Transpilation to MSL from SPIRV has uniforms and storage buffers flipped

[christopherliu830]

My game has a storage buffer for model matrices and a uniform buffer for global scene data. Given this sample glsl file:

#version 450

layout(std430, binding = 0, set = 0) readonly buffer Objects {
    mat4 _data[];
} objects;

layout(std140, binding = 0, set = 1) uniform Scene
{
    mat4 view_proj;
} scene;

layout(location = 0) in vec3 input_position;

void main()
{
    gl_Position = scene.view_proj * objects._data[gl_InstanceIndex] * vec4(input_position, 1.0);
}

Then running these commands:

glslangValidator -V standard.vert
shadercross vert.spv -t vertex -d msl -o vert.metal 

I get this output:

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

using namespace metal;

struct Scene
{
    float4x4 view_proj;
};

struct Objects
{
    float4x4 _data[1];
};

struct main0_out
{
    float4 gl_Position [[position]];
};

struct main0_in
{
    float3 input_position [[attribute(0)]];
};

vertex main0_out main0(main0_in in [[stage_in]], const device Objects& objects [[buffer(0)]], constant Scene& scene [[buffer(1)]], uint gl_InstanceIndex [[instance_id]])
{
    main0_out out = {};
    out.gl_Position = (scene.view_proj * objects._data[int(gl_InstanceIndex)]) * float4(in.input_position, 1.0);
    return out;
}

I believe that scene [[buffer(1)]] and objects [[buffer(0)]] should be flipped, as per SDL_gpu documentation:

/*
 * For MSL/metallib, use the following order:
 *
 * - [[texture]]: Sampled textures, followed by storage textures
 * - [[sampler]]: Samplers with indices corresponding to the sampled textures
 * - [[buffer]]: Uniform buffers, followed by storage buffers. Vertex buffer 0
 *   is bound at [[buffer(14)]], vertex buffer 1 at [[buffer(15)]], and so on.
 *   Rather than manually authoring vertex buffer indices, use the
 *   [[stage_in]] attribute which will automatically use the vertex input
 *   information from the SDL_GPUGraphicsPipeline.
 */

And when I swap 1s and 0s for the buffer indices, my shader works correctly.

[karabatov]

Hit the same issue where I have 3 structured buffers and 1 uniform in the vertex shader and 2 structured buffers and 1 uniform in the fragment shader.

The uniform is declared as buffer(3) in MSL instead of buffer(0) and as a result the bindings are shifted down by one: should be uniform (0) followed by 3 buffers.

SDL itself sets the uniform in slot 0 as expected (the unnamed buffer is the uniform):

Exactly the same happens in the fragment shader:

When I manually go to the MSL transpiled file and up-shift the structured buffers by 1 and put the uniform as buffer(0) everything starts working.

[thatcosmonaut]

Should be fixed by #87 if you wouldn't mind testing.