[d3d11] Optimize index buffer binding with offset

Do not rebind the buffer if only the offset changes. Instead,
adjust StartIndexLocation in indexed draw calls. For indirect
draws, this will be disabled on the fly.

This may save a whole bunch of work in the backend, and reduces
the number of commands being sent to the CS thread in the first
place, which is why this optimization is not being done in the
backend itself but rather on the client API side.

src/d3d11/d3d11_context.cpp

@@ -1459,6 +1459,7 @@ namespace dxvk {
           UINT            StartIndexLocation,
           INT             BaseVertexLocation) {
     D3D10DeviceLock lock = LockContext();
+    StartIndexLocation += m_state.ia.indexBuffer.firstIndex;
     
     EmitCs([=] (DxvkContext* ctx) {
       ctx->drawIndexed(
@@ -1493,6 +1494,7 @@ namespace dxvk {
           INT             BaseVertexLocation,
           UINT            StartInstanceLocation) {
     D3D10DeviceLock lock = LockContext();
+    StartIndexLocation += m_state.ia.indexBuffer.firstIndex;
     
     EmitCs([=] (DxvkContext* ctx) {
       ctx->drawIndexed(
@@ -1667,6 +1669,12 @@ namespace dxvk {
     if (needsUpdate)
       m_state.ia.indexBuffer.buffer = newBuffer;
 
+    if (likely(m_state.ia.indexBuffer.optimized)) {
+      uint32_t shift = Format == DXGI_FORMAT_R16_UINT ? 1 : 2;
+      m_state.ia.indexBuffer.firstIndex = Offset >> shift;
+      Offset = 0;
+    }
+
     needsUpdate |= m_state.ia.indexBuffer.offset != Offset
                 || m_state.ia.indexBuffer.format != Format;
 
@@ -3377,7 +3385,13 @@ namespace dxvk {
 
   void D3D11DeviceContext::BindFramebuffer() {
     DxvkRenderTargets attachments;
-    
+
+    // Re-enable index buffer optimization here. Some games will
+    // use indirect draws for actual scene rendering, but then
+    // use direct draws for things like the user interface.
+    if (!m_state.ia.indexBuffer.optimized)
+      SetIndexBufferOptimized(true);
+
     // D3D11 doesn't have the concept of a framebuffer object,
     // so we'll just create a new one every time the render
     // target bindings are updated. Set up the attachments.
@@ -3572,12 +3586,39 @@ namespace dxvk {
   }
 
 
+  void D3D11DeviceContext::SetIndexBufferOptimized(
+          BOOL                              Enable) {
+    uint32_t shift = m_state.ia.indexBuffer.format == DXGI_FORMAT_R16_UINT ? 1 : 2;
+
+    if (Enable) {
+      m_state.ia.indexBuffer.firstIndex = m_state.ia.indexBuffer.offset >> shift;
+      m_state.ia.indexBuffer.offset = 0;
+    } else {
+      m_state.ia.indexBuffer.offset += m_state.ia.indexBuffer.firstIndex << shift;
+      m_state.ia.indexBuffer.firstIndex = 0;
+    }
+
+    m_state.ia.indexBuffer.optimized = Enable;
+
+    BindIndexBuffer(
+      m_state.ia.indexBuffer.buffer.ptr(),
+      m_state.ia.indexBuffer.offset,
+      m_state.ia.indexBuffer.format);
+  }
+
+
   void D3D11DeviceContext::SetDrawBuffers(
           ID3D11Buffer*                     pBufferForArgs,
           ID3D11Buffer*                     pBufferForCount) {
     auto argBuffer = static_cast<D3D11Buffer*>(pBufferForArgs);
     auto cntBuffer = static_cast<D3D11Buffer*>(pBufferForCount);
 
+    // Bind index buffer with the actual offset since otherwise the
+    // first index members of the indirect draw structures would be
+    // incorrect.
+    if (unlikely(m_state.ia.indexBuffer.optimized))
+      SetIndexBufferOptimized(false);
+
     if (m_state.id.argBuffer != argBuffer
      || m_state.id.cntBuffer != cntBuffer) {
       m_state.id.argBuffer = argBuffer;

src/d3d11/d3d11_context.h

@@ -786,6 +786,9 @@ namespace dxvk {
             ID3D11Resource*                   pResource,
             UINT                              Subresource);
     
+    void SetIndexBufferOptimized(
+            BOOL                              Enable);
+    
     void SetDrawBuffers(
             ID3D11Buffer*                     pBufferForArgs,
             ID3D11Buffer*                     pBufferForCount);

src/d3d11/d3d11_context_state.h

@@ -103,6 +103,8 @@ namespace dxvk {
     Com<D3D11Buffer> buffer = nullptr;
     UINT             offset = 0;
     DXGI_FORMAT      format = DXGI_FORMAT_UNKNOWN;
+    UINT             firstIndex = 0;
+    BOOL             optimized  = true;
   };