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[libc++] Optimize string operator[] for known large inputs #69500

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Merged
merged 1 commit into from
Oct 26, 2023
Merged

[libc++] Optimize string operator[] for known large inputs #69500

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6 changes: 6 additions & 0 deletions libcxx/include/string
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Original file line number Diff line number Diff line change
Expand Up @@ -1198,11 +1198,17 @@ public:

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 const_reference operator[](size_type __pos) const _NOEXCEPT {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
return *(__get_long_pointer() + __pos);
}
return *(data() + __pos);
}

_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 reference operator[](size_type __pos) _NOEXCEPT {
_LIBCPP_ASSERT_VALID_ELEMENT_ACCESS(__pos <= size(), "string index out of bounds");
if (__builtin_constant_p(__pos) && !__fits_in_sso(__pos)) {
return *(__get_long_pointer() + __pos);
}
return *(__get_pointer() + __pos);
}

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21 changes: 21 additions & 0 deletions libcxx/test/std/strings/basic.string/string.access/index.pass.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -34,10 +34,31 @@ TEST_CONSTEXPR_CXX20 void test_string() {
assert(s2[0] == '\0');
}

// Same, but for the string that doesn't fit into SSO.
template <class S>
TEST_CONSTEXPR_CXX20 void test_string_long() {
S s("0123456789012345678901234567890123456789");
const S& cs = s;
ASSERT_SAME_TYPE(decltype(s[0]), typename S::reference);
ASSERT_SAME_TYPE(decltype(cs[0]), typename S::const_reference);
LIBCPP_ASSERT_NOEXCEPT(s[0]);
LIBCPP_ASSERT_NOEXCEPT(cs[0]);
for (typename S::size_type i = 0; i < cs.size(); ++i) {
assert(s[i] == static_cast<char>('0' + (i % 10)));
assert(cs[i] == s[i]);
}
assert(s[33] == static_cast<char>('0' + (33 % 10)));
assert(cs[34] == s[34]);
assert(cs[cs.size()] == '\0');
const S s2 = S();
assert(s2[0] == '\0');
}

TEST_CONSTEXPR_CXX20 bool test() {
test_string<std::string>();
#if TEST_STD_VER >= 11
test_string<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
test_string_long<std::basic_string<char, std::char_traits<char>, min_allocator<char>>>();
#endif

return true;
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