shadow textures but you can use a textureGrad instead with vec2(0) as the gradients, internally naga will convert it to a texture sample with Zero lod

float textureGrad(sampler2DArrayShadow, sampler, vec4 P, vec2 dPdx, vec2 dPdy)

[cwfitzgerald]

This is unfortunately not available through glslang, there was a PR, but it went stale and got closed.

[JCapucho]

wdym?

This worked for me on glslang

#version 460

layout (location = 0) out vec4 fragColor;
layout (set = 0, binding = 0) uniform texture2DArray tex;
layout (set = 0, binding = 0) uniform sampler samp;

void main()
{
    float a = textureGrad(sampler2DArrayShadow(tex,samp), vec4(0), vec2(0), vec2(0));
	fragColor = vec4(a);
}

http://shader-playground.timjones.io/50f33d0eb84ce1eb744225e701d882fc

[cwfitzgerald]

Well hot diggity dog, I didn't realize that worked, I must have thought I tried it, but didn't.

[teoxoy]

Let's track this in #4369.