[SPIR-V] SV_Position can no longer be declared to be min10float4, min16float4, float16_t4 or half4 after fix to issue 3736 (works with DXIL)

[Dredhog]

Title

[SPIR-V] SV_Position can no longer be declared to be min10float4, min16float4, float16_t4 or half4 after fix to issue 3736

Functional impact

• Vertex shaders using these types for SV_Position written for FXC will not be compatible with DXC
• Cannot declare SV_Position to be relaxed or true 16-bit precision in the SPIR-V backend (can with the DXIL backend)

Minimal repro steps

1. Compile any of the following vertex shaders with with dxc.exe -T vs_6_0 -E Vert -spirv repro_shader.txt:

min10float4 Vert () : SV_Position
{
    return 1;
}

min16float4 Vert () : SV_Position
{
    return 1;
}

half4 Vert () : SV_Position
{
    return 1;
}

2. Compile the following vertex shader using explicit fp16 support dxc.exe -T vs_6_2 -E Vert -spirv -enable-16bit-types repro_shader.txt:

float16_t4 Vert () : SV_Position
{
    return 1;
}

Expected result

Compilation succeeds

Actual result

The compiler produces the corresponding error messages:

repro_shader.txt:1:24: error: semantic Position must be float4 or a composite type recursively including only float4
min10float4  Vert () : SV_Position
                       ^

repro_shader.txt:1:24: error: semantic Position must be float4 or a composite type recursively including only float4
min16float4  Vert () : SV_Position
                       ^

repro_shader.txt:1:18: error: semantic Position must be float4 or a composite type recursively including only float4
half4  Vert () : SV_Position
                 ^

The vertex shader with -enable-16-bit-types produces the following error:

repro_shader4.txt:1:22: error: semantic Position must be float4 or a composite type recursively including only float4
float16_t4 Vert () : SV_Position
                     ^

Further technical details

• Seems to have been caused by the fix to issue 3736. The fix to only allow float4 seems to have been overly conservative.
• Used dxc.exe downloaded from release v1.6.2112 (Dec 09, 2021).
• Note, the DXIL validator appears to have a small typo, where it prints error: SV_Position must be float4. when the component count is not 4 for SV_Position, which seems to not account for the other 4 component floating-point types that are supported.

[jaebaek]

Thank you for reporting this issue. We will take a look. Since now we are experiencing intense workloads, it would take some time.