[libc++][math] Add constexpr for std::abs() #146633

arjunUpatel · 2025-07-02T04:39:21Z

Implement constexpr for std::abs() as defined by P0533R9 (new C++23 feature). Progress tracked by #105174

github-actions · 2025-07-02T04:39:40Z

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llvmbot · 2025-07-02T04:40:08Z

@llvm/pr-subscribers-libcxx

Author: Arjun Patel (arjunUpatel)

Changes

Implement constexpr for std::abs() as defined by P0533R9 (new C++23 feature). Progress tracked by #105174

Full diff: https://github.com/llvm/llvm-project/pull/146633.diff

3 Files Affected:

(modified) libcxx/include/__math/abs.h (+6-6)
(modified) libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp (+6-6)
(modified) libcxx/test/std/numerics/c.math/abs.pass.cpp (+19-11)

diff --git a/libcxx/include/__math/abs.h b/libcxx/include/__math/abs.h
index b780159f11ebf..a1d5854ac8878 100644
--- a/libcxx/include/__math/abs.h
+++ b/libcxx/include/__math/abs.h
@@ -41,25 +41,25 @@ template <class _A1, __enable_if_t<is_integral<_A1>::value, int> = 0>
 
 // abs
 
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI inline float abs(float __x) _NOEXCEPT { return __builtin_fabsf(__x); }
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI inline double abs(double __x) _NOEXCEPT { return __builtin_fabs(__x); }
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline float abs(float __x) _NOEXCEPT { return __builtin_fabsf(__x); }
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline double abs(double __x) _NOEXCEPT { return __builtin_fabs(__x); }
 
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI inline long double abs(long double __x) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline long double abs(long double __x) _NOEXCEPT {
   return __builtin_fabsl(__x);
 }
 
 template <class = int>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI inline int abs(int __x) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline int abs(int __x) _NOEXCEPT {
   return __builtin_abs(__x);
 }
 
 template <class = int>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI inline long abs(long __x) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline long abs(long __x) _NOEXCEPT {
   return __builtin_labs(__x);
 }
 
 template <class = int>
-[[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI inline long long abs(long long __x) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline long long abs(long long __x) _NOEXCEPT {
   return __builtin_llabs(__x);
 }
 
diff --git a/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp b/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp
index 3f17f21e8c108..64164240c8564 100644
--- a/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp
+++ b/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp
@@ -40,12 +40,12 @@ int main(int, char**) {
   double DummyDouble;
   long double DummyLongDouble;
 
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1) == 1);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1L) == 1L);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1LL) == 1LL);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1) == 1);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1L) == 1L);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1LL) == 1LL);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
 
   ASSERT_NOT_CONSTEXPR_CXX23(std::labs(-1L) == 1L);
   ASSERT_NOT_CONSTEXPR_CXX23(std::llabs(-1LL) == 1LL);
diff --git a/libcxx/test/std/numerics/c.math/abs.pass.cpp b/libcxx/test/std/numerics/c.math/abs.pass.cpp
index 51aee6e986836..908620b2a3290 100644
--- a/libcxx/test/std/numerics/c.math/abs.pass.cpp
+++ b/libcxx/test/std/numerics/c.math/abs.pass.cpp
@@ -20,21 +20,29 @@ struct correct_size_int {
 };
 
 template <class Source, class Result>
-void test_abs() {
-  Source neg_val = -5;
-  Source pos_val = 5;
-  Result res     = 5;
+TEST_CONSTEXPR_CXX23 void test_abs() {
+  TEST_CONSTEXPR_CXX23 Source neg_val = -5;
+  TEST_CONSTEXPR_CXX23 Source pos_val = 5;
+  TEST_CONSTEXPR_CXX23 Result res     = 5;
 
   ASSERT_SAME_TYPE(decltype(std::abs(neg_val)), Result);
-
-  assert(std::abs(neg_val) == res);
-  assert(std::abs(pos_val) == res);
+  #if TEST_STD_VER >= 23
+    static_assert(std::abs(neg_val) == res);
+    static_assert(std::abs(pos_val) == res);
+  #else 
+    assert(std::abs(neg_val) == res);
+    assert(std::abs(pos_val) == res);
+  #endif
 }
 
-void test_big() {
-  long long int big_value          = std::numeric_limits<long long int>::max(); // a value too big for ints to store
-  long long int negative_big_value = -big_value;
-  assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
+TEST_CONSTEXPR_CXX23 void test_big() {
+  TEST_CONSTEXPR_CXX23 long long int big_value          = std::numeric_limits<long long int>::max(); // a value too big for ints to store
+  TEST_CONSTEXPR_CXX23 long long int negative_big_value = -big_value;
+  #if TEST_STD_VER >= 23
+    static_assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
+  #else
+    assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
+  #endif
 }
 
 // The following is helpful to keep in mind:

github-actions · 2025-07-02T06:12:05Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions h,cpp -- libcxx/include/__math/abs.h libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-gcc.pass.cpp libcxx/test/std/numerics/c.math/abs.pass.cpp

View the diff from clang-format here.

diff --git a/libcxx/include/__math/abs.h b/libcxx/include/__math/abs.h
index a1d5854ac..cf047df68 100644
--- a/libcxx/include/__math/abs.h
+++ b/libcxx/include/__math/abs.h
@@ -41,10 +41,15 @@ template <class _A1, __enable_if_t<is_integral<_A1>::value, int> = 0>
 
 // abs
 
-[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline float abs(float __x) _NOEXCEPT { return __builtin_fabsf(__x); }
-[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline double abs(double __x) _NOEXCEPT { return __builtin_fabs(__x); }
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline float abs(float __x) _NOEXCEPT {
+  return __builtin_fabsf(__x);
+}
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline double abs(double __x) _NOEXCEPT {
+  return __builtin_fabs(__x);
+}
 
-[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline long double abs(long double __x) _NOEXCEPT {
+[[__nodiscard__]] _LIBCPP_CONSTEXPR_SINCE_CXX23 _LIBCPP_HIDE_FROM_ABI inline long double
+abs(long double __x) _NOEXCEPT {
   return __builtin_fabsl(__x);
 }
 
diff --git a/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp b/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp
index 675270264..8988491f9 100644
--- a/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp
+++ b/libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp
@@ -40,37 +40,37 @@ int main(int, char**) {
   double DummyDouble;
   long double DummyLongDouble;
 
-  // TODO(LLVM 22): Remove `__has_constexpr_builtin` conditional once support for Clang 19 is dropped.
-  #if __has_constexpr_builtin(__builtin_abs)
-    ASSERT_CONSTEXPR_CXX23(std::abs(-1) == 1);
-  #else
-    ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1) == 1);
-  #endif
-  #if __has_constexpr_builtin(__builtin_labs)
-    ASSERT_CONSTEXPR_CXX23(std::abs(-1L) == 1L);
-  #else
-    ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1L) == 1L);
-  #endif
-  #if __has_constexpr_builtin(__builtin_llabs)
-    ASSERT_CONSTEXPR_CXX23(std::abs(-1LL) == 1LL);
-  #else
-    ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1LL) == 1LL);
-  #endif
-  #if __has_constexpr_builtin(__builtin_fabsf)
-    ASSERT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
-  #else
-    ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
-  #endif
-  #if __has_constexpr_builtin(__builtin_fabs)
-    ASSERT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
-  #else
-    ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
-  #endif
-  #if __has_constexpr_builtin(__builtin_fabsl)
-    ASSERT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
-  #else
-    ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
-  #endif
+// TODO(LLVM 22): Remove `__has_constexpr_builtin` conditional once support for Clang 19 is dropped.
+#if __has_constexpr_builtin(__builtin_abs)
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1) == 1);
+#else
+  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1) == 1);
+#endif
+#if __has_constexpr_builtin(__builtin_labs)
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1L) == 1L);
+#else
+  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1L) == 1L);
+#endif
+#if __has_constexpr_builtin(__builtin_llabs)
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1LL) == 1LL);
+#else
+  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1LL) == 1LL);
+#endif
+#if __has_constexpr_builtin(__builtin_fabsf)
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
+#else
+  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
+#endif
+#if __has_constexpr_builtin(__builtin_fabs)
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
+#else
+  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
+#endif
+#if __has_constexpr_builtin(__builtin_fabsl)
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
+#else
+  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
+#endif
 
   ASSERT_NOT_CONSTEXPR_CXX23(std::labs(-1L) == 1L);
   ASSERT_NOT_CONSTEXPR_CXX23(std::llabs(-1LL) == 1LL);
diff --git a/libcxx/test/std/numerics/c.math/abs.pass.cpp b/libcxx/test/std/numerics/c.math/abs.pass.cpp
index 908620b2a..6c9456f0d 100644
--- a/libcxx/test/std/numerics/c.math/abs.pass.cpp
+++ b/libcxx/test/std/numerics/c.math/abs.pass.cpp
@@ -26,23 +26,24 @@ TEST_CONSTEXPR_CXX23 void test_abs() {
   TEST_CONSTEXPR_CXX23 Result res     = 5;
 
   ASSERT_SAME_TYPE(decltype(std::abs(neg_val)), Result);
-  #if TEST_STD_VER >= 23
-    static_assert(std::abs(neg_val) == res);
-    static_assert(std::abs(pos_val) == res);
-  #else 
-    assert(std::abs(neg_val) == res);
-    assert(std::abs(pos_val) == res);
-  #endif
+#if TEST_STD_VER >= 23
+  static_assert(std::abs(neg_val) == res);
+  static_assert(std::abs(pos_val) == res);
+#else
+  assert(std::abs(neg_val) == res);
+  assert(std::abs(pos_val) == res);
+#endif
 }
 
 TEST_CONSTEXPR_CXX23 void test_big() {
-  TEST_CONSTEXPR_CXX23 long long int big_value          = std::numeric_limits<long long int>::max(); // a value too big for ints to store
+  TEST_CONSTEXPR_CXX23 long long int big_value =
+      std::numeric_limits<long long int>::max(); // a value too big for ints to store
   TEST_CONSTEXPR_CXX23 long long int negative_big_value = -big_value;
-  #if TEST_STD_VER >= 23
-    static_assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
-  #else
-    assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
-  #endif
+#if TEST_STD_VER >= 23
+  static_assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
+#else
+  assert(std::abs(negative_big_value) == big_value); // make sure it doesn't get casted to a smaller type
+#endif
 }
 
 // The following is helpful to keep in mind:

frederick-vs-ja · 2025-07-02T06:43:24Z

libcxx/test/libcxx/numerics/c.math/constexpr-cxx23-clang.pass.cpp

-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0f) == 1.0f);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0) == 1.0);
-  ASSERT_NOT_CONSTEXPR_CXX23(std::abs(-1.0L) == 1.0L);
+  ASSERT_CONSTEXPR_CXX23(std::abs(-1) == 1);


This line is problematic. Currently, this abs overload is from the underlying C standard library and not constexpr-since-C++23 yet. I guess we can avoid testing this overload to make this PR proceed more quickly.

philnik777 · 2025-07-02T08:07:05Z

I'm really not sure whether we want to do this right now. It seems very weird to me to sometimes have constexpr support and sometimes not, depending on what you include.

arjunUpatel added 3 commits July 2, 2025 00:28

Update abs test

5c135a1

Make abs traits constexpr

32d133a

Update clang cpp23 constexpr tests

c843467

arjunUpatel requested a review from a team as a code owner July 2, 2025 04:39

llvmbot added the libc++ libc++ C++ Standard Library. Not GNU libstdc++. Not libc++abi. label Jul 2, 2025

frederick-vs-ja reviewed Jul 2, 2025

View reviewed changes

arjunUpatel added 2 commits July 2, 2025 14:56

Update gcc cpp23 constexpr tests

4f49c87

Check constexpr builtins for clang

c07b0d8

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