From 08be65a7dd76dc1a3074f1d2d83b8d8c5a4282ab Mon Sep 17 00:00:00 2001
From: Artur Bieniek <abieniek@internships.antmicro.com>
Date: Fri, 19 Sep 2025 15:36:57 +0200
Subject: [PATCH] Optimize dead functions in more cases (#6430)

Signed-off-by: Artur Bieniek <abieniek@internships.antmicro.com>
---
 src/V3Dead.cpp                    | 26 ++++++++++++++++++--------
 test_regress/t/t_opt_dead_task.py |  2 +-
 test_regress/t/t_opt_dead_task.v  | 14 ++++++++++++++
 3 files changed, 33 insertions(+), 9 deletions(-)

diff --git a/src/V3Dead.cpp b/src/V3Dead.cpp
index cf548f880..97cd5d36e 100644
--- a/src/V3Dead.cpp
+++ b/src/V3Dead.cpp
@@ -39,6 +39,7 @@
 
 #include "V3Stats.h"
 
+#include <queue>
 #include <vector>
 
 VL_DEFINE_DEBUG_FUNCTIONS;
@@ -53,7 +54,7 @@ class DeadVisitor final : public VNVisitor {
     //  AstVar::user1()         -> int. Count of number of references
     //  AstVarScope::user1()    -> int. Count of number of references
     //  AstNodeDType::user1()   -> int. Count of number of references
-    //  AstNodeFTask::user1()   -> int. Non-zero if ever referenced (called)
+    //  AstNodeFTask::user1()   -> int. Count of number of references (via AstNodeFTaskRefs)
     const VNUser1InUse m_inuser1;
 
     // TYPES
@@ -72,7 +73,7 @@ class DeadVisitor final : public VNVisitor {
     std::vector<AstCell*> m_cellsp;
     std::vector<AstClass*> m_classesp;
     std::vector<AstTypedef*> m_typedefsp;
-    std::vector<AstNodeFTask*> m_tasksp;  // All the tasks that could be removed if not called
+    std::queue<AstNodeFTask*> m_tasksp;  // All the tasks that could be removed if not called
     AssignMap m_assignMap;  // List of all simple assignments for each variable
     bool m_sideEffect = false;  // Side effects discovered in assign RHS
 
@@ -173,7 +174,7 @@ class DeadVisitor final : public VNVisitor {
     void visit(AstNodeFTaskRef* nodep) override {
         iterateChildren(nodep);
         checkAll(nodep);
-        if (nodep->taskp()) nodep->taskp()->user1(1);
+        if (nodep->taskp()) nodep->taskp()->user1Inc();
         if (nodep->classOrPackagep()) {
             if (m_elimCells) {
                 nodep->classOrPackagep(nullptr);
@@ -325,7 +326,7 @@ class DeadVisitor final : public VNVisitor {
         iterateChildren(nodep);
         checkAll(nodep);
         if (!nodep->taskPublic() && !nodep->dpiExport() && !nodep->dpiImport())
-            m_tasksp.push_back(nodep);
+            m_tasksp.push(nodep);
         if (nodep->classOrPackagep()) {
             if (m_elimCells) {
                 nodep->classOrPackagep(nullptr);
@@ -365,8 +366,17 @@ class DeadVisitor final : public VNVisitor {
     }
 
     void deadCheckTasks() {
-        for (AstNodeFTask* taskp : m_tasksp) {
-            if (!taskp->user1() && !taskp->classMethod()) {
+        while (!m_tasksp.empty()) {
+            AstNodeFTask* taskp = m_tasksp.front();
+            m_tasksp.pop();
+            if (taskp->user1() == 0 && !taskp->classMethod()) {
+                taskp->foreach([this](AstNodeFTaskRef* ftaskrefp) {
+                    AstNodeFTask* task2p = ftaskrefp->taskp();
+                    if (!task2p) return;
+                    task2p->user1Inc(-1);
+                    if (task2p->user1() == 0) m_tasksp.push(task2p);
+                });
+                taskp->user1(-1);  // we don't want to try deleting twice
                 deleting(taskp);
                 m_statFTasksDeadified++;
             }
@@ -611,12 +621,12 @@ void V3Dead::deadifyDTypesScoped(AstNetlist* nodep) {
 
 void V3Dead::deadifyAll(AstNetlist* nodep) {
     UINFO(2, __FUNCTION__ << ":");
-    { DeadVisitor{nodep, true, true, false, true, false, false}; }  // Destruct before checking
+    { DeadVisitor{nodep, true, true, false, true, false, true}; }  // Destruct before checking
     V3Global::dumpCheckGlobalTree("deadAll", 0, dumpTreeEitherLevel() >= 3);
 }
 
 void V3Dead::deadifyAllScoped(AstNetlist* nodep) {
     UINFO(2, __FUNCTION__ << ":");
-    { DeadVisitor{nodep, true, true, true, true, false, false}; }  // Destruct before checking
+    { DeadVisitor{nodep, true, true, true, true, false, true}; }  // Destruct before checking
     V3Global::dumpCheckGlobalTree("deadAllScoped", 0, dumpTreeEitherLevel() >= 3);
 }
diff --git a/test_regress/t/t_opt_dead_task.py b/test_regress/t/t_opt_dead_task.py
index 95f680736..84ef5c5e2 100755
--- a/test_regress/t/t_opt_dead_task.py
+++ b/test_regress/t/t_opt_dead_task.py
@@ -14,7 +14,7 @@ test.scenarios('simulator')
 test.compile(verilator_flags2=["--stats"])
 
 if test.vlt_all:
-    test.file_grep(test.stats, r'Optimizations, deadified FTasks\s+(\d+)', 2)
+    test.file_grep(test.stats, r'Optimizations, deadified FTasks\s+(\d+)', 6)
 
 test.execute()
 
diff --git a/test_regress/t/t_opt_dead_task.v b/test_regress/t/t_opt_dead_task.v
index 0be4aa58a..fe9e1aeba 100644
--- a/test_regress/t/t_opt_dead_task.v
+++ b/test_regress/t/t_opt_dead_task.v
@@ -22,7 +22,21 @@ module t (input clk);
 
    // These should be deadified
    function deadfunc();
+      deeptask2();
    endfunction
    task deadtask;
+      deeptask1();
+   endtask
+   // A chain of dead tasks calling each other to ensure V3Dead can remove chained dead tasks
+   task deeptask1;
+      deeptask2();
+   endtask
+   task deeptask2;
+      deeptask3();
+   endtask
+   task deeptask3;
+      deeptask4();
+   endtask
+   task deeptask4;
    endtask
 endmodule