From 9455dddab4c477c0e33d364bea735c541f6ca982 Mon Sep 17 00:00:00 2001
From: Geza Lore <glore@tenstorrent.com>
Date: Fri, 29 May 2026 11:26:07 +0100
Subject: [PATCH] Optimize if branches with same trailing statements (#7674)

If the same statements appears in both branches of an 'if', put a single
copy after the 'if', apply recursively. This also has the effect of
getting rid of conditionals with identical branches, but is more widely
applicable.
---
 src/V3Ast.h     |  8 ++++++++
 src/V3Const.cpp | 19 +++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/src/V3Ast.h b/src/V3Ast.h
index 7e0d2a73d..2963987f2 100644
--- a/src/V3Ast.h
+++ b/src/V3Ast.h
@@ -541,6 +541,14 @@ public:
     AstNode* backp() const VL_MT_STABLE { return m_backp; }
     AstNode* abovep() const;  // Get parent node above, only for list head and tail
     AstNode* aboveLoopp() const;  // Get parent node above, may have performance issues as loops
+    AstNode* lastp() const {  // Get last node in list, only for list head
+        UASSERT_OBJ(m_backp->m_nextp != this, this, "lastp() only allowed on head of list");
+        return m_headtailp;
+    }
+    AstNode* prevp() const {  // Previous node in list, nullptr for head of list
+        if (m_backp->m_nextp != this) return nullptr;
+        return m_backp;
+    }
     AstNode* op1p() const VL_MT_STABLE { return m_op1p; }
     AstNode* op2p() const VL_MT_STABLE { return m_op2p; }
     AstNode* op3p() const VL_MT_STABLE { return m_op3p; }
diff --git a/src/V3Const.cpp b/src/V3Const.cpp
index 2d5a07faf..949ef9f79 100644
--- a/src/V3Const.cpp
+++ b/src/V3Const.cpp
@@ -1230,6 +1230,24 @@ class ConstVisitor final : public VNVisitor {
         }
     }
 
+    void matchIfSameLast(AstNodeIf* nodep) {
+        if (!nodep->thensp()) return;
+        if (!nodep->elsesp()) return;
+        // Iterate bodies in reverse
+        AstNode* tp = nodep->thensp()->lastp();
+        AstNode* ep = nodep->elsesp()->lastp();
+        while (tp && ep && tp->sameTree(ep)) {
+            UASSERT_OBJ(!tp->nextp(), tp, "Expected no nextp");
+            UASSERT_OBJ(!ep->nextp(), ep, "Expected no nextp");
+            AstNode* const tPrevp = tp->prevp();
+            AstNode* const ePrevp = ep->prevp();
+            nodep->addNextHere(tp->unlinkFrBack());
+            VL_DO_DANGLING(pushDeletep(ep->unlinkFrBack()), ep);
+            tp = tPrevp;
+            ep = ePrevp;
+        }
+    }
+
     bool matchMaskedOr(AstAnd* nodep) {
         // Masking an OR with terms that have no bits set under the mask is replaced with masking
         // only the remaining terms. Canonical example as generated by V3Expand is:
@@ -3636,6 +3654,7 @@ class ConstVisitor final : public VNVisitor {
             } else {
                 // Optimizations that don't reform the IF itself
                 if (operandBoolShift(nodep->condp())) replaceBoolShift(nodep->condp());
+                matchIfSameLast(nodep);
                 matchIfCondCond(nodep);
             }
         }