Fix vectorized `ranges::find` with `unreachable_sentinel` to properly mask the beginning and handle unaligned pointers (#4450)

stl/src/vector_algorithms.cpp

@@ -1844,7 +1844,10 @@ namespace {
     template <class _Traits, class _Ty>
     const void* __stdcall __std_find_trivial_unsized_impl(const void* _First, const _Ty _Val) noexcept {
 #ifndef _M_ARM64EC
-        if (_Use_avx2()) {
+        if ((reinterpret_cast<uintptr_t>(_First) & (sizeof(_Ty) - 1)) != 0) {
+            // _First isn't aligned to sizeof(_Ty), so we need to use the scalar fallback below.
+            // This can happen with 8-byte elements on x86's 4-aligned stack. It can also happen with packed structs.
+        } else if (_Use_avx2()) {
             _Zeroupper_on_exit _Guard; // TRANSITION, DevCom-10331414
 
             // We read by vector-sized pieces, and we align pointers to vector-sized boundary.
@@ -1862,16 +1865,8 @@ namespace {
             unsigned int _Bingo = static_cast<unsigned int>(_mm256_movemask_epi8(_Traits::_Cmp_avx(_Data, _Comparand)));
 
             _Bingo &= _Mask;
-            if (_Bingo != 0) {
-                unsigned long _Offset = _tzcnt_u32(_Bingo);
-                _Advance_bytes(_First, _Offset);
-                return _First;
-            }
 
             for (;;) {
-                _Data  = _mm256_load_si256(static_cast<const __m256i*>(_First));
-                _Bingo = static_cast<unsigned int>(_mm256_movemask_epi8(_Traits::_Cmp_avx(_Data, _Comparand)));
-
                 if (_Bingo != 0) {
                     unsigned long _Offset = _tzcnt_u32(_Bingo);
                     _Advance_bytes(_First, _Offset);
@@ -1879,10 +1874,11 @@ namespace {
                 }
 
                 _Advance_bytes(_First, 32);
-            }
-        }
 
-        if (_Traits::_Sse_available()) {
+                _Data  = _mm256_load_si256(static_cast<const __m256i*>(_First));
+                _Bingo = static_cast<unsigned int>(_mm256_movemask_epi8(_Traits::_Cmp_avx(_Data, _Comparand)));
+            }
+        } else if (_Traits::_Sse_available()) {
             // We read by vector-sized pieces, and we align pointers to vector-sized boundary.
             // From start partial piece we mask out matches that don't belong to the range.
             // This makes sure we never cross page boundary, thus we read 'as if' sequentially.
@@ -1898,17 +1894,8 @@ namespace {
             unsigned int _Bingo = static_cast<unsigned int>(_mm_movemask_epi8(_Traits::_Cmp_sse(_Data, _Comparand)));
 
             _Bingo &= _Mask;
-            if (_Bingo != 0) {
-                unsigned long _Offset;
-                _BitScanForward(&_Offset, _Bingo); // lgtm [cpp/conditionallyuninitializedvariable]
-                _Advance_bytes(_First, _Offset);
-                return _First;
-            }
 
             for (;;) {
-                _Data  = _mm_load_si128(static_cast<const __m128i*>(_First));
-                _Bingo = static_cast<unsigned int>(_mm_movemask_epi8(_Traits::_Cmp_sse(_Data, _Comparand)));
-
                 if (_Bingo != 0) {
                     unsigned long _Offset;
                     _BitScanForward(&_Offset, _Bingo); // lgtm [cpp/conditionallyuninitializedvariable]
@@ -1917,6 +1904,9 @@ namespace {
                 }
 
                 _Advance_bytes(_First, 16);
+
+                _Data  = _mm_load_si128(static_cast<const __m128i*>(_First));
+                _Bingo = static_cast<unsigned int>(_mm_movemask_epi8(_Traits::_Cmp_sse(_Data, _Comparand)));
             }
         }
 #endif // !_M_ARM64EC

tests/std/tests/VSO_0000000_vector_algorithms/test.cpp

@@ -140,6 +140,53 @@ void test_find(mt19937_64& gen) {
     }
 }
 
+#if _HAS_CXX20
+template <class T, size_t N>
+struct NormalArrayWrapper {
+    T m_arr[N];
+};
+
+// Also test GH-4454 "vector_algorithms.cpp: __std_find_trivial_unsized_impl assumes N-byte elements are N-aligned"
+#pragma pack(push, 1)
+template <class T, size_t N>
+struct PackedArrayWrapper {
+    uint8_t m_ignored; // to misalign the following array
+    T m_arr[N];
+};
+#pragma pack(pop)
+
+// GH-4449 <xutility>: ranges::find with unreachable_sentinel / __std_find_trivial_unsized_1 gives wrong result
+template <class T, template <class, size_t> class ArrayWrapper>
+void test_gh_4449_impl() {
+    constexpr T desired_val{11};
+    constexpr T unwanted_val{22};
+
+    ArrayWrapper<T, 256> wrapper;
+    auto& arr = wrapper.m_arr;
+
+    constexpr int mid1 = 64;
+    constexpr int mid2 = 192;
+
+    ranges::fill(arr, arr + mid1, desired_val);
+    ranges::fill(arr + mid1, arr + mid2, unwanted_val);
+    ranges::fill(arr + mid2, end(arr), desired_val);
+
+    for (int idx = mid1; idx <= mid2; ++idx) { // when idx == mid2, the value is immediately found
+        const auto where = ranges::find(arr + idx, unreachable_sentinel, desired_val);
+
+        assert(where == arr + mid2);
+
+        arr[idx] = desired_val; // get ready for the next iteration
+    }
+}
+
+template <class T>
+void test_gh_4449() {
+    test_gh_4449_impl<T, NormalArrayWrapper>();
+    test_gh_4449_impl<T, PackedArrayWrapper>();
+}
+#endif // _HAS_CXX20
+
 #if _HAS_CXX23
 template <class T>
 void test_case_find_last(const vector<T>& input, T v) {
@@ -371,6 +418,13 @@ void test_vector_algorithms(mt19937_64& gen) {
     test_find<long long>(gen);
     test_find<unsigned long long>(gen);
 
+#if _HAS_CXX20
+    test_gh_4449<uint8_t>();
+    test_gh_4449<uint16_t>();
+    test_gh_4449<uint32_t>();
+    test_gh_4449<uint64_t>();
+#endif // _HAS_CXX20
+
 #if _HAS_CXX23
     test_find_last<char>(gen);
     test_find_last<signed char>(gen);