Fix T70325 EEVEE: Performance regression with large nodetrees

This was caused by nodeChainIter which is not linear complexity.
Introduce nodeChainIterBackwards to iterate backwards faster.

source/blender/blenkernel/BKE_node.h

@@ -591,6 +591,10 @@ void nodeChainIter(const bNodeTree *ntree,
                    bool (*callback)(bNode *, bNode *, void *, const bool),
                    void *userdata,
                    const bool reversed);
+void nodeChainIterBackwards(const bNodeTree *ntree,
+                            const bNode *node_start,
+                            bool (*callback)(bNode *, bNode *, void *),
+                            void *userdata);
 void nodeParentsIter(bNode *node, bool (*callback)(bNode *, void *), void *userdata);
 
 struct bNodeLink *nodeFindLink(struct bNodeTree *ntree,

source/blender/blenkernel/intern/node.c

@@ -947,6 +947,57 @@ void nodeChainIter(const bNodeTree *ntree,
   }
 }
 
+static void iter_backwards_ex(const bNodeTree *ntree,
+                              const bNode *node_start,
+                              bool (*callback)(bNode *, bNode *, void *),
+                              void *userdata)
+{
+  LISTBASE_FOREACH (bNodeSocket *, sock, &node_start->inputs) {
+    bNodeLink *link = sock->link;
+    if (link == NULL) {
+      continue;
+    }
+    if ((link->flag & NODE_LINK_VALID) == 0) {
+      /* Skip links marked as cyclic. */
+      continue;
+    }
+    if (link->fromnode->iter_flag) {
+      /* Only iter on nodes once. */
+      continue;
+    }
+    else {
+      link->fromnode->iter_flag = 1;
+    }
+
+    if (!callback(link->fromnode, link->tonode, userdata)) {
+      return;
+    }
+    iter_backwards_ex(ntree, link->fromnode, callback, userdata);
+  }
+}
+
+/**
+ * Iterate over a chain of nodes, starting with \a node_start, executing
+ * \a callback for each node (which can return false to end iterator).
+ *
+ * Faster than nodeChainIter. Iter only once per node.
+ *
+ * \note Needs updated socket links (ntreeUpdateTree).
+ * \note Recursive
+ */
+void nodeChainIterBackwards(const bNodeTree *ntree,
+                            const bNode *node_start,
+                            bool (*callback)(bNode *, bNode *, void *),
+                            void *userdata)
+{
+  /* Reset flag. */
+  LISTBASE_FOREACH (bNode *, node, &ntree->nodes) {
+    node->iter_flag = 0;
+  }
+
+  iter_backwards_ex(ntree, node_start, callback, userdata);
+}
+
 /**
  * Iterate over all parents of \a node, executing \a callback for each parent
  * (which can return false to end iterator)

source/blender/makesdna/DNA_node_types.h

@@ -277,7 +277,9 @@ typedef struct bNode {
   /** Used at runtime when going through the tree. Initialize before use. */
   short tmp_flag;
   /** Used at runtime to tag derivatives branches. EEVEE only. */
-  short branch_tag;
+  char branch_tag;
+  /** Used at runtime when iterating over node branches. */
+  char iter_flag;
   /** Runtime during drawing. */
   struct uiBlock *block;
 

source/blender/nodes/shader/node_shader_tree.c

@@ -597,10 +597,7 @@ static void ntree_shader_bypass_tagged_bump_nodes(bNodeTree *ntree)
   ntreeUpdateTree(G.main, ntree);
 }
 
-static bool ntree_branch_count_and_tag_nodes(bNode *fromnode,
-                                             bNode *tonode,
-                                             void *userdata,
-                                             const bool UNUSED(reversed))
+static bool ntree_branch_count_and_tag_nodes(bNode *fromnode, bNode *tonode, void *userdata)
 {
   int *node_count = (int *)userdata;
   if (fromnode->tmp_flag == -1) {
@@ -629,7 +626,7 @@ static bNode *ntree_shader_copy_branch(bNodeTree *ntree,
   /* Count and tag all nodes inside the displacement branch of the tree. */
   start_node->tmp_flag = 0;
   int node_count = 1;
-  nodeChainIter(ntree, start_node, ntree_branch_count_and_tag_nodes, &node_count, true);
+  nodeChainIterBackwards(ntree, start_node, ntree_branch_count_and_tag_nodes, &node_count);
   /* Make a full copy of the branch */
   bNode **nodes_copy = MEM_mallocN(sizeof(bNode *) * node_count, __func__);
   LISTBASE_FOREACH (bNode *, node, &ntree->nodes) {
@@ -763,10 +760,7 @@ static void node_tag_branch_as_derivative(bNode *node, int dx)
   }
 }
 
-static bool ntree_shader_bump_branches(bNode *UNUSED(fromnode),
-                                       bNode *tonode,
-                                       void *userdata,
-                                       const bool UNUSED(reversed))
+static bool ntree_shader_bump_branches(bNode *UNUSED(fromnode), bNode *tonode, void *userdata)
 {
   bNodeTree *ntree = (bNodeTree *)userdata;
 
@@ -794,17 +788,8 @@ static bool ntree_shader_bump_branches(bNode *UNUSED(fromnode),
   return true;
 }
 
-static bool ntree_tag_bsdf_cb(bNode *fromnode,
-                              bNode *UNUSED(tonode),
-                              void *userdata,
-                              const bool UNUSED(reversed))
+static bool ntree_tag_bsdf_cb(bNode *fromnode, bNode *UNUSED(tonode), void *userdata)
 {
-  /* Don't evaluate nodes more than once. */
-  if (fromnode->tmp_flag) {
-    return true;
-  }
-  fromnode->tmp_flag = 1;
-
   switch (fromnode->type) {
     case SH_NODE_BSDF_ANISOTROPIC:
     case SH_NODE_EEVEE_SPECULAR:
@@ -844,12 +829,7 @@ void ntree_shader_tag_nodes(bNodeTree *ntree, bNode *output_node, nTreeTags *tag
   /* Make sure sockets links pointers are correct. */
   ntreeUpdateTree(G.main, ntree);
 
-  /* Reset visit flag. */
-  for (bNode *node = ntree->nodes.first; node; node = node->next) {
-    node->tmp_flag = 0;
-  }
-
-  nodeChainIter(ntree, output_node, ntree_tag_bsdf_cb, tags, true);
+  nodeChainIterBackwards(ntree, output_node, ntree_tag_bsdf_cb, tags);
 }
 
 /* This one needs to work on a local tree. */
@@ -878,7 +858,7 @@ void ntreeGPUMaterialNodes(bNodeTree *localtree,
 
   /* Duplicate bump height branches for manual derivatives.
    */
-  nodeChainIter(localtree, output, ntree_shader_bump_branches, localtree, true);
+  nodeChainIterBackwards(localtree, output, ntree_shader_bump_branches, localtree);
 
   /* TODO(fclem): consider moving this to the gpu shader tree evaluation. */
   nTreeTags tags = {