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

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merged 4 commits into from
Mar 8, 2024
Merged

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

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8cdc78f
Add test coverage.
StephanTLavavej Mar 5, 2024
458fea6
Fix bug and improve perf with better control flow.
StephanTLavavej Mar 5, 2024
7da8f4b
Add PackedArrayWrapper test coverage.
StephanTLavavej Mar 6, 2024
a6781b6
Solve the alignment problem.
StephanTLavavej Mar 6, 2024
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32 changes: 11 additions & 21 deletions stl/src/vector_algorithms.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -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.
Expand All @@ -1862,27 +1865,20 @@ 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);
return _First;
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I observe that since we have only one return from AVX2 code path, we can remove _Zeroupper_on_exit _Guard; and put explicit _mm256_zeroupper(); here. Sure even if it is worth doing, can be done as a follow-up

}

_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.
Expand All @@ -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]
Expand All @@ -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
Expand Down
54 changes: 54 additions & 0 deletions tests/std/tests/VSO_0000000_vector_algorithms/test.cpp
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Expand Up @@ -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) {
Expand Down Expand Up @@ -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);
Expand Down