[libc] Implement vector 'split' and 'concat' routines #157537

jhuber6 · 2025-09-08T20:12:59Z

Summary:
This provides some helpers for the split and concatenation routines for
changing the size of an existing vector. This includes a simple tuple
type to do the splitting. The tuple doesn't support structured bindings
yet.

The concat function is more limited than what would be ideal, but the
shufflevector builtin requires things of equivalent sizes and I
didn't think it was worth wrangling with that just yet.

llvmbot · 2025-09-08T20:13:30Z

@llvm/pr-subscribers-libc

Author: Joseph Huber (jhuber6)

Changes

Summary:
This provides some helpers for the split and concatenation routines for
changing the size of an existing vector. This includes a simple tuple
type to do the splitting. The tuple doesn't support structured bindings
yet.

The concat function is more limited than what would be ideal, but the
shufflevector builtin requires things of equivalent sizes and I
didn't think it was worth wrangling with that just yet.

Full diff: https://github.com/llvm/llvm-project/pull/157537.diff

4 Files Affected:

(modified) libc/src/__support/CPP/CMakeLists.txt (+11)
(modified) libc/src/__support/CPP/simd.h (+37)
(added) libc/src/__support/CPP/tuple.h (+106)
(modified) libc/src/__support/CPP/utility/integer_sequence.h (+10)

diff --git a/libc/src/__support/CPP/CMakeLists.txt b/libc/src/__support/CPP/CMakeLists.txt
index a389a6d1702fe..53b0b17774dac 100644
--- a/libc/src/__support/CPP/CMakeLists.txt
+++ b/libc/src/__support/CPP/CMakeLists.txt
@@ -211,8 +211,19 @@ add_object_library(
     libc.src.__support.macros.properties.os
 )
 
+add_header_library(
+  tuple
+  HDRS
+    tuple.h
+  DEPENDS
+    utility
+)
+
 add_header_library(
   simd
   HDRS
     simd.h
+  DEPENDS
+    utility
+    tuple
 )
diff --git a/libc/src/__support/CPP/simd.h b/libc/src/__support/CPP/simd.h
index 449455c5c0390..f4cdcdb820acb 100644
--- a/libc/src/__support/CPP/simd.h
+++ b/libc/src/__support/CPP/simd.h
@@ -16,7 +16,9 @@
 #include "hdr/stdint_proxy.h"
 #include "src/__support/CPP/algorithm.h"
 #include "src/__support/CPP/limits.h"
+#include "src/__support/CPP/tuple.h"
 #include "src/__support/CPP/type_traits.h"
+#include "src/__support/CPP/utility/integer_sequence.h"
 #include "src/__support/macros/attributes.h"
 #include "src/__support/macros/config.h"
 
@@ -51,6 +53,7 @@ template <typename T> inline constexpr size_t native_vector_size = 1;
 template <typename T> LIBC_INLINE constexpr T poison() {
   return __builtin_nondeterministic_value(T());
 }
+
 } // namespace internal
 
 // Type aliases.
@@ -267,6 +270,40 @@ LIBC_INLINE constexpr simd<T, N> select(simd<bool, N> m, simd<T, N> x,
   return m ? x : y;
 }
 
+namespace internal {
+template <typename T, size_t N, size_t M, size_t... I>
+LIBC_INLINE constexpr cpp::simd<T, N + M>
+concat(cpp::simd<T, N> x, cpp::simd<T, M> y, index_sequence<I...>) {
+  return __builtin_shufflevector(x, y, I...);
+}
+
+template <typename T, size_t N, size_t Count, size_t Offset, size_t... I>
+LIBC_INLINE constexpr cpp::simd<T, Count> slice(cpp::simd<T, N> x,
+                                                cpp::index_sequence<I...>) {
+  return __builtin_shufflevector(x, x, (Offset + I)...);
+}
+template <typename T, size_t N, size_t Offset, size_t Head, size_t... Tail>
+LIBC_INLINE constexpr auto split(cpp::simd<T, N> x) {
+  auto first = cpp::make_tuple(
+      slice<T, N, Head, Offset>(x, cpp::make_index_sequence<Head>{}));
+  if constexpr (sizeof...(Tail) > 0)
+    return cpp::tuple_cat(first, split<T, N, Offset + Head, Tail...>(x));
+  else
+    return first;
+}
+
+} // namespace internal
+
+// Shuffling helpers.
+template <typename T, size_t N>
+cpp::simd<T, N + N> concat(cpp::simd<T, N> x, cpp::simd<T, N> y) {
+  return internal::concat(x, y, make_index_sequence<N + N>{});
+}
+template <size_t... Sizes, typename T, size_t N> auto split(cpp::simd<T, N> x) {
+  static_assert((... + Sizes) == N, "split sizes must sum to vector size");
+  return internal::split<T, N, 0, Sizes...>(x);
+}
+
 // TODO: where expressions, scalar overloads, ABI types.
 
 } // namespace cpp
diff --git a/libc/src/__support/CPP/tuple.h b/libc/src/__support/CPP/tuple.h
new file mode 100644
index 0000000000000..01d374a0841aa
--- /dev/null
+++ b/libc/src/__support/CPP/tuple.h
@@ -0,0 +1,106 @@
+//===-- tuple utility -------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC___SUPPORT_CPP_UTILITY_TUPLE_H
+#define LLVM_LIBC_SRC___SUPPORT_CPP_UTILITY_TUPLE_H
+
+#include "src/__support/CPP/type_traits/decay.h"
+#include "src/__support/CPP/utility/integer_sequence.h"
+
+namespace LIBC_NAMESPACE_DECL {
+namespace cpp {
+
+template <typename... Ts> struct tuple;
+template <> struct tuple<> {};
+
+template <typename Head, typename... Tail>
+struct tuple<Head, Tail...> : tuple<Tail...> {
+  Head head;
+
+  LIBC_INLINE constexpr tuple() = default;
+
+  LIBC_INLINE constexpr tuple(const Head &h, const Tail &...t)
+      : tuple<Tail...>(t...), head(h) {}
+
+  LIBC_INLINE constexpr Head &get_head() { return head; }
+  LIBC_INLINE constexpr const Head &get_head() const { return head; }
+
+  LIBC_INLINE constexpr tuple<Tail...> &get_tail() { return *this; }
+  LIBC_INLINE constexpr const tuple<Tail...> &get_tail() const { return *this; }
+};
+
+template <typename... Ts> LIBC_INLINE constexpr auto make_tuple(Ts &&...args) {
+  return tuple<cpp::decay_t<Ts>...>(static_cast<Ts &&>(args)...);
+}
+template <typename... Ts> LIBC_INLINE constexpr auto tie(Ts &...args) {
+  return tuple<Ts &...>(args...);
+}
+
+template <size_t I, typename Head, typename... Tail>
+LIBC_INLINE constexpr auto &get(tuple<Head, Tail...> &t) {
+  if constexpr (I == 0)
+    return t.get_head();
+  else
+    return get<I - 1>(t.get_tail());
+}
+template <size_t I, typename Head, typename... Tail>
+LIBC_INLINE constexpr const auto &get(const tuple<Head, Tail...> &t) {
+  if constexpr (I == 0)
+    return t.get_head();
+  else
+    return get<I - 1>(t.get_tail());
+}
+
+template <typename T> struct tuple_size;
+template <typename... Ts> struct tuple_size<tuple<Ts...>> {
+  static constexpr size_t value = sizeof...(Ts);
+};
+
+template <size_t I, typename T> struct tuple_element;
+template <size_t I, typename Head, typename... Tail>
+struct tuple_element<I, tuple<Head, Tail...>>
+    : tuple_element<I - 1, tuple<Tail...>> {};
+template <typename Head, typename... Tail>
+struct tuple_element<0, tuple<Head, Tail...>> {
+  using type = Head;
+};
+
+namespace internal {
+template <typename... As, typename... Bs, size_t... I, size_t... J>
+LIBC_INLINE constexpr auto
+tuple_cat(const tuple<As...> &a, const tuple<Bs...> &b,
+          cpp::index_sequence<I...>, cpp::index_sequence<J...>) {
+  return tuple<As..., Bs...>(get<I>(a)..., get<J>(b)...);
+}
+
+template <typename First, typename Second, typename... Rest>
+LIBC_INLINE constexpr auto tuple_cat(const First &f, const Second &s,
+                                     const Rest &...rest) {
+  auto concat =
+      tuple_cat(f, s, cpp::make_index_sequence<tuple_size<First>::value>{},
+                cpp::make_index_sequence<tuple_size<Second>::value>{});
+  if constexpr (sizeof...(Rest))
+    return tuple_cat(concat, rest...);
+  else
+    return concat;
+}
+} // namespace internal
+
+template <typename... Tuples>
+LIBC_INLINE constexpr auto tuple_cat(const Tuples &...tuples) {
+  static_assert(sizeof...(Tuples) > 0, "need at least one element");
+  if constexpr (sizeof...(Tuples) == 1)
+    return (tuples, ...);
+  else
+    return internal::tuple_cat(tuples...);
+}
+
+} // namespace cpp
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC___SUPPORT_CPP_UTILITY_TUPLE_H
diff --git a/libc/src/__support/CPP/utility/integer_sequence.h b/libc/src/__support/CPP/utility/integer_sequence.h
index 06643d505aca0..37faa4f1d5aac 100644
--- a/libc/src/__support/CPP/utility/integer_sequence.h
+++ b/libc/src/__support/CPP/utility/integer_sequence.h
@@ -5,12 +5,15 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
+
 #ifndef LLVM_LIBC_SRC___SUPPORT_CPP_UTILITY_INTEGER_SEQUENCE_H
 #define LLVM_LIBC_SRC___SUPPORT_CPP_UTILITY_INTEGER_SEQUENCE_H
 
 #include "src/__support/CPP/type_traits/is_integral.h"
 #include "src/__support/macros/config.h"
 
+#include <stddef.h>
+
 namespace LIBC_NAMESPACE_DECL {
 namespace cpp {
 
@@ -30,10 +33,17 @@ template <typename T> struct make_integer_sequence<T, -1> {
 };
 } // namespace detail
 
+// index sequence
 template <typename T, int N>
 using make_integer_sequence =
     typename detail::make_integer_sequence<T, N - 1>::type;
 
+template <size_t... Ints>
+using index_sequence = integer_sequence<size_t, Ints...>;
+template <int N>
+using make_index_sequence =
+    typename detail::make_integer_sequence<size_t, N - 1>::type;
+
 } // namespace cpp
 } // namespace LIBC_NAMESPACE_DECL

michaelrj-google

given that these SIMD utilities are getting more complex, it would be best to add unit tests to ensure that they work. It'd probably be best to do that in a separate patch to avoid having one massive patch.

michaelrj-google · 2025-09-09T19:16:42Z

libc/src/__support/CPP/tuple.h

+} // namespace LIBC_NAMESPACE_DECL
+
+// For structured binding support.
+namespace std {


could you add a bit more detail on why this needs to be in namespace std?

C++ standard says so

will this clash with libc++ if being included?

It's a specialization on the cpp::tuple_element variant I believe, it should overload. It's not always present since we're freestanding hence the forward declaration.

jhuber6 · 2025-09-09T19:19:34Z

given that these SIMD utilities are getting more complex, it would be best to add unit tests to ensure that they work. It'd probably be best to do that in a separate patch to avoid having one massive patch.

Sure, I can make a base test and then merge them into this one if they can get reviewed real quick.

jhuber6 · 2025-09-09T21:17:42Z

Split into #157739 and #157739. Those are the first two commits, to see just the changes here go to the last commit.

Summary: This provides some helpers for the split and concatenation routines for changing the size of an existing vector. This includes a simple tuple type to do the splitting. The tuple doesn't support structured bindings yet. The concat function is more limited than what would be ideal, but the shufflevector builtin requires things of equivalent sizes and I didn't think it was worth wrangling with that just yet.

michaelrj-google · 2025-09-10T16:54:47Z

libc/src/__support/CPP/simd.h

 }

+namespace internal {
+template <typename T, size_t N, size_t O, size_t... I>


There are too many template arguments with one-character names. Could you give at least some of these more descriptive names here and below?

Sure, I'll add some basic comments as well since I feel like this is impossible to follow if you don't know what it's doing.

michaelrj-google · 2025-09-10T17:16:12Z

libc/src/__support/CPP/simd.h

+  // Recursively resize an input vector to the target size, increasing its size
+  // by at most double the input size each step.


Why are the intermediate steps necessary? Does the compiler not let you expand a vector by more than 2x per step?

The interface __builtin_shufflevector(x, x, {...}) cannot possibly express more indices than 2x.

michaelrj-google · 2025-09-10T17:20:04Z

libc/src/__support/CPP/simd.h

+      x, x, (Indices < OriginalSize ? static_cast<int>(Indices) : -1)...);
+}
+
+template <typename T, size_t N, size_t TargetSize, size_t OriginalSize>


the value of OriginalSize is never used

It is just below,

return extend<T, 2 * N, TargetSize, OriginalSize>( extend<T, N, 2 * N>(x, cpp::make_index_sequence<2 * N>{}));

It's used to recurse there, but I don't see how the value is used since that's just another call to the same function.

The function right above this is called, I guess I could call it 'do_extend' if you think that'd make it clearer, but it's just basic recurse until you get to the version of the function that doesn't expand.

Nevermind, renaming the function breaks everything because I remembered I wrote it this way on purpose. It'll just need to be confusing.

All good, in that case I'd say add a comment explaining that it's confusing but that's how it has to be and call it good.

michaelrj-google · 2025-09-10T17:40:46Z

libc/src/__support/CPP/simd.h

+  return __builtin_shufflevector(x, x, (Offset + Indices)...);
+}
+
+template <typename T, size_t N, size_t Offset, size_t Head, size_t... Tail>


Could you add a comment explaining that first is a tuple of one simd vector instead of a tuple containing the values in that simd vector? I misunderstood this the first time I read it.

michaelrj-google

LGTM after minor fixes

michaelrj-google · 2025-09-10T20:59:52Z

libc/src/__support/CPP/simd.h

+      x, x, (Indices < OriginalSize ? static_cast<int>(Indices) : -1)...);
+}
+
+template <typename T, size_t N, size_t TargetSize, size_t OriginalSize>


All good, in that case I'd say add a comment explaining that it's confusing but that's how it has to be and call it good.

libc/test/src/__support/CPP/simd_test.cpp

jhuber6 requested review from gchatelet, lntue, michaelrj-google, philnik777 and Sterling-Augustine September 8, 2025 20:13

llvmbot added the libc label Sep 8, 2025

jhuber6 force-pushed the shuffles branch 2 times, most recently from 4a4e39e to 92c05a6 Compare September 9, 2025 16:31

michaelrj-google reviewed Sep 9, 2025

View reviewed changes

jhuber6 force-pushed the shuffles branch from d9cce7c to 04e187a Compare September 9, 2025 21:16

jhuber6 force-pushed the shuffles branch from 04e187a to 79fcbc2 Compare September 10, 2025 00:12

jhuber6 force-pushed the shuffles branch from 79fcbc2 to f5f8920 Compare September 10, 2025 00:19

michaelrj-google reviewed Sep 10, 2025

View reviewed changes

jhuber6 added 2 commits September 10, 2025 12:07

cleanup and comments

0b9c4e7

Use max

1d3c8fd

michaelrj-google reviewed Sep 10, 2025

View reviewed changes

comments

944418d

michaelrj-google approved these changes Sep 10, 2025

View reviewed changes

Better test

b0d3b71

jhuber6 merged commit 0b87d27 into llvm:main Sep 10, 2025
19 checks passed

jtstogel added a commit to jtstogel/llvm-project that referenced this pull request Sep 11, 2025

Fix bazel target broken in llvm#157537

e6a3b53

jhuber6 pushed a commit that referenced this pull request Sep 11, 2025

Fix bazel target broken in #157537 (#157982)

cf5ae4d

		// Recursively resize an input vector to the target size, increasing its size
		// by at most double the input size each step.

[libc] Implement vector 'split' and 'concat' routines #157537

[libc] Implement vector 'split' and 'concat' routines #157537

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