DXC can sink and duplicate SetMeshOutputCounts if the inputs are computed within a scalar branch

[karltechno]

Description
DXC can sink a call to SetMeshOutputCounts into a scalar branch if the branches are used to compute the number of vertices/indices. Anecdotally I have been able to fix it by wrapping the branch condition with WaveReadLaneFirst, despite the branch being scalar anyway.

Steps to Reproduce
See https://godbolt.org/z/oca77vqGh

Actual Behavior
DXC validation fails after SetMeshOutputCounts is duplicated in resulting DXIL:
<source>:32:5: error: SetMeshOUtputCounts cannot be called multiple times.

Environment

• DXC version: DXC 1.8.2505 on compiler explorer
• Host Operating System Observed on: windows and linux (via compiler explorer)

[damyanp]

A possible workaround for this is to change the SetMeshOutputs call to:

    SetMeshOutputCounts(WaveReadLaneFirst(nverts), nprims);

See https://godbolt.org/z/W4bh6dErW

[llvm-beanz]

The correct fix here is that we should just annotate these operations with noduplicate.

[copilot-swe-agent]

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