Fix swapped mergeIf arguments in no-else case in Dataflow IR

src/dataflow/graph.h

@@ -274,7 +274,7 @@ struct Graph : public UnifiedExpressionVisitor<Graph, Node*> {
       auto afterIfFalseState = locals; // TODO: optimize
       mergeIf(afterIfTrueState, afterIfFalseState, condition, curr, locals);
     } else {
-      mergeIf(initialState, afterIfTrueState, condition, curr, locals);
+      mergeIf(afterIfTrueState, initialState, condition, curr, locals);
     }
     parent = oldParent;
     return &bad;

test/gtest/CMakeLists.txt

@@ -9,6 +9,7 @@ set(unittest_SOURCES
   arena.cpp
   cast-check.cpp
   cfg.cpp
+  dataflow.cpp
   dfa_minimization.cpp
   disjoint_sets.cpp
   glbs.cpp

test/gtest/dataflow.cpp

@@ -0,0 +1,76 @@
+#include "dataflow/graph.h"
+#include "dataflow/node.h"
+#include "parser/wat-parser.h"
+#include "wasm.h"
+#include "gtest/gtest.h"
+
+using namespace wasm;
+using namespace wasm::DataFlow;
+
+class DataflowTest : public ::testing::Test {
+protected:
+  void parseWast(Module& wasm, const std::string& wast) {
+    auto parsed = WATParser::parseModule(wasm, wast);
+    if (auto* err = parsed.getErr()) {
+      Fatal() << err->msg << "\n";
+    }
+  }
+};
+
+// Regression test for https://github.com/WebAssembly/binaryen/issues/8273
+// In the no-else case, mergeIf arguments were swapped: the initial state
+// (before the if body) was paired with the ifTrue condition and the
+// afterIfTrue state was paired with ifFalse. This test verifies the fix.
+TEST_F(DataflowTest, IfNoElseMergeOrder) {
+  auto moduleText = R"wasm(
+    (module
+      (func $test (param $cond i32)
+        (local $x i32)
+        (local.set $x (i32.const 10))
+        (if (local.get $cond)
+          (then
+            (local.set $x (i32.const 42))
+          )
+        )
+        (drop (local.get $x))
+      )
+    )
+  )wasm";
+
+  Module wasm;
+  parseWast(wasm, moduleText);
+
+  auto* func = wasm.getFunction("test");
+  Graph graph;
+  graph.build(func, &wasm);
+
+  // Find the phi node for local $x (index 1).
+  Node* phi = nullptr;
+  for (auto& node : graph.nodes) {
+    if (node->isPhi() && node->index == 1) {
+      phi = node.get();
+      break;
+    }
+  }
+  ASSERT_NE(phi, nullptr) << "Expected a phi node for local $x";
+
+  // Phi structure: values[0] = block, values[1] = ifTrue value,
+  // values[2] = ifFalse value.
+  ASSERT_EQ(phi->values.size(), 3u);
+
+  auto* ifTrueNode = phi->values[1];
+  auto* ifFalseNode = phi->values[2];
+
+  ASSERT_TRUE(ifTrueNode->isConst());
+  ASSERT_TRUE(ifFalseNode->isConst());
+
+  int32_t ifTrueValue = ifTrueNode->expr->cast<Const>()->value.geti32();
+  int32_t ifFalseValue = ifFalseNode->expr->cast<Const>()->value.geti32();
+
+  // When condition is true (body ran), $x should be 42.
+  EXPECT_EQ(ifTrueValue, 42)
+    << "When condition is TRUE, phi should select 42 (set in ifTrue body)";
+  // When condition is false (body skipped), $x should be 10 (initial value).
+  EXPECT_EQ(ifFalseValue, 10)
+    << "When condition is FALSE, phi should select 10 (initial value)";
+}