DecodeState, the draw precheck and DecodePixelStageInfo rejected every
nonzero CONSERVATIVE_Z_EXPORT (DB_SHADER_CONTROL bits 13-14).
EXPORT_LESS_THAN_Z (1) and EXPORT_GREATER_THAN_Z (2) promise that the
exported Z stays on one side of the interpolated Z. The value now
reaches the recompiler as a pixel stage field, and a pixel shader that
writes FragDepth declares DepthLess or DepthGreater next to
DepthReplacing, the SPIR-V modes RADV maps to these values. Vulkan lets
those modes only add a depth test of the interpolated Z before shading,
so a shader that keeps the promise renders the same. The reserved value
3 still throws.
The field is part of the recompile cache key, the draw key and the
serialized request (version 7), and ValidateShaders accepts both modes.
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