startvertexlocation-not-vs-input

Pre-v0 status: this rule page is published ahead of the implementation. Behaviour described here reflects the design intent; the rule is not yet enforced by the tool.

(via ADR 0010)

What it detects

A use of the SM 6.8 SV_StartVertexLocation (or the analogous SV_StartInstanceLocation) system-value semantic anywhere other than as a vertex-shader input parameter. The SM 6.8 spec restricts these semantics to VS-input position only: they expose the per-draw BaseVertexLocation / StartInstanceLocation value the runtime passes to Draw* calls. Putting them on a VS output, a PS input, or a compute parameter is invalid. Slang reflection identifies the parameter's stage role; the rule fires on misplacement.

Why it matters on a GPU

SV_StartVertexLocation is the per-draw constant the rasterizer broadcasts to all VS invocations: it is the BaseVertexLocation argument of DrawIndexed*, materialised as a per-vertex input. On NVIDIA Ada Lovelace and AMD RDNA 2/3, the value is delivered through the shader-input pipeline as a uniform-across-the-wave value; Intel Xe-HPG implements the same path. The hardware delivers it only at VS-input — the value has no defined meaning at any later stage because the rasterizer does not propagate it through the inter-stage parameter cache.

Putting the semantic on a VS output ("forward this to PS") or a PS input is a hard validator error: the semantic is not in the legal-IO table for those stages. DXC catches the simplest forms; the lint catches the rest, including the case where the semantic is on a struct member that is then passed across the VS->PS boundary.

The fix is to read the value at VS input and forward it explicitly through a TEXCOORD slot if PS needs it, or to skip it entirely if the per-draw value isn't actually consumed.

Examples

Bad

hlsl

// SV_StartVertexLocation as a VS output is invalid.
struct VSOut {
    float4 pos     : SV_Position;
    uint   baseVtx : SV_StartVertexLocation;  // ERROR: not allowed on output
};

VSOut main(uint vtx : SV_VertexID, uint baseVtx : SV_StartVertexLocation) {
    VSOut o;
    o.pos     = float4(0, 0, 0, 1);
    o.baseVtx = baseVtx;
    return o;
}

Good

hlsl

// Read at VS input; forward via TEXCOORD if PS needs it.
struct VSOut {
    float4 pos    : SV_Position;
    uint   baseVtxFwd : TEXCOORD0;
};

VSOut main(uint vtx : SV_VertexID, uint baseVtx : SV_StartVertexLocation) {
    VSOut o;
    o.pos        = float4(0, 0, 0, 1);
    o.baseVtxFwd = baseVtx;
    return o;
}

Options

none

Fix availability

none — Restructuring the IO is authorial.

startvertexlocation-not-vs-input ​

What it detects ​

Why it matters on a GPU ​

Examples ​

Bad ​

Good ​

Options ​

Fix availability ​

See also ​