diff --git a/gerbview/gerbview_frame.cpp b/gerbview/gerbview_frame.cpp
index 6046bc8b8c..5d8d421f9b 100644
--- a/gerbview/gerbview_frame.cpp
+++ b/gerbview/gerbview_frame.cpp
@@ -780,7 +780,7 @@ LSET GERBVIEW_FRAME::GetVisibleLayers() const
 }
 
 
-void GERBVIEW_FRAME::SetVisibleLayers( LSET aLayerMask )
+void GERBVIEW_FRAME::SetVisibleLayers( const LSET& aLayerMask )
 {
     if( GetCanvas() )
     {
diff --git a/gerbview/gerbview_frame.h b/gerbview/gerbview_frame.h
index c00aa45540..3912786594 100644
--- a/gerbview/gerbview_frame.h
+++ b/gerbview/gerbview_frame.h
@@ -117,7 +117,7 @@ public:
      *
      * @param aLayerMask The new set of visible layers
      */
-    void SetVisibleLayers( LSET aLayerMask );
+    void SetVisibleLayers( const LSET& aLayerMask );
 
     /**
      * Test whether a given layer is visible.
diff --git a/include/board_design_settings.h b/include/board_design_settings.h
index 24a5720ecf..c9b1eac9dc 100644
--- a/include/board_design_settings.h
+++ b/include/board_design_settings.h
@@ -583,7 +583,7 @@ public:
      *
      * @param aMask = The new bit-mask of enabled layers.
      */
-    void SetEnabledLayers( LSET aMask );
+    void SetEnabledLayers( const LSET& aMask );
 
     /**
      * Test whether a given layer \a aLayerId is enabled.
diff --git a/include/pcb_base_frame.h b/include/pcb_base_frame.h
index 8f6da7c719..9042e12c80 100644
--- a/include/pcb_base_frame.h
+++ b/include/pcb_base_frame.h
@@ -337,7 +337,8 @@ public:
      * @param aDlgPosition is the position of dialog (default is centered).
      * @return the selected layer id.
      */
-    PCB_LAYER_ID SelectOneLayer( PCB_LAYER_ID aDefaultLayer, LSET aNotAllowedLayersMask = LSET(),
+    PCB_LAYER_ID SelectOneLayer( PCB_LAYER_ID aDefaultLayer,
+                                 const LSET& aNotAllowedLayersMask = LSET(),
                                  wxPoint aDlgPosition = wxDefaultPosition );
 
     /**
diff --git a/include/render_settings.h b/include/render_settings.h
index 315378e8b4..e9b38e6ed0 100644
--- a/include/render_settings.h
+++ b/include/render_settings.h
@@ -117,7 +117,7 @@ public:
     void SetLayerName( const wxString& aLayerName ) { m_layerName = aLayerName; }
 
     LSET GetPrintLayers() const { return m_printLayers; }
-    void SetPrintLayers( LSET aLayerSet ) { m_printLayers = aLayerSet; }
+    void SetPrintLayers( const LSET& aLayerSet ) { m_printLayers = aLayerSet; }
 
     /**
      * Clear the list of active layers.
diff --git a/pcbnew/autorouter/ar_autoplacer.cpp b/pcbnew/autorouter/ar_autoplacer.cpp
index b63366a2ac..469e89ed81 100644
--- a/pcbnew/autorouter/ar_autoplacer.cpp
+++ b/pcbnew/autorouter/ar_autoplacer.cpp
@@ -251,7 +251,7 @@ bool AR_AUTOPLACER::fillMatrix()
 }
 
 
-void AR_AUTOPLACER::addFpBody( const VECTOR2I& aStart, const VECTOR2I& aEnd, LSET aLayerMask )
+void AR_AUTOPLACER::addFpBody( const VECTOR2I& aStart, const VECTOR2I& aEnd, const LSET& aLayerMask )
 {
     // Add a polygonal shape (rectangle) to m_fpAreaFront and/or m_fpAreaBack
     if( aLayerMask[ F_Cu ] )
diff --git a/pcbnew/autorouter/ar_autoplacer.h b/pcbnew/autorouter/ar_autoplacer.h
index b8154927d9..d2a39add07 100644
--- a/pcbnew/autorouter/ar_autoplacer.h
+++ b/pcbnew/autorouter/ar_autoplacer.h
@@ -114,7 +114,7 @@ private:
     const PAD* nearestPad( FOOTPRINT* aRefFP, PAD* aRefPad, const VECTOR2I& aOffset );
 
     // Add a polygonal shape (rectangle) to m_fpAreaFront and/or m_fpAreaBack
-    void addFpBody( const VECTOR2I& aStart, const VECTOR2I& aEnd, LSET aLayerMask );
+    void addFpBody( const VECTOR2I& aStart, const VECTOR2I& aEnd, const LSET& aLayerMask );
 
     // Add a polygonal shape (rectangle) to m_fpAreaFront and/or m_fpAreaBack
     void addPad( PAD* aPad, int aClearance );
diff --git a/pcbnew/autorouter/ar_matrix.cpp b/pcbnew/autorouter/ar_matrix.cpp
index 8a539ff545..fa1f9585c9 100644
--- a/pcbnew/autorouter/ar_matrix.cpp
+++ b/pcbnew/autorouter/ar_matrix.cpp
@@ -437,8 +437,8 @@ void AR_MATRIX::traceCircle( int ux0, int uy0, int ux1, int uy1, int lg, int lay
 }
 
 
-void AR_MATRIX::traceFilledCircle(
-        int cx, int cy, int radius, LSET aLayerMask, int color, AR_MATRIX::CELL_OP op_logic )
+void AR_MATRIX::traceFilledCircle( int cx, int cy, int radius, const LSET& aLayerMask, int color,
+                                   AR_MATRIX::CELL_OP op_logic )
 {
     int    row, col;
     int    ux0, uy0, ux1, uy1;
@@ -602,7 +602,7 @@ void AR_MATRIX::traceArc( int ux0, int uy0, int ux1, int uy1, const EDA_ANGLE& a
 
 
 void AR_MATRIX::TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, double angle,
-                                      LSET aLayerMask, int color, AR_MATRIX::CELL_OP op_logic )
+                                      const LSET& aLayerMask, int color, AR_MATRIX::CELL_OP op_logic )
 {
     int row, col;
     int cx, cy; // Center of rectangle
@@ -689,7 +689,7 @@ void AR_MATRIX::TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, double
 }
 
 
-void AR_MATRIX::TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, LSET aLayerMask,
+void AR_MATRIX::TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, const LSET& aLayerMask,
                                       int color, AR_MATRIX::CELL_OP op_logic )
 {
     int row, col;
@@ -793,8 +793,8 @@ void AR_MATRIX::TraceSegmentPcb( PCB_SHAPE* aShape, int aColor, int aMargin,
  *  (aKeepOut ... 0). The decreasing value depends on the distance to the first rectangle
  *  Therefore the cost is high in rect x0,y0 to x1,y1, and decrease outside this rectangle
  */
-void AR_MATRIX::CreateKeepOutRectangle(
-        int ux0, int uy0, int ux1, int uy1, int marge, int aKeepOut, LSET aLayerMask )
+void AR_MATRIX::CreateKeepOutRectangle( int ux0, int uy0, int ux1, int uy1, int marge,
+                                        int aKeepOut, const LSET& aLayerMask )
 {
     int       row, col;
     int       row_min, row_max, col_min, col_max, pmarge;
diff --git a/pcbnew/autorouter/ar_matrix.h b/pcbnew/autorouter/ar_matrix.h
index 9a7c052930..19dd5fbded 100644
--- a/pcbnew/autorouter/ar_matrix.h
+++ b/pcbnew/autorouter/ar_matrix.h
@@ -112,14 +112,14 @@ public:
     void TraceSegmentPcb( PCB_SHAPE* aShape, int aColor, int aMargin, AR_MATRIX::CELL_OP op_logic );
 
     void CreateKeepOutRectangle( int ux0, int uy0, int ux1, int uy1, int marge, int aKeepOut,
-                                 LSET aLayerMask );
+                                 const LSET& aLayerMask );
 
     void PlacePad( PAD* aPad, int color, int marge, AR_MATRIX::CELL_OP op_logic );
 
-    void TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, double angle, LSET aLayerMask,
+    void TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, double angle, const LSET& aLayerMask,
                                int color, AR_MATRIX::CELL_OP op_logic );
 
-    void TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, LSET aLayerMask, int color,
+    void TraceFilledRectangle( int ux0, int uy0, int ux1, int uy1, const LSET& aLayerMask, int color,
                                AR_MATRIX::CELL_OP op_logic );
 
 private:
@@ -129,7 +129,7 @@ private:
     void traceCircle( int ux0, int uy0, int ux1, int uy1, int lg, int layer, int color,
                       AR_MATRIX::CELL_OP op_logic );
 
-    void traceFilledCircle( int cx, int cy, int radius, LSET aLayerMask, int color,
+    void traceFilledCircle( int cx, int cy, int radius, const LSET& aLayerMask, int color,
                             AR_MATRIX::CELL_OP op_logic );
 
     void traceArc( int ux0, int uy0, int ux1, int uy1, const EDA_ANGLE& arcAngle, int lg,
diff --git a/pcbnew/board.cpp b/pcbnew/board.cpp
index 6955b87c9a..b5b2728b10 100644
--- a/pcbnew/board.cpp
+++ b/pcbnew/board.cpp
@@ -841,7 +841,7 @@ LSET BOARD::GetVisibleLayers() const
 }
 
 
-void BOARD::SetEnabledLayers( LSET aLayerSet )
+void BOARD::SetEnabledLayers( const LSET& aLayerSet )
 {
     GetDesignSettings().SetEnabledLayers( aLayerSet );
 }
@@ -853,7 +853,7 @@ bool BOARD::IsLayerEnabled( PCB_LAYER_ID aLayer ) const
 }
 
 
-void BOARD::SetVisibleLayers( LSET aLayerSet )
+void BOARD::SetVisibleLayers( const LSET& aLayerSet )
 {
     if( m_project )
         m_project->GetLocalSettings().m_VisibleLayers = aLayerSet;
@@ -2209,17 +2209,14 @@ int BOARD::SetAreasNetCodesFromNetNames()
 }
 
 
-PAD* BOARD::GetPad( const VECTOR2I& aPosition, LSET aLayerSet ) const
+PAD* BOARD::GetPad( const VECTOR2I& aPosition, const LSET& aLayerSet ) const
 {
-    if( !aLayerSet.any() )
-        aLayerSet = LSET::AllCuMask();
-
     for( FOOTPRINT* footprint : m_footprints )
     {
         PAD* pad = nullptr;
 
         if( footprint->HitTest( aPosition ) )
-            pad = footprint->GetPad( aPosition, aLayerSet );
+            pad = footprint->GetPad( aPosition, aLayerSet.any() ? aLayerSet : LSET::AllCuMask() );
 
         if( pad )
             return pad;
@@ -2239,7 +2236,7 @@ PAD* BOARD::GetPad( const PCB_TRACK* aTrace, ENDPOINT_T aEndPoint ) const
 }
 
 
-PAD* BOARD::GetPadFast( const VECTOR2I& aPosition, LSET aLayerSet ) const
+PAD* BOARD::GetPadFast( const VECTOR2I& aPosition, const LSET& aLayerSet ) const
 {
     for( FOOTPRINT* footprint : Footprints() )
     {
@@ -2258,7 +2255,7 @@ PAD* BOARD::GetPadFast( const VECTOR2I& aPosition, LSET aLayerSet ) const
 }
 
 
-PAD* BOARD::GetPad( std::vector<PAD*>& aPadList, const VECTOR2I& aPosition, LSET aLayerSet ) const
+PAD* BOARD::GetPad( std::vector<PAD*>& aPadList, const VECTOR2I& aPosition, const LSET& aLayerSet ) const
 {
     // Search aPadList for aPosition
     // aPadList is sorted by X then Y values, and a fast binary search is used
@@ -2398,9 +2395,9 @@ std::tuple<int, double, double> BOARD::GetTrackLength( const PCB_TRACK& aTrack )
     double length = 0.0;
     double package_length = 0.0;
 
-    auto              connectivity = GetBoard()->GetConnectivity();
-    BOARD_STACKUP&    stackup      = GetDesignSettings().GetStackupDescriptor();
-    bool              useHeight    = GetDesignSettings().m_UseHeightForLengthCalcs;
+    auto           connectivity = GetBoard()->GetConnectivity();
+    BOARD_STACKUP& stackup      = GetDesignSettings().GetStackupDescriptor();
+    bool           useHeight    = GetDesignSettings().m_UseHeightForLengthCalcs;
 
     for( BOARD_CONNECTED_ITEM* item : connectivity->GetConnectedItems( &aTrack, EXCLUDE_ZONES ) )
     {
diff --git a/pcbnew/board.h b/pcbnew/board.h
index 2385eb3045..b01bf85e6a 100644
--- a/pcbnew/board.h
+++ b/pcbnew/board.h
@@ -616,7 +616,7 @@ public:
      *
      * @param aLayerMask the new bit-mask of enabled layers.
      */
-    void SetEnabledLayers( LSET aLayerMask );
+    void SetEnabledLayers( const LSET& aLayerMask );
     void SetLayerSet( const LSET& aLayerMask ) override { SetEnabledLayers( aLayerMask ); }
 
     /**
@@ -649,7 +649,7 @@ public:
      *
      * @param aLayerMask is the new bit-mask of visible layers.
      */
-    void SetVisibleLayers( LSET aLayerMask );
+    void SetVisibleLayers( const LSET& aLayerMask );
 
     // these 2 functions are not tidy at this time, since there are PCB_LAYER_IDs that
     // are not stored in the bitmap.
@@ -1110,7 +1110,7 @@ public:
      * @param aLayerMask A layer or layers to mask the hit test.
      * @return A pointer to a PAD object if found or NULL if not found.
      */
-    PAD* GetPad( const VECTOR2I& aPosition, LSET aLayerMask ) const;
+    PAD* GetPad( const VECTOR2I& aPosition, const LSET& aLayerMask ) const;
     PAD* GetPad( const VECTOR2I& aPosition ) const
     {
         return GetPad( aPosition, LSET().set() );
@@ -1134,7 +1134,7 @@ public:
      * @param aLayerMask A layer or layers to mask the hit test.
      * @return A pointer to a PAD object if found or NULL if not found.
      */
-    PAD* GetPadFast( const VECTOR2I& aPosition, LSET aLayerMask ) const;
+    PAD* GetPadFast( const VECTOR2I& aPosition, const LSET& aLayerMask ) const;
 
     /**
      * Locate the pad connected at \a aPosition on \a aLayer starting at list position
@@ -1150,7 +1150,7 @@ public:
      * @param aLayerMask A layer or layers to mask the hit test.
      * @return a PAD object pointer to the connected pad.
      */
-    PAD* GetPad( std::vector<PAD*>& aPadList, const VECTOR2I& aPosition, LSET aLayerMask ) const;
+    PAD* GetPad( std::vector<PAD*>& aPadList, const VECTOR2I& aPosition, const LSET& aLayerMask ) const;
 
     /**
      * First empties then fills the vector with all pads and sorts them by increasing x
diff --git a/pcbnew/board_design_settings.cpp b/pcbnew/board_design_settings.cpp
index 768691fd36..a2a5cb127b 100644
--- a/pcbnew/board_design_settings.cpp
+++ b/pcbnew/board_design_settings.cpp
@@ -1473,16 +1473,16 @@ void BOARD_DESIGN_SETTINGS::SetUserDefinedLayerCount( int aNewLayerCount )
 }
 
 
-void BOARD_DESIGN_SETTINGS::SetEnabledLayers( LSET aMask )
+void BOARD_DESIGN_SETTINGS::SetEnabledLayers( const LSET& aMask )
 {
+    m_enabledLayers = aMask;
+
     // Ensures mandatory back and front layers are always enabled regardless of board file
     // configuration.
-    aMask.set( B_Cu ).set( F_Cu )
-         .set( B_CrtYd ).set( F_CrtYd )
-         .set( Edge_Cuts )
-         .set( Margin );
-
-    m_enabledLayers = aMask;
+    m_enabledLayers.set( B_Cu ).set( F_Cu )
+                   .set( B_CrtYd ).set( F_CrtYd )
+                   .set( Edge_Cuts )
+                   .set( Margin );
 
     // update layer counts to ensure their consistency with m_EnabledLayers
     LSET copperLayers = aMask;
diff --git a/pcbnew/board_stackup_manager/panel_board_stackup.cpp b/pcbnew/board_stackup_manager/panel_board_stackup.cpp
index df1f1a7169..13f3f5592e 100644
--- a/pcbnew/board_stackup_manager/panel_board_stackup.cpp
+++ b/pcbnew/board_stackup_manager/panel_board_stackup.cpp
@@ -1349,7 +1349,7 @@ void PANEL_SETUP_BOARD_STACKUP::ImportSettingsFrom( BOARD* aBoard )
 }
 
 
-void PANEL_SETUP_BOARD_STACKUP::OnLayersOptionsChanged( LSET aNewLayerSet )
+void PANEL_SETUP_BOARD_STACKUP::OnLayersOptionsChanged( const LSET& aNewLayerSet )
 {
     // Can be called spuriously from events before the layers page is even created
     if( !m_panelLayers->IsInitialized() )
diff --git a/pcbnew/board_stackup_manager/panel_board_stackup.h b/pcbnew/board_stackup_manager/panel_board_stackup.h
index 63503db096..01f24cf324 100644
--- a/pcbnew/board_stackup_manager/panel_board_stackup.h
+++ b/pcbnew/board_stackup_manager/panel_board_stackup.h
@@ -103,7 +103,7 @@ public:
      * Rebuild the Layer Stack Panel if the new layer set differs
      * from the current layet set
      */
-    void OnLayersOptionsChanged( LSET aNewLayerSet );
+    void OnLayersOptionsChanged( const LSET& aNewLayerSet );
 
     /// @return the number of copper layers configured for the board stackup
     int GetCopperLayerCount() const;
diff --git a/pcbnew/collectors.h b/pcbnew/collectors.h
index 24de8eeb0c..979b370423 100644
--- a/pcbnew/collectors.h
+++ b/pcbnew/collectors.h
@@ -327,7 +327,7 @@ public:
      * @param aVisibleLayerMask is the current visible layers (bit mask).
      * @param aPreferredLayer is the layer to search first.
      */
-    GENERAL_COLLECTORS_GUIDE( LSET aVisibleLayerMask, PCB_LAYER_ID aPreferredLayer,
+    GENERAL_COLLECTORS_GUIDE( const LSET& aVisibleLayerMask, PCB_LAYER_ID aPreferredLayer,
                               KIGFX::VIEW* aView )
     {
         static const VECTOR2I one( 1, 1 );
@@ -375,7 +375,7 @@ public:
     {
         m_visibleLayers.set( aLayerId, isVisible );
     }
-    void SetLayerVisibleBits( LSET aLayerBits ) { m_visibleLayers = aLayerBits; }
+    void SetLayerVisibleBits( const LSET& aLayerBits ) { m_visibleLayers = aLayerBits; }
 
     /**
      * @return int - the preferred layer for HitTest()ing.
diff --git a/pcbnew/drc/drc_test_provider.cpp b/pcbnew/drc/drc_test_provider.cpp
index f11364ce0f..559612c303 100644
--- a/pcbnew/drc/drc_test_provider.cpp
+++ b/pcbnew/drc/drc_test_provider.cpp
@@ -151,7 +151,7 @@ void DRC_TEST_PROVIDER::reportRuleStatistics()
 }
 
 
-int DRC_TEST_PROVIDER::forEachGeometryItem( const std::vector<KICAD_T>& aTypes, LSET aLayers,
+int DRC_TEST_PROVIDER::forEachGeometryItem( const std::vector<KICAD_T>& aTypes, const LSET& aLayers,
                                             const std::function<bool( BOARD_ITEM*)>& aFunc )
 {
     BOARD *brd = m_drcEngine->GetBoard();
diff --git a/pcbnew/drc/drc_test_provider.h b/pcbnew/drc/drc_test_provider.h
index 57d4a22a97..22ae1c4aa7 100644
--- a/pcbnew/drc/drc_test_provider.h
+++ b/pcbnew/drc/drc_test_provider.h
@@ -102,7 +102,7 @@ public:
     virtual const wxString GetDescription() const;
 
 protected:
-    int forEachGeometryItem( const std::vector<KICAD_T>& aTypes, LSET aLayers,
+    int forEachGeometryItem( const std::vector<KICAD_T>& aTypes, const LSET& aLayers,
                              const std::function<bool(BOARD_ITEM*)>& aFunc );
 
     // Do not use a wxString with a vararg list: it is a complex thing and can create issues.
diff --git a/pcbnew/exporters/export_gencad_writer.cpp b/pcbnew/exporters/export_gencad_writer.cpp
index 7b00b55a1d..e0915e9151 100644
--- a/pcbnew/exporters/export_gencad_writer.cpp
+++ b/pcbnew/exporters/export_gencad_writer.cpp
@@ -107,7 +107,7 @@ static wxString escapeString( const wxString& aString )
 }
 
 
-static std::string fmt_mask( LSET aSet )
+static std::string fmt_mask( const LSET& aSet )
 {
     return ( aSet & LSET::AllCuMask() ).to_string();
 }
diff --git a/pcbnew/footprint.cpp b/pcbnew/footprint.cpp
index 05aaf9c3fe..a9fc6c691d 100644
--- a/pcbnew/footprint.cpp
+++ b/pcbnew/footprint.cpp
@@ -1454,7 +1454,7 @@ const BOX2I FOOTPRINT::GetBoundingBox( bool aIncludeText ) const
 }
 
 
-const BOX2I FOOTPRINT::GetLayerBoundingBox( LSET aLayers ) const
+const BOX2I FOOTPRINT::GetLayerBoundingBox( const LSET& aLayers ) const
 {
     std::vector<PCB_TEXT*> texts;
     const BOARD* board = GetBoard();
@@ -1943,7 +1943,7 @@ PAD* FOOTPRINT::FindPadByNumber( const wxString& aPadNumber, PAD* aSearchAfterMe
 }
 
 
-PAD* FOOTPRINT::GetPad( const VECTOR2I& aPosition, LSET aLayerMask )
+PAD* FOOTPRINT::GetPad( const VECTOR2I& aPosition, const LSET& aLayerMask )
 {
     for( PAD* pad : m_pads )
     {
diff --git a/pcbnew/footprint.h b/pcbnew/footprint.h
index 281cde525f..daa971a0c1 100644
--- a/pcbnew/footprint.h
+++ b/pcbnew/footprint.h
@@ -144,7 +144,7 @@ public:
     }
 
     LSET GetPrivateLayers() const { return m_privateLayers; }
-    void SetPrivateLayers( LSET aLayers ) { m_privateLayers = aLayers; }
+    void SetPrivateLayers( const LSET& aLayers ) { m_privateLayers = aLayers; }
 
     ///< @copydoc BOARD_ITEM_CONTAINER::Add()
     void Add( BOARD_ITEM* aItem, ADD_MODE aMode = ADD_MODE::INSERT,
@@ -199,7 +199,7 @@ public:
     /**
      * Return the bounding box of the footprint on a given set of layers
     */
-    const BOX2I GetLayerBoundingBox( LSET aLayers ) const;
+    const BOX2I GetLayerBoundingBox( const LSET& aLayers ) const;
 
     VECTOR2I GetCenter() const override { return GetBoundingBox( false ).GetCenter(); }
 
@@ -769,7 +769,7 @@ public:
      * @param aLayerMask A layer or layers to mask the hit test.
      * @return A pointer to a #PAD object if found otherwise NULL.
      */
-    PAD* GetPad( const VECTOR2I& aPosition, LSET aLayerMask = LSET::AllLayersMask() );
+    PAD* GetPad( const VECTOR2I& aPosition, const LSET& aLayerMask = LSET::AllLayersMask() );
 
     std::vector<const PAD*> GetPads( const wxString& aPadNumber,
                                      const PAD* aIgnore = nullptr ) const;
diff --git a/pcbnew/grid_layer_box_helpers.cpp b/pcbnew/grid_layer_box_helpers.cpp
index 5908ba09e3..f249ebd2c2 100644
--- a/pcbnew/grid_layer_box_helpers.cpp
+++ b/pcbnew/grid_layer_box_helpers.cpp
@@ -110,7 +110,7 @@ void GRID_CELL_LAYER_RENDERER::Draw( wxGrid& aGrid, wxGridCellAttr& aAttr, wxDC&
 // Note: this implementation is an adaptation of wxGridCellChoiceEditor
 
 
-GRID_CELL_LAYER_SELECTOR::GRID_CELL_LAYER_SELECTOR( PCB_BASE_FRAME* aFrame, LSET aMask,
+GRID_CELL_LAYER_SELECTOR::GRID_CELL_LAYER_SELECTOR( PCB_BASE_FRAME* aFrame, const LSET& aMask,
                                                     bool aShowNonActivated ) :
         m_frame( aFrame ),
         m_mask( aMask ),
diff --git a/pcbnew/grid_layer_box_helpers.h b/pcbnew/grid_layer_box_helpers.h
index 921c5d76b7..7c7198b54b 100644
--- a/pcbnew/grid_layer_box_helpers.h
+++ b/pcbnew/grid_layer_box_helpers.h
@@ -56,7 +56,7 @@ private:
 class GRID_CELL_LAYER_SELECTOR : public wxGridCellEditor
 {
 public:
-    GRID_CELL_LAYER_SELECTOR( PCB_BASE_FRAME* aFrame, LSET forbiddenLayers,
+    GRID_CELL_LAYER_SELECTOR( PCB_BASE_FRAME* aFrame, const LSET& forbiddenLayers,
                               bool aShowNonActivated = false );
 
     wxGridCellEditor* Clone() const override;
diff --git a/pcbnew/pad.cpp b/pcbnew/pad.cpp
index 7e9f7a100b..fa23c29a54 100644
--- a/pcbnew/pad.cpp
+++ b/pcbnew/pad.cpp
@@ -330,7 +330,7 @@ PCB_LAYER_ID PAD::GetPrincipalLayer() const
 }
 
 
-bool PAD::FlashLayer( LSET aLayers ) const
+bool PAD::FlashLayer( const LSET& aLayers ) const
 {
     for( PCB_LAYER_ID layer : aLayers.Seq() )
     {
diff --git a/pcbnew/pad.h b/pcbnew/pad.h
index 0fc3df7058..c108638651 100644
--- a/pcbnew/pad.h
+++ b/pcbnew/pad.h
@@ -814,7 +814,7 @@ public:
      * @return true if connected by pad or track (or optionally zone) on any of the associated
      *         layers
      */
-    bool FlashLayer( LSET aLayers ) const;
+    bool FlashLayer( const LSET& aLayers ) const;
 
     bool HitTest( const VECTOR2I& aPosition, int aAccuracy = 0 ) const override;
     bool HitTest( const BOX2I& aRect, bool aContained, int aAccuracy = 0 ) const override;
diff --git a/pcbnew/pcb_layer_box_selector.h b/pcbnew/pcb_layer_box_selector.h
index e91a0e312e..1426e1d032 100644
--- a/pcbnew/pcb_layer_box_selector.h
+++ b/pcbnew/pcb_layer_box_selector.h
@@ -54,7 +54,7 @@ public:
     void SetBoardFrame( PCB_BASE_FRAME* aFrame );
 
     // SetLayerSet allows disabling some layers, which are not shown in list
-    void SetNotAllowedLayerSet( LSET aMask ) { m_layerMaskDisable = aMask; }
+    void SetNotAllowedLayerSet( const LSET& aMask ) { m_layerMaskDisable = aMask; }
 
     // If the UNDEFINED_LAYER should be selectable, give it a name here.  Usually either
     // INDETERMINATE_STATE or INDETERMINATE_ACTION.
diff --git a/pcbnew/pcb_plot_params.h b/pcbnew/pcb_plot_params.h
index 3dcf0217ab..dffb56e442 100644
--- a/pcbnew/pcb_plot_params.h
+++ b/pcbnew/pcb_plot_params.h
@@ -163,11 +163,11 @@ public:
     void        SetSubtractMaskFromSilk( bool aSubtract ) { m_subtractMaskFromSilk = aSubtract; }
     bool        GetSubtractMaskFromSilk() const { return m_subtractMaskFromSilk; }
 
-    void        SetLayerSelection( LSET aSelection )    { m_layerSelection = aSelection; }
-    LSET        GetLayerSelection() const               { return m_layerSelection; }
+    void        SetLayerSelection( const LSET& aSelection ) { m_layerSelection = aSelection; }
+    LSET        GetLayerSelection() const { return m_layerSelection; }
 
     void        SetPlotOnAllLayersSequence( LSEQ aSeq ) { m_plotOnAllLayersSequence = aSeq; }
-    LSEQ        GetPlotOnAllLayersSequence() const      { return m_plotOnAllLayersSequence; }
+    LSEQ        GetPlotOnAllLayersSequence() const { return m_plotOnAllLayersSequence; }
 
     void        SetUseAuxOrigin( bool aAux ) { m_useAuxOrigin = aAux; }
     bool        GetUseAuxOrigin() const { return m_useAuxOrigin; }
diff --git a/pcbnew/pcb_track.cpp b/pcbnew/pcb_track.cpp
index a1272967f6..6e90110092 100644
--- a/pcbnew/pcb_track.cpp
+++ b/pcbnew/pcb_track.cpp
@@ -1344,7 +1344,7 @@ void PCB_VIA::SanitizeLayers()
 }
 
 
-bool PCB_VIA::FlashLayer( LSET aLayers ) const
+bool PCB_VIA::FlashLayer( const LSET& aLayers ) const
 {
     for( size_t ii = 0; ii < aLayers.size(); ++ii )
     {
diff --git a/pcbnew/pcb_track.h b/pcbnew/pcb_track.h
index 94fcbcc6c0..7058c57b82 100644
--- a/pcbnew/pcb_track.h
+++ b/pcbnew/pcb_track.h
@@ -634,7 +634,7 @@ public:
      * @return true if connected by pad or track (or optionally zone) on any of the associated
      *         layers.
      */
-    bool FlashLayer( LSET aLayers ) const;
+    bool FlashLayer( const LSET& aLayers ) const;
 
     /**
      * Return the top-most and bottom-most connected layers.
diff --git a/pcbnew/pcbplot.h b/pcbnew/pcbplot.h
index 3c81a73046..c26a37755e 100644
--- a/pcbnew/pcbplot.h
+++ b/pcbnew/pcbplot.h
@@ -83,7 +83,7 @@ public:
     }
 
     // Basic functions to plot a board item
-    void SetLayerSet( LSET aLayerMask ) { m_layerMask = aLayerMask; }
+    void SetLayerSet( const LSET& aLayerMask ) { m_layerMask = aLayerMask; }
     void PlotFootprintGraphicItems( const FOOTPRINT* aFootprint );
     void PlotFootprintTextItems( const FOOTPRINT* aFootprint );
 
@@ -215,7 +215,7 @@ void PlotOneBoardLayer( BOARD* aBoard, PLOTTER* aPlotter, PCB_LAYER_ID aLayer,
  *    SetDrillMarksType( DrillMarksType aVal )
  *        aVal = no hole, small hole, actual hole size
  */
-void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
+void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, const LSET& aLayerMask,
                         const PCB_PLOT_PARAMS& aPlotOpt );
 
 /**
@@ -226,7 +226,7 @@ void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
  * @param aLayerMask is the mask to define the layers to plot.
  * @param aPlotOpt is the plot options. Has meaning for some formats only.
  */
-void PlotLayerOutlines( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
+void PlotLayerOutlines( BOARD* aBoard, PLOTTER* aPlotter, const LSET& aLayerMask,
                         const PCB_PLOT_PARAMS& aPlotOpt );
 
 /**
diff --git a/pcbnew/plot_board_layers.cpp b/pcbnew/plot_board_layers.cpp
index be24e9727e..414da58e7b 100644
--- a/pcbnew/plot_board_layers.cpp
+++ b/pcbnew/plot_board_layers.cpp
@@ -90,7 +90,7 @@ void PlotPolySet( BOARD* aBoard, PLOTTER* aPlotter, const PCB_PLOT_PARAMS& aPlot
  * Solder mask layers have a minimum thickness value and cannot be drawn like standard layers,
  * unless the minimum thickness is 0.
  */
-void PlotSolderMaskLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
+void PlotSolderMaskLayer( BOARD* aBoard, PLOTTER* aPlotter, const LSET& aLayerMask,
                           const PCB_PLOT_PARAMS& aPlotOpt )
 {
     if( aBoard->GetDesignSettings().m_SolderMaskMinWidth == 0 )
@@ -109,7 +109,7 @@ void PlotSolderMaskLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
 }
 
 
-void PlotClippedSilkLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
+void PlotClippedSilkLayer( BOARD* aBoard, PLOTTER* aPlotter, const LSET& aLayerMask,
                            const PCB_PLOT_PARAMS& aPlotOpt )
 {
     SHAPE_POLY_SET silkscreen, solderMask;
@@ -329,7 +329,7 @@ void PlotOneBoardLayer( BOARD *aBoard, PLOTTER* aPlotter, PCB_LAYER_ID aLayer,
 /**
  * Plot any layer EXCEPT a solder-mask with an enforced minimum width.
  */
-void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
+void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, const LSET& aLayerMask,
                         const PCB_PLOT_PARAMS& aPlotOpt )
 {
     BRDITEMS_PLOTTER itemplotter( aPlotter, aBoard, aPlotOpt );
@@ -840,7 +840,7 @@ void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
 /**
  * Plot outlines.
  */
-void PlotLayerOutlines( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
+void PlotLayerOutlines( BOARD* aBoard, PLOTTER* aPlotter, const LSET& aLayerMask,
                         const PCB_PLOT_PARAMS& aPlotOpt )
 {
     BRDITEMS_PLOTTER itemplotter( aPlotter, aBoard, aPlotOpt );
diff --git a/pcbnew/sel_layer.cpp b/pcbnew/sel_layer.cpp
index ed6d7ee4b5..7e3bdd4b54 100644
--- a/pcbnew/sel_layer.cpp
+++ b/pcbnew/sel_layer.cpp
@@ -299,7 +299,8 @@ void PCB_ONE_LAYER_SELECTOR::OnRightGridCellClick( wxGridEvent& event )
 }
 
 
-PCB_LAYER_ID PCB_BASE_FRAME::SelectOneLayer( PCB_LAYER_ID aDefaultLayer, LSET aNotAllowedLayersMask,
+PCB_LAYER_ID PCB_BASE_FRAME::SelectOneLayer( PCB_LAYER_ID aDefaultLayer,
+                                             const LSET& aNotAllowedLayersMask,
                                              wxPoint aDlgPosition )
 {
     PCB_ONE_LAYER_SELECTOR dlg( this, aDefaultLayer, aNotAllowedLayersMask, true );
diff --git a/pcbnew/tools/pcb_picker_tool.h b/pcbnew/tools/pcb_picker_tool.h
index 55a6fb6041..25f06f9bc9 100644
--- a/pcbnew/tools/pcb_picker_tool.h
+++ b/pcbnew/tools/pcb_picker_tool.h
@@ -71,7 +71,7 @@ public:
     /**
      * Set the tool's snap layer set.
      */
-    inline void SetLayerSet( LSET aLayerSet ) { m_layerMask = aLayerSet; }
+    inline void SetLayerSet( const LSET& aLayerSet ) { m_layerMask = aLayerSet; }
 
     int SelectPointInteractively( const TOOL_EVENT& aEvent );
     int SelectItemInteractively( const TOOL_EVENT& aEvent );
diff --git a/pcbnew/widgets/appearance_controls.cpp b/pcbnew/widgets/appearance_controls.cpp
index f1e5491d5d..5b708a3d52 100644
--- a/pcbnew/widgets/appearance_controls.cpp
+++ b/pcbnew/widgets/appearance_controls.cpp
@@ -1294,7 +1294,7 @@ void APPEARANCE_CONTROLS::SetObjectVisible( GAL_LAYER_ID aLayer, bool isVisible
 }
 
 
-void APPEARANCE_CONTROLS::setVisibleLayers( LSET aLayers )
+void APPEARANCE_CONTROLS::setVisibleLayers( const LSET& aLayers )
 {
     KIGFX::VIEW* view = m_frame->GetCanvas()->GetView();
 
diff --git a/pcbnew/widgets/appearance_controls.h b/pcbnew/widgets/appearance_controls.h
index cabafda362..df329c5df0 100644
--- a/pcbnew/widgets/appearance_controls.h
+++ b/pcbnew/widgets/appearance_controls.h
@@ -333,7 +333,7 @@ private:
 
     void onObjectVisibilityChanged( GAL_LAYER_ID aLayer, bool isVisible, bool isFinal );
 
-    void setVisibleLayers( LSET aLayers );
+    void setVisibleLayers( const LSET& aLayers );
 
     void setVisibleObjects( GAL_SET aObjects );