.

[goldvitaly]

Clang-Tidy unnecessary-value-param value param will be triggered for
templated functions if at least one instantiontion with expensive to
copy type is present in translation unit.

It is relatively common mistake to write lambda functions with auto
arguments for expensive to copy types.

Example:
Copy of the vectors will happen on every comparison.

void SortBySize(std::vector<std::vector>& a) {
std::sort(
a.begin(), a.end(),
[](auto x, auto y) { return a.size() < b.size()});
}

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[llvmbot]

@llvm/pr-subscribers-clang-tidy

@llvm/pr-subscribers-clang-tools-extra

Author: Vitaly Goldshteyn (goldvitaly)

Changes

Clang-Tidy unnecessary-value-param value param will be triggered for
templated functions if at least one instantiontion with expensive to
copy type is present in translation unit.

It is relatively common mistake to write lambda functions with auto
arguments for expensive to copy types.

Example:
Copy of the vectors will happen on every comparison.

void SortBySize(std::vector<std::vector<int>>& a) {
std::sort(
a.begin(), a.end(),
[](auto x, auto y) { return a.size() < b.size()});
}

---

Full diff: https://github.com/llvm/llvm-project/pull/97766.diff

1 Files Affected:

• (added) clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp (+92)

diff --git a/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp b/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp
new file mode 100644
index 0000000000000..4e6ec401f17c8
--- /dev/null
+++ b/clang-tools-extra/test/clang-tidy/checkers/performance/unnecessary-value-param-templates.cpp
@@ -0,0 +1,92 @@
+// RUN: %check_clang_tidy  -std=c++14-or-later %s performance-unnecessary-value-param %t
+
+struct ExpensiveToCopyType {
+  virtual ~ExpensiveToCopyType();
+};
+
+template <typename T> void templateWithNonTemplatizedParameter(const ExpensiveToCopyType S, T V) {
+  // CHECK-MESSAGES: [[@LINE-1]]:90: warning: the const qualified parameter 'S'
+  // CHECK-MESSAGES: [[@LINE-2]]:95: warning: the parameter 'V'
+  // CHECK-FIXES: template <typename T> void templateWithNonTemplatizedParameter(const ExpensiveToCopyType& S, const T& V) {
+}
+
+void instantiatedWithExpensiveValue() {
+  templateWithNonTemplatizedParameter(
+      ExpensiveToCopyType(), ExpensiveToCopyType());
+  templateWithNonTemplatizedParameter(ExpensiveToCopyType(), 5);
+}
+
+template <typename T> void templateWithNonTemplatizedParameterCheapTemplate(const ExpensiveToCopyType S, T V) {
+  // CHECK-MESSAGES: [[@LINE-1]]:103: warning: the const qualified parameter 'S'
+  // CHECK-FIXES: template <typename T> void templateWithNonTemplatizedParameterCheapTemplate(const ExpensiveToCopyType& S, T V) {
+}
+
+void instantiatedWithCheapValue() {
+  templateWithNonTemplatizedParameterCheapTemplate(ExpensiveToCopyType(), 5);
+}
+
+template <typename T> void nonInstantiatedTemplateWithConstValue(const T S) {}
+template <typename T> void nonInstantiatedTemplateWithNonConstValue(T S) {}
+
+template <typename T> void instantiatedTemplateSpecialization(T NoSpecS) {}
+template <>
+void instantiatedTemplateSpecialization<ExpensiveToCopyType>(
+    ExpensiveToCopyType SpecS) {
+  // CHECK-MESSAGES: [[@LINE-1]]:25: warning: the parameter 'SpecS'
+  // CHECK-FIXES: const T& NoSpecS
+  // CHECK-FIXES: const ExpensiveToCopyType& SpecS
+}
+
+void instantiatedTemplateSpecialization() {
+  instantiatedTemplateSpecialization(ExpensiveToCopyType());
+}
+
+template <typename T> void instantiatedTemplateWithConstValue(const T S) {
+  // CHECK-MESSAGES: [[@LINE-1]]:71: warning: the const qualified parameter 'S'
+  // CHECK-FIXES: template <typename T> void instantiatedTemplateWithConstValue(const T& S) {
+}
+
+void instantiatedConstValue() {
+  instantiatedTemplateWithConstValue(ExpensiveToCopyType());
+}
+
+template <typename T> void instantiatedTemplateWithNonConstValue(T S) {
+  // CHECK-MESSAGES: [[@LINE-1]]:68: warning: the parameter 'S'
+  // CHECK-FIXES: template <typename T> void instantiatedTemplateWithNonConstValue(const T& S) {
+}
+
+void instantiatedNonConstValue() {
+  instantiatedTemplateWithNonConstValue(ExpensiveToCopyType());
+}
+
+void lambdaConstValue() {
+  auto fn = [](const ExpensiveToCopyType S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:42: warning: the const qualified parameter 'S'
+    // CHECK-FIXES: auto fn = [](const ExpensiveToCopyType& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
+
+void lambdaNonConstValue() {
+  auto fn = [](ExpensiveToCopyType S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:36: warning: the parameter 'S'
+    // CHECK-FIXES: auto fn = [](const ExpensiveToCopyType& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
+
+void lambdaConstAutoValue() {
+  auto fn = [](const auto S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:27: warning: the const qualified parameter 'S'
+    // CHECK-FIXES: auto fn = [](const auto& S) {
+  };
+  fn(ExpensiveToCopyType());
+}
+
+void lambdaNonConstAutoValue() {
+  auto fn = [](auto S) {
+    // CHECK-MESSAGES: [[@LINE-1]]:21: warning: the parameter 'S'
+    // CHECK-FIXES: auto fn = [](const auto& S) {
+  };
+  fn(ExpensiveToCopyType());
+}