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Allow for SSO for constexpr string
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@miscco
miscco committed Jun 16, 2022
1 parent ea32e86 commit 6368f07
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Showing 2 changed files with 20 additions and 81 deletions.
87 changes: 15 additions & 72 deletions stl/inc/xstring
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Original file line number Diff line number Diff line change
Expand Up @@ -2279,11 +2279,6 @@ public:
}

_CONSTEXPR20 bool _Large_string_engaged() const noexcept {
#if _HAS_CXX20
if (_STD is_constant_evaluated()) {
return true;
}
#endif // _HAS_CXX20
return _BUF_SIZE <= _Myres;
}

Expand Down Expand Up @@ -2311,7 +2306,7 @@ public:
}

union _Bxty { // storage for small buffer or pointer to larger one
_CONSTEXPR20 _Bxty() noexcept : _Ptr() {} // user-provided, for fancy pointers
_CONSTEXPR20 _Bxty() noexcept : _Buf() {} // user-provided, for fancy pointers

_CONSTEXPR20 ~_Bxty() noexcept {} // user-provided, for fancy pointers

Expand Down Expand Up @@ -2689,19 +2684,8 @@ private:
auto&& _Alproxy = _GET_PROXY_ALLOCATOR(_Alty, _Al);
_Container_proxy_ptr<_Alty> _Proxy(_Alproxy, _My_data);

if (_Count < _BUF_SIZE) {
#if _HAS_CXX20
if (_STD is_constant_evaluated()) {
_My_data._Myres = _BUF_SIZE; // TRANSITION: constexpr SSO
}

const bool _Stay_small = _Count < _BUF_SIZE && !_STD is_constant_evaluated();
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
const bool _Stay_small = _Count < _BUF_SIZE;
#endif // _HAS_CXX20

if (_Stay_small) {
#if _HAS_CXX20
// TRANSITION: This is currently unused until SSO support is merged
if (_STD is_constant_evaluated()) {
_Construct_in_place(_My_data._Bx);
}
Expand Down Expand Up @@ -2767,7 +2751,6 @@ private:
#if _HAS_CXX20
if (_STD is_constant_evaluated()) {
_Construct_in_place(_My_data._Bx);
_My_data._Myres = _BUF_SIZE; // TRANSITION: constexpr SSO
}
#endif // _HAS_CXX20

Expand All @@ -2777,12 +2760,7 @@ private:
_Xlen_string(); // result too long
}

#if _HAS_CXX20
const bool _Become_large = _Count >= _BUF_SIZE || _STD is_constant_evaluated();
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
const bool _Become_large = _Count >= _BUF_SIZE;
#endif // _HAS_CXX20
if (_Become_large) {
if (_Count >= _BUF_SIZE) {
const size_type _New_capacity = _Calculate_growth(_Count);
const pointer _New_ptr = _Al.allocate(_New_capacity + 1); // throws
_Construct_in_place(_My_data._Bx._Ptr, _New_ptr);
Expand Down Expand Up @@ -2873,13 +2851,7 @@ public:
auto&& _Alproxy = _GET_PROXY_ALLOCATOR(_Alty, _Getal());
_Container_proxy_ptr<_Alty> _Proxy(_Alproxy, _My_data); // throws

#if _HAS_CXX20
const bool _Activate_large_mode = _New_capacity < _New_size || _STD is_constant_evaluated();
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
const bool _Activate_large_mode = _New_capacity < _New_size;
#endif // _HAS_CXX20

if (_Activate_large_mode) {
if (_New_capacity < _New_size) {
// we should never allocate less than _BUF_SIZE space (_New_size could be small if constant evaluated)
const size_type _Requested_size = (_STD max)(_New_size, _BUF_SIZE);
_New_capacity = _Calculate_growth(_Requested_size, _BUF_SIZE - 1, max_size());
Expand Down Expand Up @@ -3222,9 +3194,6 @@ public:
private:
_CONSTEXPR20 void _Copy_assign_val_from_small(const basic_string& _Right) {
// TRANSITION, VSO-761321; inline into only caller when that's fixed
#if _HAS_CXX20
_STL_ASSERT(!_STD is_constant_evaluated(), "SSO should be disabled in a constexpr context");
#endif // _HAS_CXX20
_Tidy_deallocate();
if constexpr (_Can_memcpy_val) {
#if _HAS_CXX20
Expand Down Expand Up @@ -4007,22 +3976,12 @@ public:
return;
}

#if _HAS_CXX20
const bool _Do_become_small = _My_data._Mysize < _BUF_SIZE && !_STD is_constant_evaluated();
#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
const bool _Do_become_small = _My_data._Mysize < _BUF_SIZE;
#endif // _HAS_CXX20
if (_Do_become_small) {
if (_My_data._Mysize < _BUF_SIZE) {
_Become_small();
return;
}

size_type _Target_capacity = (_STD min)(_My_data._Mysize | _ALLOC_MASK, max_size());
#if _HAS_CXX20
// must allocate at least _BUF_SIZE space
_Target_capacity = (_STD max)(_Target_capacity, _BUF_SIZE);
#endif // _HAS_CXX20

const size_type _Target_capacity = (_STD min)(_My_data._Mysize | _ALLOC_MASK, max_size());
if (_Target_capacity < _My_data._Myres) { // worth shrinking, do it
auto& _Al = _Getal();
const pointer _New_ptr = _Al.allocate(_Target_capacity + 1); // throws
Expand Down Expand Up @@ -4872,24 +4831,15 @@ private:
_CONSTEXPR20 void _Tidy_init() noexcept { // initialize basic_string data members
auto& _My_data = _Mypair._Myval2;
_My_data._Mysize = 0;

_My_data._Myres = _BUF_SIZE - 1;
#if _HAS_CXX20
if (_STD is_constant_evaluated()) {
_My_data._Myres = _BUF_SIZE; // SSO disabled in constexpr context
auto& _Al = _Getal();
const pointer _New_ptr = _Al.allocate(_BUF_SIZE + 1); // throws
_My_data._Bx._Ptr = _New_ptr;

_Elem* const _Raw_new = _Unfancy(_New_ptr);
_Traits::assign(_Raw_new, _BUF_SIZE + 1, _Elem());
} else
#endif // _HAS_CXX20
{
_My_data._Myres = _BUF_SIZE - 1;
// the _Traits::assign is last so the codegen doesn't think the char write can alias this
_Traits::assign(_My_data._Bx._Buf[0], _Elem());
if (_STD is_constant_evaluated()) { // begin the lifetime of the array elements before copying into them
_Construct_in_place(_My_data._Bx);
}
#endif // _HAS_CXX20

// the _Traits::assign is last so the codegen doesn't think the char write can alias this
_Traits::assign(_My_data._Bx._Buf[0], _Elem());
_ASAN_STRING_CREATE(*this);
}

Expand All @@ -4910,16 +4860,9 @@ private:
}

_My_data._Mysize = 0;
#if _HAS_CXX20
if (_STD is_constant_evaluated()) {
_My_data._Myres = 0;
} else
#endif // _HAS_CXX20
{
_My_data._Myres = _BUF_SIZE - 1;
// the _Traits::assign is last so the codegen doesn't think the char write can alias this
_Traits::assign(_My_data._Bx._Buf[0], _Elem());
}
_My_data._Myres = _BUF_SIZE - 1;
// the _Traits::assign is last so the codegen doesn't think the char write can alias this
_Traits::assign(_My_data._Bx._Buf[0], _Elem());
}

public:
Expand Down
14 changes: 5 additions & 9 deletions tests/std/tests/P0980R1_constexpr_strings/test.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -613,16 +613,12 @@ constexpr bool test_interface() {
literal_constructed.shrink_to_fit();

const auto c4 = literal_constructed.capacity();
if (is_constant_evaluated()) { // check minimum allocation of _BUF_SIZE when constant evaluated
assert(c4 == 16 / sizeof(CharType));
if constexpr (is_same_v<CharType, char16_t> || is_same_v<CharType, wchar_t>) {
assert(c4 == 7);
} else if constexpr (is_same_v<CharType, char32_t>) {
assert(c4 == 3);
} else {
if constexpr (is_same_v<CharType, char16_t> || is_same_v<CharType, wchar_t>) {
assert(c4 == 7);
} else if constexpr (is_same_v<CharType, char32_t>) {
assert(c4 == 3);
} else {
assert(c4 == 15);
}
assert(c4 == 15);
}
}

Expand Down

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