From 7affb3e580ba72cffeb457aaca512aeb05d24080 Mon Sep 17 00:00:00 2001
From: Wilson Snyder <wsnyder@wsnyder.org>
Date: Sat, 23 Aug 2025 21:10:46 -0400
Subject: [PATCH] Internals: Add $past variable statistic

---
 src/V3Assert.cpp         |  8 +++++++-
 src/V3AssertPre.cpp      | 24 ++++++++++++------------
 test_regress/t/t_past.py |  5 ++++-
 3 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/src/V3Assert.cpp b/src/V3Assert.cpp
index 72279d547..a8a3b984f 100644
--- a/src/V3Assert.cpp
+++ b/src/V3Assert.cpp
@@ -23,7 +23,7 @@
 VL_DEFINE_DEBUG_FUNCTIONS;
 
 //######################################################################
-// Assert class functions
+// AssertVisitor
 
 class AssertVisitor final : public VNVisitor {
     // CONSTANTS
@@ -48,6 +48,7 @@ class AssertVisitor final : public VNVisitor {
     VDouble0 m_statAsNotImm;  // Statistic tracking
     VDouble0 m_statAsImm;  // Statistic tracking
     VDouble0 m_statAsFull;  // Statistic tracking
+    VDouble0 m_statPastVars;  // Statistic tracking
     bool m_inSampled = false;  // True inside a sampled expression
 
     // METHODS
@@ -466,9 +467,13 @@ class AssertVisitor final : public VNVisitor {
             = new AstAlways{nodep->fileline(), VAlwaysKwd::ALWAYS, sentreep, nullptr};
         m_modp->addStmtsp(alwaysp);
         for (uint32_t i = 0; i < ticks; ++i) {
+            // TODO recognize AstVarRef is getting delayed and share variables between
+            // $pasts with same reference (or same expression).  Saves downstream
+            // optimizations from identifying and removing duplication.
             AstVar* const outvarp = new AstVar{
                 nodep->fileline(), VVarType::MODULETEMP,
                 "_Vpast_" + cvtToStr(m_modPastNum++) + "_" + cvtToStr(i), inp->dtypep()};
+            ++m_statPastVars;
             m_modp->addStmtsp(outvarp);
             AstNode* const assp = new AstAssignDly{
                 nodep->fileline(), new AstVarRef{nodep->fileline(), outvarp, VAccess::WRITE}, inp};
@@ -719,6 +724,7 @@ public:
         V3Stats::addStat("Assertions, assert immediate statements", m_statAsImm);
         V3Stats::addStat("Assertions, cover statements", m_statCover);
         V3Stats::addStat("Assertions, full/parallel case", m_statAsFull);
+        V3Stats::addStat("Assertions, $past variables", m_statPastVars);
     }
 };
 
diff --git a/src/V3AssertPre.cpp b/src/V3AssertPre.cpp
index 7b6a21bc0..89fdc83b1 100644
--- a/src/V3AssertPre.cpp
+++ b/src/V3AssertPre.cpp
@@ -461,9 +461,9 @@ private:
         if (exprp->width() > 1) exprp = new AstSel{fl, exprp, 0, 1};
         AstSenTree* sentreep = nodep->sentreep();
         if (sentreep) sentreep->unlinkFrBack();
-        AstNodeExpr* const past = new AstPast{fl, exprp};
-        past->dtypeFrom(exprp);
-        exprp = new AstAnd{fl, past, new AstNot{fl, exprp->cloneTreePure(false)}};
+        AstNodeExpr* const pastp = new AstPast{fl, exprp};
+        pastp->dtypeFrom(exprp);
+        exprp = new AstAnd{fl, pastp, new AstNot{fl, exprp->cloneTreePure(false)}};
         exprp->dtypeSetBit();
         nodep->replaceWith(exprp);
         nodep->sentreep(newSenTree(nodep, sentreep));
@@ -482,9 +482,9 @@ private:
         if (exprp->width() > 1) exprp = new AstSel{fl, exprp, 0, 1};
         AstSenTree* sentreep = nodep->sentreep();
         if (sentreep) sentreep->unlinkFrBack();
-        AstNodeExpr* const past = new AstPast{fl, exprp};
-        past->dtypeFrom(exprp);
-        exprp = new AstAnd{fl, new AstNot{fl, past}, exprp->cloneTreePure(false)};
+        AstNodeExpr* const pastp = new AstPast{fl, exprp};
+        pastp->dtypeFrom(exprp);
+        exprp = new AstAnd{fl, new AstNot{fl, pastp}, exprp->cloneTreePure(false)};
         exprp->dtypeSetBit();
         nodep->replaceWith(exprp);
         nodep->sentreep(newSenTree(nodep, sentreep));
@@ -497,9 +497,9 @@ private:
         AstNodeExpr* exprp = nodep->exprp()->unlinkFrBack();
         AstSenTree* sentreep = nodep->sentreep();
         if (sentreep) sentreep->unlinkFrBack();
-        AstNodeExpr* const past = new AstPast{fl, exprp};
-        past->dtypeFrom(exprp);
-        exprp = new AstEq{fl, past, exprp->cloneTreePure(false)};
+        AstNodeExpr* const pastp = new AstPast{fl, exprp};
+        pastp->dtypeFrom(exprp);
+        exprp = new AstEq{fl, pastp, exprp->cloneTreePure(false)};
         exprp->dtypeSetBit();
         nodep->replaceWith(exprp);
         nodep->sentreep(newSenTree(nodep, sentreep));
@@ -515,9 +515,9 @@ private:
 
         if (m_disablep) lhsp = new AstAnd{fl, new AstNot{fl, m_disablep}, lhsp};
 
-        AstNodeExpr* const past = new AstPast{fl, lhsp};
-        past->dtypeFrom(lhsp);
-        AstNodeExpr* const exprp = new AstOr{fl, new AstNot{fl, past}, rhsp};
+        AstNodeExpr* const pastp = new AstPast{fl, lhsp};
+        pastp->dtypeFrom(lhsp);
+        AstNodeExpr* const exprp = new AstOr{fl, new AstNot{fl, pastp}, rhsp};
         exprp->dtypeSetBit();
         nodep->replaceWith(exprp);
         nodep->sentreep(newSenTree(nodep));
diff --git a/test_regress/t/t_past.py b/test_regress/t/t_past.py
index d4f986441..bd043aa76 100755
--- a/test_regress/t/t_past.py
+++ b/test_regress/t/t_past.py
@@ -11,8 +11,11 @@ import vltest_bootstrap
 
 test.scenarios('simulator')
 
-test.compile()
+test.compile(verilator_flags2=['--stats'])
 
 test.execute()
 
+# Check that $past shared common variables
+test.file_grep(test.stats, r'Assertions, \$past variables\s+(\d+)', 11)
+
 test.passes()