From afe91c84e49b5ba0a0da75612f9e207258887dd8 Mon Sep 17 00:00:00 2001
From: AntoinePrv <AntoinePrv@users.noreply.github.com>
Date: Wed, 5 Aug 2026 18:58:49 +0200
Subject: [PATCH 1/3] Migrate recursive template instantiations

---
 include/xsimd/arch/xsimd_avx512f.hpp         | 26 +++++++++-----------
 include/xsimd/arch/xsimd_avx512vl_128.hpp    |  2 +-
 include/xsimd/arch/xsimd_avx512vl_256.hpp    |  2 +-
 include/xsimd/config/xsimd_arch.hpp          | 21 ++--------------
 include/xsimd/types/xsimd_batch_constant.hpp | 20 ++++-----------
 include/xsimd/utils/bits.hpp                 | 13 +++-------
 6 files changed, 24 insertions(+), 60 deletions(-)

diff --git a/include/xsimd/arch/xsimd_avx512f.hpp b/include/xsimd/arch/xsimd_avx512f.hpp
index 658b7d448..8bfc1feb5 100644
--- a/include/xsimd/arch/xsimd_avx512f.hpp
+++ b/include/xsimd/arch/xsimd_avx512f.hpp
@@ -2258,7 +2258,7 @@ namespace xsimd
             using register_type = typename batch_bool<T, A>::register_type;
             register_type r = 0;
             unsigned shift = 0;
-            (void)std::initializer_list<register_type> { (r |= register_type(values ? 1 : 0) << (shift++))... };
+            ((r |= register_type(values ? 1 : 0) << (shift++)), ...);
             return r;
         }
 
@@ -2656,21 +2656,19 @@ namespace xsimd
 
         namespace detail
         {
-            template <class T, class A, T... Idx>
-            struct is_pair_of_contiguous_indices;
-
-            template <class T, class A>
-            struct is_pair_of_contiguous_indices<T, A> : std::true_type
+            template <class T, T... Idx, std::size_t... Is>
+            constexpr bool is_pair_of_contiguous_indices_impl(std::index_sequence<Is...>) noexcept
             {
-            };
+                constexpr T idx[] = { Idx... };
+                return (... && (idx[2 * Is] % 2 == 0 && idx[2 * Is] + 1 == idx[2 * Is + 1]));
+            }
 
-            template <class T, class A, T Idx0, T Idx1, T... Idx>
-            struct is_pair_of_contiguous_indices<T, A, Idx0, Idx1, Idx...> : std::conditional_t<(Idx0 % 2 == 0) && (Idx0 + 1 == Idx1), is_pair_of_contiguous_indices<T, A, Idx...>, std::false_type>
+            template <class T, T... Idx>
+            constexpr bool is_pair_of_contiguous_indices() noexcept
             {
-            };
-
-            template <class T, class A, T... Idx>
-            inline constexpr bool is_pair_of_contiguous_indices_v = is_pair_of_contiguous_indices<T, A, Idx...>::value;
+                static_assert(sizeof...(Idx) % 2 == 0, "indices come in pairs");
+                return is_pair_of_contiguous_indices_impl<T, Idx...>(std::make_index_sequence<sizeof...(Idx) / 2>());
+            }
 
             template <class A, uint16_t I0, uint16_t I1, uint16_t I2, uint16_t I3, uint16_t I4, uint16_t I5, uint16_t I6, uint16_t I7,
                       uint16_t I8, uint16_t I9, uint16_t I10, uint16_t I11, uint16_t I12, uint16_t I13, uint16_t I14, uint16_t I15,
@@ -2703,7 +2701,7 @@ namespace xsimd
         template <class A, uint16_t... Idx>
         XSIMD_INLINE batch<uint16_t, A> swizzle(batch<uint16_t, A> const& self, batch_constant<uint16_t, A, Idx...> mask, requires_arch<avx512f>) noexcept
         {
-            if constexpr (detail::is_pair_of_contiguous_indices_v<uint16_t, A, Idx...>)
+            if constexpr (detail::is_pair_of_contiguous_indices<uint16_t, Idx...>())
             {
                 constexpr typename detail::fold_batch_constant<A, Idx...>::type mask32;
                 return _mm512_permutexvar_epi32(static_cast<batch<uint32_t, A>>(mask32), self);
diff --git a/include/xsimd/arch/xsimd_avx512vl_128.hpp b/include/xsimd/arch/xsimd_avx512vl_128.hpp
index 9922182fc..c2894c818 100644
--- a/include/xsimd/arch/xsimd_avx512vl_128.hpp
+++ b/include/xsimd/arch/xsimd_avx512vl_128.hpp
@@ -176,7 +176,7 @@ namespace xsimd
             using register_type = typename batch_bool<T, A>::register_type;
             register_type r = 0;
             unsigned shift = 0;
-            (void)std::initializer_list<register_type> { (r |= register_type(values ? 1 : 0) << (shift++))... };
+            ((r |= register_type(values ? 1 : 0) << (shift++)), ...);
             return r;
         }
 
diff --git a/include/xsimd/arch/xsimd_avx512vl_256.hpp b/include/xsimd/arch/xsimd_avx512vl_256.hpp
index 3d58a67f5..3c3108d9c 100644
--- a/include/xsimd/arch/xsimd_avx512vl_256.hpp
+++ b/include/xsimd/arch/xsimd_avx512vl_256.hpp
@@ -176,7 +176,7 @@ namespace xsimd
             using register_type = typename batch_bool<T, A>::register_type;
             register_type r = 0;
             unsigned shift = 0;
-            (void)std::initializer_list<register_type> { (r |= register_type(values ? 1 : 0) << (shift++))... };
+            ((r |= register_type(values ? 1 : 0) << (shift++)), ...);
             return r;
         }
 
diff --git a/include/xsimd/config/xsimd_arch.hpp b/include/xsimd/config/xsimd_arch.hpp
index 329421833..ac5546bbc 100644
--- a/include/xsimd/config/xsimd_arch.hpp
+++ b/include/xsimd/config/xsimd_arch.hpp
@@ -16,7 +16,6 @@
 #include "./xsimd_config.hpp"
 #include "./xsimd_cpuid.hpp"
 
-#include <initializer_list>
 #include <type_traits>
 #include <utility>
 
@@ -40,22 +39,6 @@ namespace xsimd
 
     namespace detail
     {
-        // Checks whether T appears in Tys.
-        template <class T, class... Tys>
-        struct contains;
-
-        template <class T>
-        struct contains<T> : std::false_type
-        {
-        };
-
-        template <class T, class Ty, class... Tys>
-        struct contains<T, Ty, Tys...>
-            : std::conditional_t<std::is_same_v<Ty, T>, std::true_type,
-                                 contains<T, Tys...>>
-        {
-        };
-
         template <typename T>
         XSIMD_INLINE constexpr T max_of(T value) noexcept
         {
@@ -100,13 +83,13 @@ namespace xsimd
         template <class Arch>
         static constexpr bool contains() noexcept
         {
-            return detail::contains<Arch, Archs...>::value;
+            return (std::is_same_v<Arch, Archs> || ...);
         }
 
         template <class F>
         static XSIMD_INLINE void for_each(F&& f) noexcept
         {
-            (void)std::initializer_list<bool> { (f(Archs {}), true)... };
+            (f(Archs {}), ...);
         }
 
         static constexpr std::size_t alignment() noexcept
diff --git a/include/xsimd/types/xsimd_batch_constant.hpp b/include/xsimd/types/xsimd_batch_constant.hpp
index c6f045f85..9cbe93b43 100644
--- a/include/xsimd/types/xsimd_batch_constant.hpp
+++ b/include/xsimd/types/xsimd_batch_constant.hpp
@@ -191,25 +191,15 @@ namespace xsimd
 
     private:
         // Build a 64-bit mask from Values... (LSB = index 0)
-        template <std::size_t I, bool... Remaining>
-        struct build_bits_helper;
-
-        template <std::size_t I>
-        struct build_bits_helper<I>
-        {
-            static constexpr uint64_t value = 0u;
-        };
-
-        template <std::size_t I, bool Current, bool... Remaining>
-        struct build_bits_helper<I, Current, Remaining...>
+        template <std::size_t... Is>
+        static constexpr uint64_t build_bits(std::index_sequence<Is...>) noexcept
         {
-            static constexpr uint64_t value = (Current ? (uint64_t(1) << I) : 0u)
-                | build_bits_helper<I + 1, Remaining...>::value;
-        };
+            return (uint64_t(0) | ... | (Values ? (uint64_t(1) << Is) : uint64_t(0)));
+        }
 
         static constexpr uint64_t bits() noexcept
         {
-            return build_bits_helper<0, Values...>::value;
+            return build_bits(std::make_index_sequence<sizeof...(Values)>());
         }
         static constexpr uint64_t low_mask(std::size_t k) noexcept
         {
diff --git a/include/xsimd/utils/bits.hpp b/include/xsimd/utils/bits.hpp
index 5d2efbc6d..f789dbad3 100644
--- a/include/xsimd/utils/bits.hpp
+++ b/include/xsimd/utils/bits.hpp
@@ -19,20 +19,13 @@ namespace xsimd
 {
     namespace utils
     {
-        template <typename I>
-        constexpr I make_bit_mask(I bit)
-        {
-            static_assert(std::is_unsigned_v<I>, "Bit operations must be done on unsigned integers");
-            assert(bit < static_cast<I>(8 * sizeof(I)));
-            return static_cast<I>(I { 1 } << bit);
-        }
-
         template <typename I, typename... Args>
         constexpr I make_bit_mask(I bit, Args... bits)
         {
-            // TODO(C++17): Use fold expression
             static_assert(std::is_unsigned_v<I>, "Bit operations must be done on unsigned integers");
-            return make_bit_mask<I>(bit) | make_bit_mask<I>(static_cast<I>(bits)...);
+            [[maybe_unused]] constexpr I bit_count = static_cast<I>(8 * sizeof(I));
+            assert(((bit < bit_count) && ... && (static_cast<I>(bits) < bit_count)));
+            return static_cast<I>(((I { 1 } << bit) | ... | (I { 1 } << static_cast<I>(bits))));
         }
 
         template <int... Bits, typename I>

From 50806dbaa07aa2f9ea9ec8ee016138993c739f8a Mon Sep 17 00:00:00 2001
From: AntoinePrv <AntoinePrv@users.noreply.github.com>
Date: Mon, 10 Aug 2026 10:07:59 +0200
Subject: [PATCH 2/3] Address review

---
 include/xsimd/arch/xsimd_avx512f.hpp      |  8 +++---
 include/xsimd/arch/xsimd_avx512vl_128.hpp |  8 +++---
 include/xsimd/arch/xsimd_avx512vl_256.hpp |  8 +++---
 include/xsimd/utils/bits.hpp              | 30 +++++++++++++++++------
 test/test_utils_bits.cpp                  | 21 ++++++++++++++++
 5 files changed, 53 insertions(+), 22 deletions(-)

diff --git a/include/xsimd/arch/xsimd_avx512f.hpp b/include/xsimd/arch/xsimd_avx512f.hpp
index 8bfc1feb5..e43a0d19c 100644
--- a/include/xsimd/arch/xsimd_avx512f.hpp
+++ b/include/xsimd/arch/xsimd_avx512f.hpp
@@ -14,6 +14,7 @@
 
 #include "../types/xsimd_avx512f_register.hpp"
 #include "../types/xsimd_batch_constant.hpp"
+#include "../utils/bits.hpp"
 
 #include <complex>
 #include <limits>
@@ -2255,11 +2256,8 @@ namespace xsimd
         XSIMD_INLINE batch_bool<T, A> set(batch_bool<T, A> const&, requires_arch<avx512f>, Values... values) noexcept
         {
             static_assert(sizeof...(Values) == batch_bool<T, A>::size, "consistent init");
-            using register_type = typename batch_bool<T, A>::register_type;
-            register_type r = 0;
-            unsigned shift = 0;
-            ((r |= register_type(values ? 1 : 0) << (shift++)), ...);
-            return r;
+            using reg_t = typename batch_bool<T, A>::register_type;
+            return ::xsimd::utils::make_bit_mask_from_bools<reg_t>(values...);
         }
 
         // shuffle
diff --git a/include/xsimd/arch/xsimd_avx512vl_128.hpp b/include/xsimd/arch/xsimd_avx512vl_128.hpp
index c2894c818..3058bad45 100644
--- a/include/xsimd/arch/xsimd_avx512vl_128.hpp
+++ b/include/xsimd/arch/xsimd_avx512vl_128.hpp
@@ -15,6 +15,7 @@
 
 #include "../types/xsimd_avx512vl_register.hpp"
 #include "../types/xsimd_batch_constant.hpp"
+#include "../utils/bits.hpp"
 
 #include <type_traits>
 
@@ -173,11 +174,8 @@ namespace xsimd
         XSIMD_INLINE batch_bool<T, A> set(batch_bool<T, A> const&, requires_arch<avx512vl_128>, Values... values) noexcept
         {
             static_assert(sizeof...(Values) == batch_bool<T, A>::size, "consistent init");
-            using register_type = typename batch_bool<T, A>::register_type;
-            register_type r = 0;
-            unsigned shift = 0;
-            ((r |= register_type(values ? 1 : 0) << (shift++)), ...);
-            return r;
+            using reg_t = typename batch_bool<T, A>::register_type;
+            return ::xsimd::utils::make_bit_mask_from_bools<reg_t>(values...);
         }
 
         // store
diff --git a/include/xsimd/arch/xsimd_avx512vl_256.hpp b/include/xsimd/arch/xsimd_avx512vl_256.hpp
index 3c3108d9c..2bfefcb13 100644
--- a/include/xsimd/arch/xsimd_avx512vl_256.hpp
+++ b/include/xsimd/arch/xsimd_avx512vl_256.hpp
@@ -15,6 +15,7 @@
 
 #include "../types/xsimd_avx512vl_register.hpp"
 #include "../types/xsimd_batch_constant.hpp"
+#include "../utils/bits.hpp"
 
 #include <type_traits>
 
@@ -173,11 +174,8 @@ namespace xsimd
         XSIMD_INLINE batch_bool<T, A> set(batch_bool<T, A> const&, requires_arch<avx512vl_256>, Values... values) noexcept
         {
             static_assert(sizeof...(Values) == batch_bool<T, A>::size, "consistent init");
-            using register_type = typename batch_bool<T, A>::register_type;
-            register_type r = 0;
-            unsigned shift = 0;
-            ((r |= register_type(values ? 1 : 0) << (shift++)), ...);
-            return r;
+            using reg_t = typename batch_bool<T, A>::register_type;
+            return ::xsimd::utils::make_bit_mask_from_bools<reg_t>(values...);
         }
 
         // store
diff --git a/include/xsimd/utils/bits.hpp b/include/xsimd/utils/bits.hpp
index f789dbad3..4faa0d970 100644
--- a/include/xsimd/utils/bits.hpp
+++ b/include/xsimd/utils/bits.hpp
@@ -19,16 +19,32 @@ namespace xsimd
 {
     namespace utils
     {
-        template <typename I, typename... Args>
-        constexpr I make_bit_mask(I bit, Args... bits)
+        template <typename... Args>
+        constexpr auto make_bit_mask(Args... bits)
+        {
+            using out_type = std::common_type_t<unsigned short, std::make_unsigned_t<Args>...>;
+            [[maybe_unused]] constexpr auto bit_count = static_cast<out_type>(8 * sizeof(out_type));
+            assert((((static_cast<out_type>(bits) < bit_count) && ...)));
+            return static_cast<out_type>((0u | ... | (1u << bits)));
+        }
+
+        /**
+         * Return a mask whose bit `i` holds the truth value of the `i`th argument.
+         */
+        template <typename I, typename... Bools>
+        constexpr I make_bit_mask_from_bools(Bools... bools) noexcept
         {
             static_assert(std::is_unsigned_v<I>, "Bit operations must be done on unsigned integers");
-            [[maybe_unused]] constexpr I bit_count = static_cast<I>(8 * sizeof(I));
-            assert(((bit < bit_count) && ... && (static_cast<I>(bits) < bit_count)));
-            return static_cast<I>(((I { 1 } << bit) | ... | (I { 1 } << static_cast<I>(bits))));
+            static_assert(sizeof...(Bools) <= 8 * sizeof(I), "Not enough bits to hold all the values");
+            // GCC's -Wsequence-point does not model fold sequencing and rejects `shift++` inside
+            // the fold operand, hence the comma to separate read from increment.
+            I mask = 0;
+            unsigned shift = 0;
+            ((mask |= static_cast<I>(static_cast<I>(bools ? 1 : 0) << shift), ++shift), ...);
+            return mask;
         }
 
-        template <int... Bits, typename I>
+        template <auto... Bits, typename I>
         constexpr bool all_bits_set(I value)
         {
             static_assert(std::is_unsigned_v<I>, "Bit operations must be done on unsigned integers");
@@ -36,7 +52,7 @@ namespace xsimd
             return (value & mask) == mask;
         }
 
-        template <int Bit, typename I>
+        template <auto Bit, typename I>
         constexpr I set_bit(I value)
         {
             static_assert(std::is_unsigned_v<I>, "Bit operations must be done on unsigned integers");
diff --git a/test/test_utils_bits.cpp b/test/test_utils_bits.cpp
index c1ac5af1b..b3de91987 100644
--- a/test/test_utils_bits.cpp
+++ b/test/test_utils_bits.cpp
@@ -17,6 +17,8 @@
 
 TEST_CASE("[utils::make_bit_mask] single bit")
 {
+    CHECK_EQ(xsimd::utils::make_bit_mask(), 0x0);
+    CHECK_EQ(xsimd::utils::make_bit_mask(1), 0b0010);
     CHECK_EQ(xsimd::utils::make_bit_mask<std::uint8_t>(0), 0x01);
     CHECK_EQ(xsimd::utils::make_bit_mask<std::uint8_t>(7), 0x80);
     CHECK_EQ(xsimd::utils::make_bit_mask<std::uint32_t>(0), 0x01u);
@@ -29,6 +31,25 @@ TEST_CASE("[utils::make_bit_mask] multiple bits")
     CHECK_EQ(xsimd::utils::make_bit_mask<std::uint8_t>(0, 2, 4), 0b00010101);
 }
 
+TEST_CASE("[utils::make_bit_mask_from_bools]")
+{
+    CHECK_EQ(xsimd::utils::make_bit_mask_from_bools<std::uint8_t>(), 0x00);
+    CHECK_EQ(xsimd::utils::make_bit_mask_from_bools<std::uint8_t>(true), 0x01);
+    CHECK_EQ(xsimd::utils::make_bit_mask_from_bools<std::uint8_t>(false), 0x00);
+    // First argument goes in the lowest bit
+    CHECK_EQ(xsimd::utils::make_bit_mask_from_bools<std::uint8_t>(true, false, false), 0b001);
+    CHECK_EQ(xsimd::utils::make_bit_mask_from_bools<std::uint8_t>(false, false, true), 0b100);
+    // Full width
+    CHECK_EQ(
+        xsimd::utils::make_bit_mask_from_bools<std::uint8_t>(true, true, true, true, true, true, true, true),
+        0xFF);
+    CHECK_EQ(
+        xsimd::utils::make_bit_mask_from_bools<std::uint16_t>(
+            true, false, true, false, true, false, true, false,
+            true, false, true, false, true, false, true, false),
+        0x5555);
+}
+
 TEST_CASE("[utils::all_bits_set] basic")
 {
     CHECK(xsimd::utils::all_bits_set<0>(0x01u));

From 8c930d0e60978fc2d9f08d1094076ea66fe4e44d Mon Sep 17 00:00:00 2001
From: AntoinePrv <AntoinePrv@users.noreply.github.com>
Date: Mon, 10 Aug 2026 11:49:01 +0200
Subject: [PATCH 3/3] More use of make_mask_from_bools

---
 include/xsimd/types/xsimd_batch_constant.hpp | 19 +++----------------
 1 file changed, 3 insertions(+), 16 deletions(-)

diff --git a/include/xsimd/types/xsimd_batch_constant.hpp b/include/xsimd/types/xsimd_batch_constant.hpp
index 9cbe93b43..d7a18069e 100644
--- a/include/xsimd/types/xsimd_batch_constant.hpp
+++ b/include/xsimd/types/xsimd_batch_constant.hpp
@@ -13,6 +13,7 @@
 #define XSIMD_BATCH_CONSTANT_HPP
 
 #include "../config/xsimd_config.hpp"
+#include "../utils/bits.hpp"
 #include "./xsimd_batch.hpp"
 #include "./xsimd_utils.hpp"
 
@@ -61,7 +62,7 @@ namespace xsimd
 
         static constexpr int mask() noexcept
         {
-            return mask_helper(0, static_cast<int>(Values)...);
+            return static_cast<int>(bits());
         }
 
         static constexpr bool none() noexcept
@@ -128,14 +129,6 @@ namespace xsimd
         }
 
     private:
-        static constexpr int mask_helper(int acc) noexcept { return acc; }
-
-        template <class... Tys>
-        static constexpr int mask_helper(int acc, int mask, Tys... masks) noexcept
-        {
-            return mask_helper(acc | mask, (masks << 1)...);
-        }
-
         struct logical_or
         {
             constexpr bool operator()(bool x, bool y) const { return x || y; }
@@ -191,15 +184,9 @@ namespace xsimd
 
     private:
         // Build a 64-bit mask from Values... (LSB = index 0)
-        template <std::size_t... Is>
-        static constexpr uint64_t build_bits(std::index_sequence<Is...>) noexcept
-        {
-            return (uint64_t(0) | ... | (Values ? (uint64_t(1) << Is) : uint64_t(0)));
-        }
-
         static constexpr uint64_t bits() noexcept
         {
-            return build_bits(std::make_index_sequence<sizeof...(Values)>());
+            return utils::make_bit_mask_from_bools<uint64_t>(Values...);
         }
         static constexpr uint64_t low_mask(std::size_t k) noexcept
         {