Loop inside shader causes duplication of nointerpolation keyword in generated HLSL code

[zopsicle]

Consider the following shader:

[shader("pixel")]
void main(out float4 SV_Target : SV_Target, in nointerpolation float4 color : color)
{
    for (int i = 0; i < 10; ++i)
        ;
    SV_Target = color;
}

Let's compile it to HLSL:

$ slangc -entry main -profile sm_5_0 -o shader.hlsl shader.slang

And look at the generated code:

#pragma pack_matrix(column_major)
#ifdef SLANG_HLSL_ENABLE_NVAPI
#include "nvHLSLExtns.h"
#endif

#ifndef __DXC_VERSION_MAJOR
    // warning X3557: loop doesn't seem to do anything, forcing loop to unroll
    #pragma warning(disable: 3557)
#endif


#line 2 "shader.slang"
void main(out float4 SV_Target_0 : SV_Target, nointerpolation nointerpolation nointerpolation nointerpolation nointerpolation float4 color_0 : color)
{


    SV_Target_0 = color_0;
    return;
}


#line 20419 "hlsl.meta.slang"
struct NullDifferential_0
{
    uint dummy_0;
};

The nointerpolation keyword is repeated five times in the generated code. FXC will reject the shader with the error message "duplicate usages specified". Removing the loop from the shader fixes the problem:

[shader("pixel")]
void main(out float4 SV_Target : SV_Target, in nointerpolation float4 color : color)
{
    SV_Target = color;
}

#pragma pack_matrix(column_major)
#ifdef SLANG_HLSL_ENABLE_NVAPI
#include "nvHLSLExtns.h"
#endif

#ifndef __DXC_VERSION_MAJOR
    // warning X3557: loop doesn't seem to do anything, forcing loop to unroll
    #pragma warning(disable: 3557)
#endif


#line 2 "shader.slang"
void main(out float4 SV_Target_0 : SV_Target, nointerpolation float4 color_0 : color)
{
    SV_Target_0 = color_0;
    return;
}


#line 20419 "hlsl.meta.slang"
struct NullDifferential_0
{
    uint dummy_0;
};

Tested with Slang 2024.10. It does not appear to matter what is inside the body of the loop; there just being a loop anywhere causes nointerpolation keywords to be repeated.

[jkwak-work]

The issue is observed at the "POST IR VALIDATION" level when IR is dumped with "-dump-ir".
Still investigating.

[jkwak-work]

It appears that a same set of decoration is added to a IRParam for some reason.
And this appears to happen not only for the nointerpolation modifier but also for other modifiers too.

### POST IR VALIDATION:
[layout(%3)]
func %main      : Func(Void, Out(Vec(Float, 4 : Int)), Vec(Float, 4 : Int))
{
block %23(
                [layout(%18)]
                [nameHint("SV_Target")]
                [semantic("SV_Target", 0 : Int)]
                [output]
                [nameHint("SV_Target")]
                [semantic("SV_Target", 0 : Int)]
                [output]
                [nameHint("SV_Target")]
                [semantic("SV_Target", 0 : Int)]
                [output]
                [nameHint("SV_Target")]
                [semantic("SV_Target", 0 : Int)]
                [output]
                [nameHint("SV_Target")]
                [semantic("SV_Target", 0 : Int)]
                [output]
                param %SVx5FTarget      : Out(Vec(Float, 4 : Int)),
                [layout(%11)]
                [nameHint("color")]
                [semantic("color", 0 : Int)]
                [interpolationMode(2 : Int)]
                [input]
                [nameHint("color")]
                [semantic("color", 0 : Int)]
                [interpolationMode(2 : Int)]
                [input]
                [nameHint("color")]
                [semantic("color", 0 : Int)]
                [interpolationMode(2 : Int)]
                [input]
                [nameHint("color")]
                [semantic("color", 0 : Int)]
                [interpolationMode(2 : Int)]
                [input]
                [nameHint("color")]
                [semantic("color", 0 : Int)]
                [interpolationMode(2 : Int)]
                [input]
                param %color    : Vec(Float, 4 : Int)):
        [loopMaxIters(10 : Int)]
        loop(%24, %25, %26, 0 : Int)

I haven't been able to locate where the bug is yet.