Help the compiler vectorize adjacent_difference

benchmarks/CMakeLists.txt

@@ -106,6 +106,7 @@ function(add_benchmark name)
     target_link_libraries(benchmark-${name} PRIVATE benchmark::benchmark)
 endfunction()
 
+add_benchmark(adjacent_difference src/adjacent_difference.cpp)
 add_benchmark(bitset_from_string src/bitset_from_string.cpp)
 add_benchmark(bitset_to_string src/bitset_to_string.cpp)
 add_benchmark(efficient_nonlocking_print src/efficient_nonlocking_print.cpp)

benchmarks/src/adjacent_difference.cpp

@@ -0,0 +1,57 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <algorithm>
+#include <benchmark/benchmark.h>
+#include <cstddef>
+#include <cstdint>
+#include <limits>
+#include <numeric>
+#include <random>
+#include <type_traits>
+#include <vector>
+
+using namespace std;
+
+template <class T>
+void bm(benchmark::State& state) {
+    mt19937 gen(96337);
+
+    const size_t size = static_cast<size_t>(state.range(0));
+
+    vector<T> input(size);
+    vector<T> output(size);
+
+    if constexpr (is_floating_point_v<T>) {
+        normal_distribution<T> dis(-100.0, 100.0);
+        ranges::generate(input, [&] { return dis(gen); });
+    } else {
+        static_assert(is_unsigned_v<T>, "This avoids signed integers to avoid UB; they shouldn't perform differently");
+        uniform_int_distribution<conditional_t<sizeof(T) != 1, T, unsigned int>> dis(0, numeric_limits<T>::max());
+        ranges::generate(input, [&] { return static_cast<T>(dis(gen)); });
+    }
+
+    for (auto _ : state) {
+        benchmark::DoNotOptimize(input);
+        adjacent_difference(input.begin(), input.end(), output.begin());
+        benchmark::DoNotOptimize(output);
+    }
+}
+
+void common_args(auto bm) {
+    bm->Arg(2255);
+}
+
+#pragma warning(push)
+#pragma warning(disable : 4244) // warning C4244: '=': conversion from 'int' to 'unsigned char', possible loss of data
+BENCHMARK(bm<uint8_t>)->Apply(common_args);
+BENCHMARK(bm<uint16_t>)->Apply(common_args);
+#pragma warning(pop)
+
+BENCHMARK(bm<uint32_t>)->Apply(common_args);
+BENCHMARK(bm<uint64_t>)->Apply(common_args);
+
+BENCHMARK(bm<float>)->Apply(common_args);
+BENCHMARK(bm<double>)->Apply(common_args);
+
+BENCHMARK_MAIN();

stl/inc/numeric

@@ -461,6 +461,20 @@ _FwdIt2 transform_inclusive_scan(_ExPo&& _Exec, _FwdIt1 _First, _FwdIt1 _Last, _
     _UnaryOp _Transform_op) noexcept; // terminates
 #endif // _HAS_CXX17
 
+template <class _TyDest, class _TySrc, class _BinOp>
+void _Adjacent_difference_no_overlap(
+    _TyDest* const __restrict _Dest, _TySrc* const __restrict _Src, const ptrdiff_t _Count, _BinOp _Func) {
+    _Dest[0] = _Src[0];
+    for (ptrdiff_t _Ix = 1; _Ix != _Count; ++_Ix) {
+#if _HAS_CXX20
+        _TySrc _Tmp = _Src[_Ix - 1];
+        _Dest[_Ix]  = _Func(_Src[_Ix], _STD move(_Tmp));
+#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
+        _Dest[_Ix] = _Func(_Src[_Ix], _Src[_Ix - 1]);
+#endif // ^^^ !_HAS_CXX20 ^^^
+    }
+}
+
 _EXPORT_STD template <class _InIt, class _OutIt, class _BinOp>
 _CONSTEXPR20 _OutIt adjacent_difference(const _InIt _First, const _InIt _Last, _OutIt _Dest, _BinOp _Func) {
     // compute adjacent differences into _Dest
@@ -469,6 +483,32 @@ _CONSTEXPR20 _OutIt adjacent_difference(const _InIt _First, const _InIt _Last, _
     const auto _ULast = _STD _Get_unwrapped(_Last);
     auto _UDest       = _STD _Get_unwrapped_n(_Dest, _STD _Idl_distance<_InIt>(_UFirst, _ULast));
     if (_UFirst != _ULast) {
+        if constexpr (_Iterators_are_contiguous<decltype(_UFirst), decltype(_UDest)> && !_Iterator_is_volatile<_InIt>
+                      && is_trivially_copyable_v<_Iter_value_t<_InIt>>) {
+#if _HAS_CXX20
+            if (!_STD is_constant_evaluated())
+#endif
+            {
+                // Go with pointers and without loop-carried dependency to enable vectorization
+                const auto _First_ptr = _STD _To_address(_UFirst);
+                const auto _Last_ptr  = _STD _To_address(_ULast);
+                const auto _Dest_ptr  = _STD _To_address(_UDest);
+                const auto _Count     = _Last_ptr - _First_ptr;
+
+                // Need to perform aliasing analysis.
+                // The vectorizer is generally able to do that on its own, and would guard the vectorized code with
+                // that, but when we eliminate the loop-carried dependency we change the semantics of the unvectorized
+                // code too. So we need to perform this check manually, and after that we can tell the compiler that
+                // there's no aliasing, to avoid it checking for that again.
+                if (reinterpret_cast<uintptr_t>(_Dest_ptr + _Count) <= reinterpret_cast<uintptr_t>(_First_ptr)
+                    || reinterpret_cast<uintptr_t>(_Last_ptr) <= reinterpret_cast<uintptr_t>(_Dest_ptr)) {
+                    _STD _Adjacent_difference_no_overlap(_Dest_ptr, _First_ptr, _Count, _STD _Pass_fn(_Func));
+                    _STD _Seek_wrapped(_Dest, _UDest + _Count);
+                    return _Dest;
+                }
+            }
+        }
+
         _Iter_value_t<_InIt> _Val(*_UFirst);
         *_UDest = _Val;
         while (++_UFirst != _ULast) { // compute another difference

tests/std/tests/VSO_0000000_vector_algorithms/test.cpp

@@ -12,6 +12,7 @@
 #include <functional>
 #include <limits>
 #include <list>
+#include <numeric>
 #include <random>
 #include <stdexcept>
 #include <string>
@@ -34,6 +35,74 @@ using namespace std;
 #pragma clang diagnostic ignored "-Wc++17-extensions" // constexpr if is a C++17 extension
 #endif // __clang__
 
+template <class InIt, class OutIt, class BinOp>
+OutIt last_known_good_adj_diff(InIt first, InIt last, OutIt dest, BinOp binop) {
+    if (first == last) {
+        return dest;
+    }
+
+    auto val = *first;
+    *dest    = val;
+
+    for (++first, ++dest; first != last; ++first, ++dest) {
+        auto tmp = *first;
+        *dest    = binop(tmp, val);
+        val      = tmp;
+    }
+
+    return dest;
+}
+
+template <class T>
+void test_case_adj_diff(const vector<T>& input, vector<T>& output_expected, vector<T>& output_actual) {
+    // Avoid truncation warnings:
+    const auto subtract = [](const T& left, const T& right) { return static_cast<T>(left - right); };
+    const auto expected = last_known_good_adj_diff(input.begin(), input.end(), output_expected.begin(), subtract);
+    const auto actual   = adjacent_difference(input.begin(), input.end(), output_actual.begin(), subtract);
+
+    assert(actual - output_actual.begin() == expected - output_expected.begin());
+    assert(output_actual == output_expected);
+}
+
+template <class T>
+void test_adjacent_difference(mt19937_64& gen) {
+    using Limits = numeric_limits<T>;
+
+    uniform_int_distribution<conditional_t<sizeof(T) == 1, int, T>> dis(
+        is_signed_v<T> ? static_cast<T>(Limits::min() / 2) : Limits::min(),
+        is_signed_v<T> ? static_cast<T>(Limits::max() / 2) : Limits::max());
+
+    vector<T> input;
+    vector<T> output_expected;
+    vector<T> output_actual;
+
+    for (const auto& v : {&input, &output_expected, &output_actual}) {
+        v->reserve(dataCount);
+    }
+
+    test_case_adj_diff(input, output_expected, output_actual);
+    for (size_t attempts = 0; attempts < dataCount; ++attempts) {
+        input.push_back(static_cast<T>(dis(gen)));
+
+        for (const auto& v : {&output_expected, &output_actual}) {
+            v->assign(input.size(), 0);
+        }
+
+        test_case_adj_diff(input, output_expected, output_actual);
+    }
+}
+
+void test_adjacent_difference_with_heterogeneous_types() {
+    const vector<unsigned char> input = {10, 70, 20, 90};
+    vector<int> output(4);
+
+    const auto result = adjacent_difference(input.begin(), input.end(), output.begin());
+    assert(result == output.end());
+
+    const vector<int> expected = {10, 60, -50, 70};
+    assert(output == expected);
+}
+
 template <class FwdIt, class T>
 ptrdiff_t last_known_good_count(FwdIt first, FwdIt last, T v) {
     ptrdiff_t result = 0;
@@ -895,6 +964,18 @@ void test_swap_arrays(mt19937_64& gen) {
 }
 
 void test_vector_algorithms(mt19937_64& gen) {
+    test_adjacent_difference<char>(gen);
+    test_adjacent_difference<signed char>(gen);
+    test_adjacent_difference<unsigned char>(gen);
+    test_adjacent_difference<short>(gen);
+    test_adjacent_difference<unsigned short>(gen);
+    test_adjacent_difference<int>(gen);
+    test_adjacent_difference<unsigned int>(gen);
+    test_adjacent_difference<long long>(gen);
+    test_adjacent_difference<unsigned long long>(gen);
+
+    test_adjacent_difference_with_heterogeneous_types();
+
     test_count<char>(gen);
     test_count<signed char>(gen);
     test_count<unsigned char>(gen);