Fix hang in assertion optimization (#7707 repair)

src/V3Assert.cpp

@@ -659,6 +659,34 @@ class AssertVisitor final : public VNVisitor {
             iterateChildren(nodep);
         }
 
+        if (nodep->user2()) {
+            // Combine consecutive assertOn checks if possible
+            if (AstIf* const backp = VN_CAST(nodep->backp(), If)) {
+                if (backp->nextp() == nodep  //
+                    && backp->user2()  //
+                    && backp->condp()->sameTree(nodep->condp())) {
+                    ++m_statAssertOnCombined;
+                    backp->addThensp(nodep->thensp()->unlinkFrBackWithNext());
+                    nodep->unlinkFrBack();
+                    VL_DO_DANGLING(pushDeletep(nodep), nodep);
+                    return;
+                }
+            }
+            // Combine nested assertOn checks if possible
+            if (nodep->thensp() && !nodep->thensp()->nextp() && isEmptyStmt(nodep->elsesp())) {
+                AstIf* const checkp = VN_CAST(nodep->thensp(), If);
+                if (checkp  //
+                    && checkp->user2()  //
+                    && checkp->condp()->sameTree(nodep->condp())) {
+                    ++m_statAssertOnCombined;
+                    nodep->addThensp(checkp->thensp()->unlinkFrBackWithNext());
+                    VL_DO_DANGLING(checkp->unlinkFrBack(), checkp);
+                    return;
+                }
+            }
+            return;
+        }
+
         // Swap assertOn check with single statement 'if' statement to bubble up for combining
         // Note we can't just swap the conditions as they two Ifs have different flags,
         // so swapping the Ifs themeselves then swapping back the bodies.
@@ -684,20 +712,6 @@ class AssertVisitor final : public VNVisitor {
                 }
             }
         }
-
-        // Combine consecutive assertOn checks if possible
-        if (nodep->user2()) {
-            if (AstIf* const backp = VN_CAST(nodep->backp(), If)) {
-                if (backp->nextp() == nodep  //
-                    && backp->user2()  //
-                    && backp->condp()->sameTree(nodep->condp())) {
-                    ++m_statAssertOnCombined;
-                    backp->addThensp(nodep->thensp()->unlinkFrBackWithNext());
-                    nodep->unlinkFrBack();
-                    VL_DO_DANGLING(pushDeletep(nodep), nodep);
-                }
-            }
-        }
     }
 
     //========== Case assertions

test_regress/t/t_assert_opt_check.py

@@ -15,7 +15,7 @@ test.compile(verilator_flags2=['--binary', '--stats'])
 
 test.execute(check_finished=True)
 
-test.file_grep(test.stats, r'Assertions, assertOn checks combined\s+(\d+)', 2)
-test.file_grep(test.stats, r'Assertions, assertOn checks hoisted\s+(\d+)', 11)
+test.file_grep(test.stats, r'Assertions, assertOn checks combined\s+(\d+)', 3)
+test.file_grep(test.stats, r'Assertions, assertOn checks hoisted\s+(\d+)', 15)
 
 test.passes()

test_regress/t/t_assert_opt_check.v

@@ -58,4 +58,12 @@ module t;
     end
   end
 
+  // Should combine the 2 nested assertOn checks after hoisting
+  always @(posedge clk) begin
+    if (assertEnable) begin
+      // This is an 'assert' with another 'assert' in the fail branch
+      assert(cntB - 100 == cntA); else assert(cntB == cntA + 100);
+    end
+  end
+
 endmodule