[libcxx-commits] [libcxx] [libc++] Improve performance of std::midpoint for small types (PR #192804)

[via libcxx-commits]

https://github.com/serge-sans-paille updated https://github.com/llvm/llvm-project/pull/192804

>From a604431a4c78776b2db85fa0429e0573a76bd67e Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sat, 18 Apr 2026 20:08:10 +0200
Subject: [PATCH 1/2] [libc++] Improve performance of std::midpoint for small
 types

---
 libcxx/include/__numeric/midpoint.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index d8e73ab8cad36..736e2b152cd38 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -36,14 +36,18 @@ template <class _Tp>
 [[nodiscard]]
 _LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
   using _Up                = make_unsigned_t<_Tp>;
-  constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
+  if constexpr (sizeof(_Tp) < sizeof(unsigned)) {
+    return (static_cast<unsigned>(__a) + static_cast<unsigned>(__b)) / 2;
+  } else {
+    constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
 
-  _Up __diff     = _Up(__b) - _Up(__a);
-  _Up __sign_bit = __b < __a;
+    _Up __diff     = _Up(__b) - _Up(__a);
+    _Up __sign_bit = __b < __a;
 
-  _Up __half_diff = (__diff / 2) + (__sign_bit << __bitshift) + (__sign_bit & __diff);
+    _Up __half_diff = (__diff / 2) + (__sign_bit << __bitshift) + (__sign_bit & __diff);
 
-  return __a + __half_diff;
+    return __a + __half_diff;
+  }
 }
 
 template <class _Tp>

>From 24123590a0d64d80bacb9740c5b262763681eeb3 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sat, 18 Apr 2026 22:11:10 +0200
Subject: [PATCH 2/2] fixup! [libc++] Improve performance of std::midpoint for
 small types

---
 libcxx/include/__numeric/midpoint.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 736e2b152cd38..aa6d7096878f0 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -37,7 +37,8 @@ template <class _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
   using _Up                = make_unsigned_t<_Tp>;
   if constexpr (sizeof(_Tp) < sizeof(unsigned)) {
-    return (static_cast<unsigned>(__a) + static_cast<unsigned>(__b)) / 2;
+    using _Ip = conditional_t<is_signed_v<_Tp>, signed, unsigned>;
+    return (static_cast<_Ip>(__a) + static_cast<_Ip>(__b)) / 2;
   } else {
     constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;