Optimization: remove unreachable if/else branch

belijzajac · Apr 27, 2024 · aa33efb · aa33efb
1 parent 0e39898
commit aa33efb
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Showing 2 changed files with 58 additions and 5 deletions.
diff --git a/src/backend/intermediate/IRGenerator.cpp b/src/backend/intermediate/IRGenerator.cpp
@@ -717,7 +717,38 @@ void IRGenerator::visit(AST::IfStmt &node) {
   };
 
   node.getCondition()->accept(*this);
-  const auto &[token, _] = getExpression(*node.getCondition());
+  const auto &[token, _] = getExpression(*node.getCondition(), false);
+
+  if (token->isLiteralType()) {
+    // compile-time optimization
+
+    bool canElseBranchBeRemoved;
+    switch (token->getType()) {
+      case TType::LIT_INT:
+        canElseBranchBeRemoved = token->getValue<int>() > 0;
+        break;
+      case TType::KW_TRUE:
+        canElseBranchBeRemoved = true;
+        break;
+      case TType::KW_FALSE:
+        canElseBranchBeRemoved = false;
+        break;
+      default:
+        throw InstructionError{fmt::format("Unsupported expression in if statement in {}:{}",
+                                           token->getPosition().getFileName(),
+                                           token->getPosition().getLineNo())};
+    }
+
+    if (canElseBranchBeRemoved) {
+      node.getBody()->accept(*this);
+    } else {
+      for (const auto &stmt : node.getElseStatements()) {
+        stmt->accept(*this);
+      }
+    }
+
+    return;
+  }
 
   Operation comparisonOp = m_instructions.back().get()->getOperation();
   Operation jumpOp;

diff --git a/tests/programs/ProgramTest.cpp b/tests/programs/ProgramTest.cpp
@@ -986,12 +986,14 @@ TEST_F(ProgramTest, ConditionalBooleanFollowup) {
 TEST_F(ProgramTest, ConditionalBooleanNested) {
   constexpr auto program = R"(
   fn main() {
-    bool value = true;
-    if (value) {
+    bool value1 = true;
+    bool value2 = true;
+    bool value3 = true;
+    if (value1) {
       print("true");
-      if (value) {
+      if (value2) {
         print("true");
-        if (value) {
+        if (value3) {
           print("true");
         }
       }
@@ -1000,3 +1002,23 @@ TEST_F(ProgramTest, ConditionalBooleanNested) {
   SetUp(program);
   EXPECT_PROGRAM_OUTPUT(exec("./a.out"), "truetruetrue");
 }
+
+TEST_F(ProgramTest, ConditionalOptimizationRemoveElseBranch) {
+  constexpr auto program = R"(
+  fn main() {
+    if (5)    { print("true"); } else { print("false"); }
+    if (true) { print("true"); } else { print("false"); }
+  })"sv;
+  SetUp(program);
+  EXPECT_PROGRAM_OUTPUT(exec("./a.out"), "truetrue");
+}
+
+TEST_F(ProgramTest, ConditionalOptimizationRemoveIfBranch) {
+  constexpr auto program = R"(
+  fn main() {
+    if (0)     { print("true"); } else { print("false"); }
+    if (false) { print("true"); } else { print("false"); }
+  })"sv;
+  SetUp(program);
+  EXPECT_PROGRAM_OUTPUT(exec("./a.out"), "falsefalse");
+}