[libcxx-commits] [libcxx] [libc++] Improve performance of std::midpoint for small types (PR #192804)
via libcxx-commits
libcxx-commits at lists.llvm.org
Tue May 5 06:57:59 PDT 2026
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 01/10] [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 02/10] 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;
>From 22be4aa8bc72308c2aec9f64876146a30ec9ccb1 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sat, 18 Apr 2026 22:34:58 +0200
Subject: [PATCH 03/10] fixup! fixup! [libc++] Improve performance of
std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 16 ++++------------
1 file changed, 4 insertions(+), 12 deletions(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index aa6d7096878f0..4b47523e565a8 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -35,19 +35,11 @@ template <class _Tp>
requires(is_integral_v<_Tp> && !is_same_v<remove_cv_t<_Tp>, bool>)
[[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>;
- if constexpr (sizeof(_Tp) < sizeof(unsigned)) {
- using _Ip = conditional_t<is_signed_v<_Tp>, signed, unsigned>;
- return (static_cast<_Ip>(__a) + static_cast<_Ip>(__b)) / 2;
+ if constexpr (is_unsigned_v<_Tp>) {
+ return ((__a ^ __b) >> 1) + (__a & __b);
} else {
- constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
-
- _Up __diff = _Up(__b) - _Up(__a);
- _Up __sign_bit = __b < __a;
-
- _Up __half_diff = (__diff / 2) + (__sign_bit << __bitshift) + (__sign_bit & __diff);
-
- return __a + __half_diff;
+ _Tp __t = ((__a ^ __b) >> 1) + (__a & __b);
+ return __t + (_Tp(__t < 0) & (__a ^ __b) & _Tp(1));
}
}
>From 6c39724828954b13cc8520da4e5d13e2d4750efa Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sat, 18 Apr 2026 23:13:14 +0200
Subject: [PATCH 04/10] fixup! fixup! fixup! [libc++] Improve performance of
std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 15 +++++++++++----
1 file changed, 11 insertions(+), 4 deletions(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 4b47523e565a8..43dc8d45e2e1c 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -35,11 +35,18 @@ template <class _Tp>
requires(is_integral_v<_Tp> && !is_same_v<remove_cv_t<_Tp>, bool>)
[[nodiscard]]
_LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
- if constexpr (is_unsigned_v<_Tp>) {
- return ((__a ^ __b) >> 1) + (__a & __b);
+ if constexpr (is_unsigned_v<_Tp> && sizeof(_Tp) < sizeof(unsigned)) {
+ return ((unsigned)__a + (unsigned)__b) / 2;
} else {
- _Tp __t = ((__a ^ __b) >> 1) + (__a & __b);
- return __t + (_Tp(__t < 0) & (__a ^ __b) & _Tp(1));
+ using _Up = make_unsigned_t<_Tp>;
+ constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
+
+ _Up __diff = _Up(__b) - _Up(__a);
+ _Up __sign_bit = __b < __a;
+
+ _Up __half_diff = (__diff / 2) + (__sign_bit << __bitshift) + (__sign_bit & __diff);
+
+ return __a + __half_diff;
}
}
>From 540ecc6919128aa374dc4dcb9200871c92a75051 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sun, 19 Apr 2026 09:00:03 +0200
Subject: [PATCH 05/10] fixup! fixup! fixup! fixup! [libc++] Improve
performance of std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 1 +
1 file changed, 1 insertion(+)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 43dc8d45e2e1c..3858c41fdf8df 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -16,6 +16,7 @@
#include <__type_traits/is_integral.h>
#include <__type_traits/is_object.h>
#include <__type_traits/is_same.h>
+#include <__type_traits/is_unsigned.h>
#include <__type_traits/is_void.h>
#include <__type_traits/make_unsigned.h>
#include <__type_traits/remove_cv.h>
>From 5dee0d69d8e9b0395388706d637b19dd7065c495 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sun, 19 Apr 2026 09:20:19 +0200
Subject: [PATCH 06/10] fixup! fixup! fixup! fixup! fixup! [libc++] Improve
performance of std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 3858c41fdf8df..119fe85f9c2d7 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -37,7 +37,7 @@ template <class _Tp>
[[nodiscard]]
_LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
if constexpr (is_unsigned_v<_Tp> && sizeof(_Tp) < sizeof(unsigned)) {
- return ((unsigned)__a + (unsigned)__b) / 2;
+ return ((unsigned)__a + (unsigned)__b + (unsigned)(__a > __b)) / 2;
} else {
using _Up = make_unsigned_t<_Tp>;
constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
>From 330b26c4e476e2b3a7f838284f756187a44ca44d Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Sun, 19 Apr 2026 22:34:55 +0200
Subject: [PATCH 07/10] fixup! fixup! fixup! fixup! fixup! fixup! [libc++]
Improve performance of std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 119fe85f9c2d7..e2bfb98950625 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -36,9 +36,17 @@ template <class _Tp>
requires(is_integral_v<_Tp> && !is_same_v<remove_cv_t<_Tp>, bool>)
[[nodiscard]]
_LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
+# if defined(_LIBCPP_COMPILER_CLANG_BASED)
+ if constexpr (is_unsigned_v<_Tp> {
+ using _Ip = unsigned _BitInt(sizeof(_Tp) * 8 + 1);
+ return (_Ip(__a) + _Ip(__b) + _Ip(__a > __b)) / 2;
+ }
+# else
if constexpr (is_unsigned_v<_Tp> && sizeof(_Tp) < sizeof(unsigned)) {
return ((unsigned)__a + (unsigned)__b + (unsigned)(__a > __b)) / 2;
- } else {
+ }
+# endif
+ else {
using _Up = make_unsigned_t<_Tp>;
constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
>From b7f46c34f8d57766cb50803dc0c0ec34d3a6451f Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Fri, 24 Apr 2026 15:50:01 +0200
Subject: [PATCH 08/10] fixup! fixup! fixup! fixup! fixup! fixup! fixup!
[libc++] Improve performance of std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 4 ----
1 file changed, 4 deletions(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index e2bfb98950625..6b2c3d5d98993 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -41,10 +41,6 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_
using _Ip = unsigned _BitInt(sizeof(_Tp) * 8 + 1);
return (_Ip(__a) + _Ip(__b) + _Ip(__a > __b)) / 2;
}
-# else
- if constexpr (is_unsigned_v<_Tp> && sizeof(_Tp) < sizeof(unsigned)) {
- return ((unsigned)__a + (unsigned)__b + (unsigned)(__a > __b)) / 2;
- }
# endif
else {
using _Up = make_unsigned_t<_Tp>;
>From dcf32121f52c89ea02b02a4e0a30a6e052f6fc52 Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Tue, 5 May 2026 12:08:48 +0200
Subject: [PATCH 09/10] fixup! fixup! fixup! fixup! fixup! fixup! fixup! fixup!
[libc++] Improve performance of std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 6b2c3d5d98993..571a41f34c678 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -37,7 +37,7 @@ template <class _Tp>
[[nodiscard]]
_LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_DISABLE_UBSAN_UNSIGNED_INTEGER_CHECK {
# if defined(_LIBCPP_COMPILER_CLANG_BASED)
- if constexpr (is_unsigned_v<_Tp> {
+ if constexpr (is_unsigned_v<_Tp>) {
using _Ip = unsigned _BitInt(sizeof(_Tp) * 8 + 1);
return (_Ip(__a) + _Ip(__b) + _Ip(__a > __b)) / 2;
}
>From 27dadb59caac40839c39596e134faa78d443ebbc Mon Sep 17 00:00:00 2001
From: serge-sans-paille <sguelton at mozilla.com>
Date: Tue, 5 May 2026 15:57:15 +0200
Subject: [PATCH 10/10] fixup! fixup! fixup! fixup! fixup! fixup! fixup! fixup!
fixup! [libc++] Improve performance of std::midpoint for small types
---
libcxx/include/__numeric/midpoint.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/libcxx/include/__numeric/midpoint.h b/libcxx/include/__numeric/midpoint.h
index 571a41f34c678..4b7a87436a873 100644
--- a/libcxx/include/__numeric/midpoint.h
+++ b/libcxx/include/__numeric/midpoint.h
@@ -40,9 +40,9 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp midpoint(_Tp __a, _Tp __b) noexcept _LIBCPP_
if constexpr (is_unsigned_v<_Tp>) {
using _Ip = unsigned _BitInt(sizeof(_Tp) * 8 + 1);
return (_Ip(__a) + _Ip(__b) + _Ip(__a > __b)) / 2;
- }
+ } else
# endif
- else {
+ {
using _Up = make_unsigned_t<_Tp>;
constexpr _Up __bitshift = numeric_limits<_Up>::digits - 1;
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