Internals: Optimzie unlinking rest of list after head (tail) (#6643)

AstNode::unlinkFrBackWithNext is O(n) if the subject node is not the
head of the list. We sometimes want to unlink the rest of the list
starting at the node after the head (e.g.: in
V3Sched::util::splitCheck), this patch makes that O(1) as well.

src/V3Ast.cpp

@@ -613,11 +613,14 @@ AstNode* AstNode::unlinkFrBackWithNext(VNRelinker* linkerp) {
         backp->m_nextp = nullptr;
         // Old list gets truncated
         // New list becomes a list upon itself
-        // Most common case is unlinking a entire operand tree
-        // (else we'd probably call unlinkFrBack without next)
-        // We may be in the middle of a list; we have no way to find head or tail!
-        AstNode* oldtailp = oldp;
-        while (oldtailp->m_nextp) oldtailp = oldtailp->m_nextp;
+        // Most common case is unlinking a entire operand tree, or all but the
+        // head (else we'd probably call unlinkFrBack without next)
+        AstNode* oldtailp = backp->m_headtailp;
+        if (!oldtailp) {
+            // We are in the middle of a list; we have no way to find head or tail in O(1)
+            oldtailp = oldp;
+            while (oldtailp->m_nextp) oldtailp = oldtailp->m_nextp;
+        }
         // Create new head/tail of old list
         AstNode* const oldheadp = oldtailp->m_headtailp;
         oldheadp->m_headtailp = oldp->m_backp;

src/V3Ast.h

@@ -427,7 +427,7 @@ class AstNode VL_NOT_FINAL {
 #endif
 
     AstNodeDType* m_dtypep = nullptr;  // Data type of output or assignment (etc)
-    AstNode* m_headtailp;  // When at begin/end of list, the opposite end of the list
+    AstNode* m_headtailp;  // When at begin/end of list, the opposite end of the list, else nullptr
     FileLine* m_fileline;  // Where it was declared
 #ifdef VL_DEBUG
     // Only keep track of the edit count in the node in the debug build.