diff --git a/include/cfg.h b/include/cfg.h
index 3a1dada..ce65f0a 100644
--- a/include/cfg.h
+++ b/include/cfg.h
@@ -5,6 +5,7 @@
 #include <vector>
 
 #include "ast.h"
+#include "constexpr.h"
 
 namespace yl {
 struct BasicBlock {
@@ -42,6 +43,7 @@ struct CFG : public Dumpable {
 };
 
 class CFGBuilder {
+  ConstantExpressionEvaluator cee;
   CFG cfg;
 
   int insertBlock(const ResolvedBlock &block, int successor);
diff --git a/include/constexpr.h b/include/constexpr.h
index c069b6e..c775185 100644
--- a/include/constexpr.h
+++ b/include/constexpr.h
@@ -8,12 +8,14 @@
 namespace yl {
 class ConstantExpressionEvaluator {
   std::optional<double>
-  evaluateBinaryOperator(const ResolvedBinaryOperator &binop);
+  evaluateBinaryOperator(const ResolvedBinaryOperator &binop,
+                         bool allowSideEffects = false);
   std::optional<double> evaluateUnaryOperator(const ResolvedUnaryOperator &op);
   std::optional<double> evaluateDeclRefExpr(const ResolvedDeclRefExpr &dre);
 
 public:
-  std::optional<double> evaluate(const ResolvedExpr &expr);
+  std::optional<double> evaluate(const ResolvedExpr &expr,
+                                 bool allowSideEffects = false);
 };
 } // namespace yl
 
diff --git a/src/cfg.cpp b/src/cfg.cpp
index 88ae220..fe95024 100644
--- a/src/cfg.cpp
+++ b/src/cfg.cpp
@@ -46,7 +46,7 @@ int CFGBuilder::insertIfStmt(const ResolvedIfStmt &stmt, int exit) {
   int trueBlock = insertBlock(*stmt.trueBlock, exit);
   int entry = cfg.insertNewBlock();
 
-  std::optional<double> val = stmt.condition->getConstantValue();
+  std::optional<double> val = cee.evaluate(*stmt.condition, true);
   cfg.insertEdge(entry, trueBlock, val != 0);
   cfg.insertEdge(entry, falseBlock, val.value_or(0) == 0);
 
@@ -61,7 +61,7 @@ int CFGBuilder::insertWhileStmt(const ResolvedWhileStmt &stmt, int exit) {
   int header = cfg.insertNewBlock();
   cfg.insertEdge(latch, header, true);
 
-  std::optional<double> val = stmt.condition->getConstantValue();
+  std::optional<double> val = cee.evaluate(*stmt.condition, true);
   cfg.insertEdge(header, body, val != 0);
   cfg.insertEdge(header, exit, val.value_or(0) == 0);
 
diff --git a/src/constexpr.cpp b/src/constexpr.cpp
index 4fb0f38..704ae90 100644
--- a/src/constexpr.cpp
+++ b/src/constexpr.cpp
@@ -14,33 +14,44 @@ std::optional<bool> toBool(std::optional<double> d) {
 
 namespace yl {
 std::optional<double> ConstantExpressionEvaluator::evaluateBinaryOperator(
-    const ResolvedBinaryOperator &binop) {
+    const ResolvedBinaryOperator &binop, bool allowSideEffects) {
   std::optional<double> lhs = evaluate(*binop.lhs);
 
+  if (!lhs && !allowSideEffects)
+    return std::nullopt;
+
   if (binop.op == TokenKind::PipePipe) {
     // If the LHS of || is true, we don't need to evaluate the RHS.
-    if (toBool(lhs).value_or(false))
+    if (toBool(lhs) == true)
+      return 1.0;
+
+    // If the LHS is false, or side effects are allowed and the RHS is true, the
+    // result is true.
+    std::optional<double> rhs = evaluate(*binop.rhs);
+    if (toBool(rhs) == true)
       return 1.0;
 
-    return toBool(evaluate(*binop.rhs));
+    // If both sides are known but none of them is true, the result is false.
+    if (lhs && rhs)
+      return 0.0;
+
+    // Otherwise one of the sides is unknown, so the result is unknown too.
+    return std::nullopt;
   }
 
   if (binop.op == TokenKind::AmpAmp) {
     // If the LHS of && is false, we don't need to evaluate the RHS.
-    if (binop.op == TokenKind::AmpAmp && !toBool(lhs).value_or(true))
+    if (toBool(lhs) == false)
       return 0.0;
 
-    // If the LHS is unknown, but the RHS is false, the expression is false.
     std::optional<double> rhs = evaluate(*binop.rhs);
-    if (!lhs) {
-      if (rhs == 0.0)
-        return rhs;
+    if (toBool(rhs) == false)
+      return 0.0;
 
-      return std::nullopt;
-    }
+    if (lhs && rhs)
+      return 1.0;
 
-    // Otherwise LHS is known to be true, so the result depends on the RHS.
-    return toBool(rhs);
+    return std::nullopt;
   }
 
   if (!lhs)
@@ -95,7 +106,8 @@ std::optional<double> ConstantExpressionEvaluator::evaluateDeclRefExpr(
 }
 
 std::optional<double>
-ConstantExpressionEvaluator::evaluate(const ResolvedExpr &expr) {
+ConstantExpressionEvaluator::evaluate(const ResolvedExpr &expr,
+                                      bool allowSideEffects) {
   // Don't evaluate the same expression multiple times.
   if (std::optional<double> val = expr.getConstantValue())
     return val;
@@ -106,11 +118,11 @@ ConstantExpressionEvaluator::evaluate(const ResolvedExpr &expr) {
 
   if (const auto *groupingExpr =
           dynamic_cast<const ResolvedGroupingExpr *>(&expr))
-    return evaluate(*groupingExpr->expr);
+    return evaluate(*groupingExpr->expr, allowSideEffects);
 
   if (const auto *binaryOperator =
           dynamic_cast<const ResolvedBinaryOperator *>(&expr))
-    return evaluateBinaryOperator(*binaryOperator);
+    return evaluateBinaryOperator(*binaryOperator, allowSideEffects);
 
   if (const auto *unaryOperator =
           dynamic_cast<const ResolvedUnaryOperator *>(&expr))
diff --git a/test/constexpr/special_cases.yl b/test/constexpr/special_cases.yl
index ec711a0..6131460 100644
--- a/test/constexpr/special_cases.yl
+++ b/test/constexpr/special_cases.yl
@@ -52,19 +52,15 @@ fn lhsUnknownRhsFalse(x: number): number {
 }
 // CHECK:    ResolvedReturnStmt
 // CHECK-NEXT:      ResolvedBinaryOperator: '&&'
-// CHECK-NEXT:      | value: 0
 // CHECK-NEXT:        ResolvedDeclRefExpr: @({{.*}}) x
 // CHECK-NEXT:        ResolvedNumberLiteral: '0'
 
 fn lhsUnknownRhsTrue(x: number): number {
-    return x && (0.0 - 1.0);
+    return x || 1.0;
 }
-// CHECK: ResolvedReturnStmt
-// CHECK-NEXT:   ResolvedBinaryOperator: '&&'
-// CHECK-NEXT:     ResolvedDeclRefExpr: @({{.*}}) x
-// CHECK-NEXT:     ResolvedGroupingExpr:
-// CHECK-NEXT:       ResolvedBinaryOperator: '-'
-// CHECK-NEXT:         ResolvedNumberLiteral: '0'
-// CHECK-NEXT:         ResolvedNumberLiteral: '1'
+// CHECK:    ResolvedReturnStmt
+// CHECK-NEXT:      ResolvedBinaryOperator: '||'
+// CHECK-NEXT:        ResolvedDeclRefExpr: @({{.*}}) x
+// CHECK-NEXT:        ResolvedNumberLiteral: '1'
 
 fn main(): void {}
diff --git a/test/sema/return_on_every_branch.yl b/test/sema/return_on_every_branch.yl
index 58f1e91..0b1dc1f 100644
--- a/test/sema/return_on_every_branch.yl
+++ b/test/sema/return_on_every_branch.yl
@@ -144,5 +144,20 @@ fn noReturnNeverRunningLoop(): number {
     while 0.0 {}
 }
 
+fn returnAllPath(x: number): number {
+    if (x || 1) {
+        return 1;
+    } else {
+
+    }
+}
+
+fn returnAllPathElse(x: number): number {
+    if (x && 0) {
+    } else {
+        return 1;
+    }
+}
+
 fn main(): void {}
 // CHECK-NOT: {{.*}}