[dxvk] Generate bit mask of used spec constants in DxvkShader

This way we can more accurately track which constants are used, and not
pass any unnecessary data at compile time. We can extend this in the
future to also skip unused constants for pipeline lookups.

Also, any spec constant with the ID of MaxNumSpecConstants will be treated
as a spec constant selector. If the shader uses this constant, it is assumed
that it does not access any other spec constants if the value of this constant
is 0. This will allow shaders with spec constants to be used with pipeline
libraries.

src/dxvk/dxvk_shader.cpp

@@ -64,8 +64,10 @@ namespace dxvk {
           bindingOffsets[varId].setOffset = ins.offset() + 3;
         }
 
-        if (ins.arg(2) == spv::DecorationSpecId && ins.arg(3) < MaxNumSpecConstants)
-          m_flags.set(DxvkShaderFlag::HasSpecConstants);
+        if (ins.arg(2) == spv::DecorationSpecId) {
+          if (ins.arg(3) <= MaxNumSpecConstants)
+            m_specConstantMask |= 1u << ins.arg(3);
+        }
 
         if (ins.arg(2) == spv::DecorationLocation && ins.arg(3) == 1) {
           m_o1LocOffset = ins.offset() + 3;
@@ -158,7 +160,8 @@ namespace dxvk {
       return false;
 
     // Ignore shaders that have user-defined spec constants
-    return !m_flags.test(DxvkShaderFlag::HasSpecConstants);
+    // and no selector to read their contents from elsewhere
+    return !m_specConstantMask || (m_specConstantMask & (1u << MaxNumSpecConstants));
   }
 
 

src/dxvk/dxvk_shader.h

@@ -27,7 +27,6 @@ namespace dxvk {
   enum DxvkShaderFlag : uint64_t {
     HasSampleRateShading,
     HasTransformFeedback,
-    HasSpecConstants,
     ExportsStencilRef,
     ExportsViewportIndexLayerFromVertexStage,
   };
@@ -112,6 +111,14 @@ namespace dxvk {
       return m_bindings;
     }
 
+    /**
+     * \brief Retrieves spec constant mask
+     * \returns Bit mask of used spec constants
+     */
+    uint32_t getSpecConstantMask() const {
+      return m_specConstantMask;
+    }
+
     /**
      * \brief Patches code using given info
      *
@@ -209,6 +216,8 @@ namespace dxvk {
     size_t                        m_o1IdxOffset = 0;
     size_t                        m_o1LocOffset = 0;
 
+    uint32_t                      m_specConstantMask = 0;
+
     std::vector<char>             m_uniformData;
     std::vector<BindingOffsets>   m_bindingOffsets;